25 June 2019

Een Geep - Belone belone - in het getijdengebied

fig.1. Geep, Belone belone belone. Neeltje Jans, Noordzee, 4-6-2019. 
Click here for an English version 

Hoe vaak maak je zoiets mee? Eén keer in je leven? Een grote Geep, Belone belone, die in een ondiep stroomgebied 'omhoog', richting land, zwemt. Ik maakte het mee bij Neeltje Jans op 4 juni 2019.


fig. 2. Detail met kop van de Geep, Belone belone belone en parasiet Caligus species (zie verderop) op de kieuwplaat.
Neeltje Jans, Noordzee, 4-6-2019. 

Ik was al een paar uur stenen aan het keren in het stroomgebied bij de getijdenpoel bij het Topshuis (fig. 3; klik hier voor een uitgebreide beschrijving van het gebied). Het was lekker weer en heerlijk stil en ik vermaakte mij prima met het fotograferen van de dieren die ik vond onder de rijk aan de onderzijde begroeide stenen. Anders dan ik gewoon ben, was ik pas rond de laagwater kentering bij het stroomgebied; doorgaans ben ik twee uur vóór de kentering ter plaatse. Die late start bleek nog minder een probleem dan ik dacht. Voordat het middelste deel van het stroomgebied weer vol begon te lopen vanuit zee (vanuit de getijdenpoel - aan de landzijde - blijft het continu lopen, vandaar 'stroom'gebied), waren vandaag drie uren verstreken. Alle tijd en rust dus om te zoeken en te fotograferen.


fig. 3. Stroomgebied van Neeltje Jans, 27-5-2016.

Echt iets bijzonders had ik nog niet gevonden, totdat ik op een paar meter afstand allerlei gespetter hoorde. Ik dacht nog even dat het kwam door het stromende water, maar het klonk toch anders. Toen  ik ging staan, zag ik veel beweging in het water en tot mijn verbazing zwom er een Geep rond (fig. 1 e.a.)!

Gepen komen naar de kust om daar te paaien en eieren af te zetten. Dit exemplaar was dan wel erg enthousiast en het leek mij niet zonder risico. Daarbij was het - zeker voor dit ondiepe en steenrijke gebied - een fors exemplaar (47 cm). Het dier had zijn lange bek zo te zien ook al een paar keer gestoten tegen de stenen, want die was een beetje beschadigd. Uiteraard wilde ik er graag foto's van maken en bij voorkeur ook onderwater.

fig. 4. Geep, Belone belone belone in de poel waar ik het dier fotografeerde. Neeltje Jans, Noordzee, 4-6-2019. 

Wat er kwam kijken bij de fotografie 
Ik was al aan het fotograferen met mijn Olympus TG-5, een erg handige camera voor bij het stenen keren (klik hier voor een bericht over de TG-5). Gelukkig had ik ook een WeeFine groothoeklens (tot 150º) meegenomen en het daarvoor benodigde onderwaterhuis. Misschien zou ik de Geep dan ook in zijn geheel onderwater kunnen fotograferen.

Het dier zo in het stroomgebied fotograferen was onmogelijk. Overdag zijn ze schuw en erg snel en zou ik hem nooit goed in beeld krijgen. Daarbij was de kans groot dat hij meteen rechtsomkeer zou maken. Ik zag een poeltje aan de rand van het stroomgebied, waar hij niet uit zou kunnen zwemmen, dat groot genoeg was en waar continu water doorheen stroomde (fig. 4). Dat laatste was belangrijk, omdat daardoor zwevende vuildeeltjes zouden worden afgevoerd. Ook waren de stenen aan de randen goed begroeid met zeewieren, zodat hij zijn snavel niet zou beschadigen. Mijn eerste poging hem te pakken mislukte. Gelukkig zwom hij een doodlopend deel van het stroomgebied in en kon ik hem alsnog in het poeltje zetten.

fig. 5. Geep, Belone belone belone. Neeltje Jans, Noordzee, 4-6-2019. 

Het werkte fantastisch. Hij zwom wel wat, maar hij had niet zoveel ruimte; hij kon zich nog net keren in de poel (fig. 4 & 5). Hij leek ook niet vreselijk bang. Anders was er het risico dat hij uit de poel zou springen en zich zou beschadigen. Het fotograferen was bepaald niet makkelijk, want het moest - net als vroeger met snorkelen - op gevoel en ervaring. Ik kon namelijk niet op het scherm kijken. Ik heb een hoekzoeker voor het onderwaterhuis, maar die had ik dus net niet bij mij.

Lastig is ook dat er water moet zitten tussen het onderwaterhuis en de groothoeklens. Door de ondiepte van de poel ging dat niet makkelijk, liep het water er uit en besloeg de tussenruimte tussen huis en lens meerdere malen. Al met al ben ik gezien de beperkingen erg tevreden met het resultaat!

Net toen ik klaar was met mijn fotowerk, stroomde het water vanuit zee de poel in en meteen draaide de Geep zich om richting zee, vond nu wel een uitgang en zwom weg.


fig. 6. Geep, Belone belone belone. Neeltje Jans, Noordzee, 4-6-2019.

Lengte
De Geep is, zoals je kunt zien, een bijzondere verschijning met die lange snavelvormige bek. De vis wordt maximaal 104 cm (Dulcic & Soldo, 2006) en gemiddeld 45 cm lang. Een exemplaar van 95 cm weegt rond de 1,3 kg. In het eerste jaar bereiken ze een lengte van maximaal 25 cm. In het tweede jaar worden ze geslachtsrijp en zijn dan zo'n 45 cm. Overigens, hoe hoger de temperatuur van het zeewater, hoe sneller de ontwikkeling en groei. Het exemplaar dat ik vond was dus minimaal twee jaar oud. Ze kunnen tot 18 jaar oud worden.

fig. 7. Opeens was de Geep uit beeld. Pas later zag ik dat een Strandkrab, Carcinus maenas er met de buit vandoor wilde gaan. 
Neeltje Jans, Noordzee, 4-6-2019. 

Jager en prooi
Gepen zijn zichtjagers en snelle zwemmers, die jagen op kleine(re) vissen, inktvissen en kreeftachtigen. Van de vissen waarop Gepen jagen, worden specifiek genoemd: Koornaarvis (Atherina species), Haring, Sprot, Ansjovis, Zandspiering, Stekelbaars en kabeljauwachtigen (Gadidae). Ze zoeken hun prooi voornamelijk overdag. Jager (2015): 'De bijzonder gespecialiseerde ogen zijn aangepast aan het jagen bij de hoge lichtintensiteit die heerst aan het wateroppervlak. Door de hoge dichtheid aan kegeltjes in het oog is het zicht zeer scherp. In het netvlies zijn twee afzonderlijke gedeelten, het ene aangepast aan de hoge lichtintensiteit in de zone boven de vis ('Snell's window'), en het andere aangepast aan de lage lichtintensiteit buiten dit venster. Het oog is bovendien erg lichtgevoelig, zodat zelfs ’s nachts door Gepen op zicht gejaagd kan worden.'


fig. 8. Geep, Belone belone belone. Neeltje Jans, Noordzee, 4-6-2019.

Voorkomen
Het is een pelagisch levende vis. Ze leven 's winters in scholen in open water, ten westen van Groot-Brittannië en trekken in maart-april - als de zee warmer wordt - de Noordzee in waar ze in ondiep water paaien. De hoofdpaaitijd is mei-juni, maar het kan doorlopen tot september. De meeste bronnen beschrijven Gepen als levend vlak onder het wateroppervlak. Louysi (2015) noteert tussen de 0 en 1 meter en maximaal 5 meter diep. Heessen et al., (2015) geven aan dat ze bij vangsten tot 600 m diep werden aangetroffen! Wel met een kanttekening: omdat het visnet meteen bij het wateroppervlak open wordt gezet en zo ook weer wordt opgehaald, kan het zijn dat de dieren op een geringere diepte in het net verzeild zijn geraakt. Die keren dat ik als duiker Gepen zag (daarover verderop meer), was dat ook nog geen halve meter diep.

Het schijnt een algemene soort te zijn. Ik sprak vorig jaar hengelaars die vertelden dat ze op de pier van Hoek van Holland regelmatig Gepen vangen. Ze zouden vrij talrijk zijn in de Oosterschelde. Nijssen (1987): 'In 1983 werd in de Zeeuwse wateren 1570 kg Geep gevangen.' In Jager (2015) kun je meer lezen over de visserij.


fig. 9. Gepen, Belone belone gracilis en hun weerspiegeling in het wateroppervlak. Playa Chica, Puerto del Carmen, Lanzarote, 3-3-2016.

Duikwaarnemingen
Door duikers worden ze in Nederland weinig gezien. Dat heeft mogelijk te maken met het feit dat ze direct onder het wateroppervlak leven; de meeste duikers gaan vrijwel meteen naar dieper water. Daarbij zijn het schuwe vissen, die moeilijk te benaderen zijn. Dan zijn het ook nog eens snelle zwemmers en is het Nederlandse duikwater doorgaans niet geweldig helder.

Wat betreft schuw en moeilijk te benaderen: dat mag overdag het geval zijn, 's avonds is dat - zoals ik twee keer heb ervaren - helemaal niet het geval. In maart 2016 maakte ik op Lanzarote, één van de Canarische Eilanden, een avondduik. Aan het einde van die duik - ik was nog net niet door mijn lucht heen en de accu's van mijn flitsers raakten duidelijk leeg - kwam ik een schooltje van zo'n 15 Gepen tegen. Die kwamen zelfs heel dichtbij tot op een tiental centimeters en lieten zich goed fotograferen! Zie figuren 9, 10 en 12.

In september datzelfde jaar kwam ik bij een avondduik in de Adriatische Zee bij Triëst een grote, dikke Geep tegen. Die zwom duidelijk op mijn navigatielicht af en ik moest hem wegduwen, omdat hij anders zijn bek en misschien mijn apparatuur zou beschadigen. Ook dat was aan het einde van mijn duik; ik kon er geen foto van maken omdat de accu's van mijn flitsers volledig leeg waren. Dus in het donker zijn ze goed te benaderen en komen ze zelfs op licht af. Dat laatste wordt bevestigd in Heessen et al. (2015). Zoals al gezegd zaten de Gepen beide keren nog geen halve meter onder het wateroppervlak.


fig. 10. Gepen, Belone belone gracilis. Playa Chica, Puerto del Carmen, Lanzarote, 3-3-2016.

Overigens wordt een drietal ondersoorten onderscheiden (Collette & Parrin, 1986). Het exemplaar dat ik bij Neeltje Jans heb gefotografeerd, is Belone belone belone. Die van Lanzarote behoren tot Belone belone gracilis.

fig. 11. Visnijntje, Caligus species op de kieuwplaat van de Geep. Helaas geen betere foto; beeld is sterk uitgesneden. 
Neeltje Jans, Noordzee, 4-6-2019. 

Visnijntje
Bij het bekijken van de foto's, zag ik parasitaire copepodes zitten, één op de kieuwplaat (fig. 11), twee op de borstvin en één op de anaalvin. Het gaat om een Caligus soort, een soort 'Visnijntje' (voorstel voor een Nederlandse naam van dit genus). Op de foto's zijn niet voldoende details te zien om de soort verder op naam te brengen en op Gepen kunnen meerdere soorten Caligus gevonden worden, onder andere Caligus belone. Je snapt meteen waarvan de soortnaam is afgeleid!


fig. 12. In Natura 10: kunstwerk gebaseerd op de gefotografeerde Gepen op Lanzarote.

Literatuur & weblinks
  • ANEMOON
  • Fishbase
  • Collette, B.B. & N.V. Parrin, 1986. Belonidae. Fishes of the North-eastern Atlantic and the Mediterranean. Volume II. p 60-609.
  • Dulcic, J. & A. Soldo, 2002. A new maximum length for the garpike, Belone belone (Belonidae). Cymbium, 30, p 382.
  • Heessen, H.J.L., N. Daan & J.R. Ellis, 2015. Fish atlas of the Celtic Sea, North Sea and Baltic Sea, p 252-254. ISBN 97889086862665.
  • Jager, Z., 2015. Factsheet Vissoorten - Geep Geïntegreerde Visserij. Klik hier voor de PDF.
  • Louisy, P., 2015. Europe and Mediterranean Marine Fish identification guide. ISBN 139782841388110.
  • Nijssen, H. & S.J. de Groot, 1987. De vissen van Nederland. ISBN 9050110061.

A garfish - Belone belone - in the tidal zone

 fig. 1. Garfish, Belone belone belone. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 
Klik hier voor een Nederlandstalige versie 

How often do you experience something like that? Once in your life? A big garfish, Belone belone, swimming upstream in a quite shallow 'rockstream'! I did at Neeltje Jans, the Netherlands (North Sea) on 4 June 2019.


 fig. 2. Garfish, Belone belone belone. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 

I was turning stones at ebb tide for a few hours, the weather was fine and I enjoyed the silence. I was taking pictures of animals I found on the rich undergrowth of the stones. Usually I start my field trips at ebb tide two hours before slack water. That gives me time to explore the rocks and rockpools just above the sublittoral zone, where you will find the more rare species. But Neeltje Jans is different: the most interesting part of the rockstream - a zone with interconnected rockpools - (fig. 3) is not situated at the lowest point. It took three hours before the tide came in, so I had all the time to explore and take photo's.


fig. 3. 'Rockstream' at Neeltje Jans, the Netherlands (North Sea), 27-5-2016.

I had not discovered something special, till I heard something splashing a few meters from where I was standing. At first I thought it was just the flowing water, but the sound was different. Then I saw a lot of movement in the water and to my surprise I saw a garfish swimming upstream (fig. 1 and others)!

Garfish migrate inshore in spring to spawn in shallow water during early summer. This one seemed over enthusiastic, because it was not without risk. It was a large specimen of 47 cm and the rockstream was quite shallow and rocky. It had already bumped its mouth; there was some damage at the front. Of course I would love to take photo's and preferably underwater.


 fig. 4. Garfish, Belone belone belone in the pool where I took the photo's. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 

What was involved in photography
I was already taking photo's with my Olympus TG-5, a very handy underwater compact camera (click here for an article about the TG-5). Fortunately I brought a WeeFine wide angle lens (to 150º) along and the underwater housing for the camera and lens. Maybe I could take photo's of the entire animal underwater!

Just taking photo's at the very spot was impossible. In daylight they are shy and fast, so getting a good look, making a composition and getting the auto focus at the right spot was not to be. And the chance it would turn around and disappear, was very probable. I saw a pool at the edge of the rockstream that was big enough, where it could not escape and with running water (fig. 4). Running water was desirable. Otherwise I would get a picture of a lot of debris instead of the garfish. Another advantage of the pool: the rocks were overgrown with seaweeds that would function as a buffer edge. I failed at my first attempt, but the garfish swam into a dead corner and I was able to put it in the pool.


 fig. 5. Garfish, Belone belone belone. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 

It worked out perfectly. The fish did swim, but the space was limited; it was just able to turn around in the pool (fig. 4 & 5). It did not seem to be very shy, otherwise there woud have been the risk of it jumping out of the pool and hurting itself. Taking photo's was not easy, because I could not see anything on the display (I have a kind of angle finder but I forgot to take it with me). So I had to take photo's in the blind and rely on my experience, just as I used to do in the past when snorkeling. The fact there has to be water between the wide angle lens and the underwater housing did not make it any easier. The pool was quite shallow, water got out and several times the space between lens and underwater housing fogged up.

All in all I am very satisfied with the results! Just as I had finished taking pictures, the pool flooded. The garfish turned around immediately, found the exit and swam away to deeper water.


 fig. 6. Garfish, Belone belone belone. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 

Length
Garfish are easily recognised by their long thin bodies and their long, needle like mouth. They can grow to a length of 104 cm (Dulcic & Soldo, 2006). Their average size is 45 cm. A garfish of 95 cm weighs about 1.3 kg. In their first year they become about 25 cm long. In their second year, when they become mature, they grow to a length of 45 cm. So the one I discovered was at least two years old. But it is not exact science: the higher the temperature of the seawater, the quicker they develop and grow. Garfish can live up to 18 years.


fig. 7. Suddenly the garfish moved out of sight. When I saw the photo's I realised that a Green crab, Carcinus maenas wanted the garfish for prey. Neeltje Jans, the Netherlands (North Sea), 4-6-2019. 

Hunters and prey
Garfish are sight hunters and fast swimmers. They hunt for prey like small(er) fish, cephalopods and arthropods. Of the fish the following are specifically mentioned: sand smelt (Atherina), herring (Clupea), sprat (Sprattus), sand eels (Ammodytes), stickleback (Gasterosteus), cod species and the like (Gadidae).

They mainly seek their prey during the day. Jager (2015): 'The highly specialized eyes are adapted to hunting at the high light intensity that prevails on the water surface. Due to the high density of cones in the eye, visibility is very sharp. In the retina, two separate sections, one adapted to the high light intensity in the area above the fish ('Snell's window'), and the other adapted to the low light intensity outside this window. Moreover, the eye is very light-sensitive, so that even at night, garfish can hunt for sight.'


fig. 8. Geep, Belone belone belone. Neeltje Jans, Noordzee, 4-6-2019.

Habitat
It is a pelagic fish. They live in open water shoals west of Great Britain in the winter and in March-April - when the sea gets warmer - head into the North Sea where they spawn in shallow water. The main spawning time is May-June, but it can continue until September. Most sources describe garfish as living just below the water surface. Louysi (2015) notes between 0 and 1 meter and a maximum of 5 meters deep. Heessen et al., (2015) indicate that they were found with catches up to 600 m deep! However, with a side note: because the fishing net is immediately opened at the surface of the water and thus brought again, it is possible that the animals have ended up in the net at a lesser depth. The times that I saw garfish as a diver (more on that later), it was not even half a meter deep.

It seems to be a general species. Last year I spoke to anglers who told me that they regularly catch garfish on the pier of Hoek van Holland. They are supposed to be quite numerous in the Oosterschelde. Nijssen (1987): 'In 1983, 1570 kg of garfish was caught in the Zeeland waters'. In Jager (2015) you can read more about the fishing on garfish.


fig. 9. Garfish, Belone belone gracilis and their reflection in the surface of the water. Playa Chica, Puerto del Carmen, Lanzarote, 3-3-2016.

Dive observations
They are rarely seen by divers. Possibly because they spend most of their time just beneath the surface; most divers descend straight away to deeper waters. Furthermore: they are quite shy at daylight, rapid swimmers and the Dutch diving waters are usually far form clear.

As regards to being shy: that may be the case during daylight, at dark they are - as I have discovered twice - easy to approach. In March 2016 I made a dive at dark at Lanzarote, one of the Canary Islands. At the end of my dive - I ran almost out of air and the power of my flash units was almost drained - I ran into a shoal of about 15 garfish. They came to me as close as ten centimeter and it was quite easy to take photo's. See figures 9, 10 and 12.

In September the same year I ran - also after dark - into a big, fat garfish at Trieste (Italy) at the Adriatic Sea. That one swam directly to my navigating light and I had to push it away or it could have damaged its mouth or my gear. That was also at the end of my dive; no more power in my flash units, hence no photo. Heessen et al. (2015) confirm my observation that they are are attracted by lights. As said, both encounters were not deeper than half a meter beneath the surface.


fig. 10. Garfish, Belone belone gracilis. Playa Chica, Puerto del Carmen, Lanzarote, 3-3-2016.

By the way: a triple of subspecies are distinguished (Collette & Parrin, 1986). The one I saw at Neeltje Jans is Belone belone belone. The garfish of Lanzarote (fig. 9, 10 & 12) and Trieste belong to Belone belone gracilis.


fig. 11. A parasitic copepod, Caligus species on the gill cover of the garfish. This - largely cropped - photo is the best I got. Neeltje Jans, Noordzee, 4-6-2019. 

Parasites
When I was looking at the photos, I saw a few parasitic copepodes, one at the gill cover, two at the pectoral fin and one on the anal fin. It is a Caligus species (fig. 11). The photo's are not detailed enough to find out what species. Garfish can be infected by several Caligus species, one with the scientific name Caligus belone; it is not hard to imagine where the name of this species derives from!


fig. 12. In Natura 10: artwork based on the garfish photographed at Lanzarote.

Literature & weblinks
  • ANEMOON
  • Fishbase
  • Collette, B.B. & N.V. Parrin, 1986. Belonidae. Fishes of the North-eastern Atlantic and the Mediterranean. Volume II. p 60-609.
  • Dulcic, J. & A. Soldo, 2002. A new maximum length for the garpike, Belone belone (Belonidae). Cymbium, 30, p 382.
  • Heessen, H.J.L., N. Daan & J.R. Ellis, 2015. Fish atlas of the Celtic Sea, North Sea and Baltic Sea, p 252-254. ISBN 97889086862665.
  • Jager, Z., 2015. Factsheet Vissoorten - Geep Geïntegreerde Visserij. Click here of a PDF (in Dutch).
  • Louisy, P., 2015. Europe and Mediterranean Marine Fish identification guide. ISBN 139782841388110.
  • Nijssen, H. & S.J. de Groot, 1987. De vissen van Nederland. ISBN 9050110061.

15 August 2018

Alive, tiny and interesting: Brachystomia scalaris, the Mussel pyramid snail


fig. 1  Two Mussel pyramid snails, Brachystomia scalaris (NL: Mosselslurper) on a Common mussel, Mytilus edulis (NL: Mossel). 
Westkapelle, 14-7-2018 (in vitro).

I googled for pictures of Odostomia scalaris, the former name of a little snail, now Brachystomia scalaris*. And what came up? Dozens of pages with carcasses. The first photo of a living specimen of this snail (at that search 81 pages later) is of the discontinued website of Marco Faasse. Why does it take so many webpages before finding one with a picture of a living animal? The answer is not too difficult: because shell collectors and (a lot of) malacologists prefer carcasses; they are collectable. I prefer the living being. Not only for its looks - isn't it a beautiful animal with its yellow-gold speckles? - but also for its behaviour as a parasite preying on Mussels.

fig. 2  Mussel pyramid snail, Brachystomia scalaris (NL: Mosselslurper). Westkapelle, 14-7-2018 (in vitro).
Brachystomia scalaris, the Mussel pyramid snail, is a tiny snail with a maximum size of about 4.5 mm and 1.8 mm width, but often they remain smaller (fig. 3). In Dutch it is aptly called Mosselslurper, translated Musselslurper. Aptly because it predates mostly on Common mussels, Mytilus edulis (NL: Mossel). More about that later.

You can find a short description of the snail at Marine Species Identification Portal. And for my Dutch readers also at the website of Stichting ANEMOON. At the website of WoRMS you can find information about its distribution.


fig. 3  The size of a batch of Mussel pyramid snails, Brachystomia scalaris (NL: Mosselslurper) 
compared to a pin-head with a diameter of 1.9 mm. Westkapelle, 14-7-2018 (in vitro).

The first time I saw one, was at a field trip with the Strandwerkgroep Waterweg-Noord to Westkapelle, the Netherlands, while turning stones at ebb tide. Marianne Ligthart, one of the participants, has eyes like a hawk and she found a few Mussel pyramid snails. The only thing I saw were some very tiny whitish dots... The next time we were at Westkapelle I tried to find one, but to no avail. She found them several times and another member of our group - also one with excellent eyes - found up to 10 specimens in one trip.

fig. 4  Common mussel, Mytilus edulis (NL: Mossel). Suspended cultivation, Neeltje Jans, the Netherlands, 22-8-2009.

The first time I found a Mussel pyramid snail was at home in my aquarium. I had collected a lump of Common mussels at Den Osse-Nieuwe Kerkweg, the Netherlands, as food for the animals in my aquarium. When I took the lump out of the water to detach a few individuals, a few tiny shell particles fell off: Mussel pyramid snails! In a tiny lump I found more than 10 specimens of this snail between the byssus threads. Recently I collected a few lumps of Mussels at Westkapelle and again out of a fairly small lump I collected 13 specimens (all figures of the snail, except fig. 16).

fig. 5  Two Mussel pyramid snails, Brachystomia scalaris (NL: Mosselslurper) on a Mussel 
beneath its byssus threads. Westkapelle, 14-7-2018 (in vitro).

Brachystomia scalaris belongs to the Pyramidellidae, a family existing of parasites preying on molluscs (snails and bivalves), worms, hydrozoans and echinoderms. It lives between the byssus threads of the Common mussel. 

fig. 6  Byssus threads of the Common mussel, Mytilus edulis (NL: Mossel). 
Den Osse-Nieuwe Kerkweg, the Netherlands, 30-3-2011.

Byssus threads are extremely tough, strong, elastic threads that secure the Mussel to its substrate (fig. 5-8). They allow the Mussel to remain sessile in areas of high flow. Enlarged they look like fiberglass (fig. 6). The threads are deposited by means of a 'tongue' (fig. 8 & 9). This tongue, in bivalve shell terms usually called the foot, has a groove on the ventral surface which is continuous with the byssus pit. In this pit, a viscous secretion is exuded, entering the groove and hardening gradually upon contact with sea water. Maybe you don't realize how extraordinary this is: have you ever tried to glue something together that is wet?!

fig. 7  Common mussel, Mytilus edulis (NL: Mossel) with its byssus threads attached to the substrate. 
Neeltje Jans, the Netherlands, 21-2-2015.

The byssal threads are also sometimes used by Mussels as a defensive measure, to tether predatory molluscs, such as the Dog whelk, Nucella lapillus (NL: Purperslak) and the Japanese oyster borer, Ocenebra inornata (NL: Japanse stekelhoren), that invade Mussel beds, immobilising them and thus starving them to death. A fate that could also await Mussel pyramid snails!

fig. 8  Byssus threads of the Common mussel, Mytilus edulis (NL: Mossel) attached to the glass of an aquarium. 
Top left you can see the cream-brown tongue with which the Mussel attached the byssus threads. Maassluis, 2-1-2018.

The snail feeds by extending its proboscis, a retractable appendage like the trunk of an elephant (fig. 9, 12, 14 & 15) and carefully sticks it between the open valves at the basal (under) side, where it reaches the soft part of the shell. There it sucks blood and tissue of the shell as food. They have no radula (rasping tongue) like most snails. Amazingly the Mussel does not close both valves in order to defend itself! If it did, the snail would loose its proboscis.

fig. 9  Mussel pyramid snail, Brachystomia scalaris (NL: Mosselslurper) with its proboscis extended. 
Next to the snail you can see the tongue of the Mussel. Westkapelle, 14-7-2018 (in vitro).

I already mentioned Marianne Ligthart; in 2013 she sent me some photo's she took of the Mussel pyramid snail and on one of the photo's the snail extended its proboscis. That is what I wanted to see and to record on photo! It took some time and patience but as you can see at fig. 9, 14 and 15 I succeeded. 

fig. 10  Common mussel, Mytilus edulis (NL: Mossel) showing the fringes at the mantle rim. 
Den Osse-Nieuwe Kerkweg, the Netherlands, 30-3-2011.

The incurrent siphon (opening) of Mussels is adorned with fringes (fig. 4, 10, 11, 13-15) to keep out alien life forms and any kind of particles, except plankton on which they live. Why are these fringes not ‘active’ when a snail is entering you with a snout and eating you? I wondered if the Mussel pyramid snail is clever enough to use the fringeless and less protected excurrent siphon (fig. 11), but Van Benthem Jutting (1933) after Pelseneer (1914) describes it as sticking in its proboscis at the basal - incurrent siphon - side.

fig. 11  Common mussel, Mytilus edulis (NL: Mossel) showing its excurrent (left) and incurrent (right) siphon. 
Den Osse-Nieuwe Kerkweg, the Netherlands, 30-3-2011.

fig. 12  Illustration from Mollusca from Van Benthem Jutting (1933) after Pelseneer (1914) 
showing several Mussel pyramid snails feeding alongside each other. 

Van Benthem Jutting (1933) shows an illustration of Pelseneer (1914) (fig. 12) of an often recorded behaviour of Mussel pyramid snails feeding alongside each other. Have a look at this funny illustration: snails eating like pigs at a trough.

fig. 13
fig. 14


fig. 15  
On fig. 13 to 15: Mussel pyramid snail, Brachystomia scalaris (NL: Mosselslurper) 
with its proboscis from withdrawn to extendend. Westkapelle, 14-7-2018 (in vitro).


As already mentioned and can be seen at fig. 13 to 15: the proboscis is an extendable appendage. The snail is probing a Mussel and extending its proboscis. The white 'curtains' are the fringes of the Mussel.

fig. 16  Mussel pyramid snail, Brachystomia scalaris (NL: Mosselslurper). Westkapelle, 13-2-2016 (in vitro).
I always thought this species to be quite rare in the Netherlands. The first native specimens were recorded in 1951 and since then they were often discovered, albeit more dead than alive. Two records in the Centraal Systeem of the Strandwerkgemeenschap (a databank of amateur and professional marine biologists) attracted my attention. Both are mentioned in articles in Het Zeepaard. The first one is the discovery of 113 specimens in total at Ameland on 21, 22 and 28 December 1976 (Adema, 1978). It is not mentioned but I expect they were empty snail-shells. The second one is a find of about 75 living specimens at De Heerenkeet on 21 October 1982 (Slager, 1984). Not only these records but my chance findings at Den Osse and Westkapelle show that they must be quite common, at least locally. Above all they are easy to overlook…


Literature & weblinks
  • Benthem Jutting, T. van, 1933. Mollusca (I). Fauna van Nederland deel VII: 126-127.
  • Fretter, V. & A. Graham, 1949. The structure and mode of life of the Pyramidellidae, parasitic Opisthobranchs. Journ. Marine Biol. Ass. Un. Kingdom, vol.23: 493-532.
  • Nordsieck, F., 1972. Die europaischen Meeresschnecken-, 85-133.
  • Pelseneer, P., 1914. Ethologie de quelques Odostomia et d'un Monstrillide parasite d'un d'eux. Buil. sci. France et Belge, vol. 48: 1-14.  
  • Slager, G. 1984. C.S.-verslag, Het Zeepaard, jrg. 44, nr. 1.
  • Strack, H., 1976. Over Odostomia rissoides (Hanley). Het Zeepaard, jrg. 36, nr. 5: 76-80. 
  • Wikipedia: https://en.wikipedia.org/wiki/Mussel
  • WoRMS: http://www.marinespecies.org/aphia.php?p=taxdetails&id=491633
*Why did I not google for Brachystomia scalaris? I did. The first searches produced websites with only some kind of snake!

13 July 2018

Thorny doris - Acanthodoris pilosa - and the advantages of a waterproof compact camera

fig. 1  Two Thorny doris, Acanthodoris pilosa (NL: Egelslak) chasing each other before mating. Neeltje Jans, the Netherlands, 19-5-2018. 

What is the connection between a sea slug and a compact camera? In short: because of the compact camera I was able to take photo's of a few Thorny doris and their mating behaviour in extreme shallow water.

Camera gear

Let's begin with the compact camera. For diving and snorkeling I use full frame camera’s like a Canon 5DMKII and a 5DSr in Ikelite underwater housings (fig. 2 & 21). See for more information about these camera’s, lenses etc. at the end of this post. I use the same camera’s at home for ‘in vitro’ photography. Both are excellent camera’s, but there are circumstances in which I need a small waterproof camera in stead of a camera in a bulky underwater housing. For example for taking photo’s in my aquarium in order to get as close as possible and to avoid all kind of reflections. And also to take photo’s while turning stones and rockpooling at ebb tide.


fig. 2  Compare the Olympus compact camera TG-5 (width 11,5 cm) with a Sea Dragon light (front) and the full frame Canon in Ikelite housing with Ikelite strobes (weight about 10 kg). 

That is why I bought an Olympus TG-5. The TG-5 is a ‘rugged’ waterproof compact camera with an excellent macro range and - a must for underwater photography - you can record in RAW. At fig. 2 you can see how small it is: compare the TG-5 (mounted on a Sealife tray with a Sea Dragon 2300 light) with my usual photo diving gear!

fig. 3  Four of the initially six Thorny doris, Acanthodoris pilosa (NL: Egelslak) on a turned stone. The dark brown patch in the middle of the photo is its prey: Alcyonidium gelatinosum. Neeltje Jans, the Netherlands, 19-5-2018. 

Thorny doris

And what about the Thorny doris, Acanthodoris pilosa (NL: Egelslak)? On May 19 this year the Strandwerkgroep Waterweg-Noord (see note) had a field trip at Neeltje Jans (North Sea coast, the Netherlands). One of our members discovered six specimens of the Thorny doris on a stone she turned (fig. 3). This nudibranch is not uncommon in the tidal zone, but six under one stone is extraordinary.


fig. 4 Pascal taking photo's with his TG-5 of the Thorny doris on the turned stone. 

With the turned stone above water level, the slugs looked like wet wine gums (fig. 3). So I changed the position of the stone in order to put the slugs back into water. This is where the TG-5 reappears in my story: even in just 5 cm of water you can take pictures of slugs and other small stuff with this camera (fig. 4). It has no tilting screen, but with some effort and a lot of photo’s, I made some nice pictures. To avoid misconception: only figures 1, 3, 4-7 are taken with the TG-5!

fig. 5  Parade of Thorny doris, Acanthodoris pilosa (NL: Egelslak). The dark brown specimen had a size of 40 mm, quite large for Dutch standards. Neeltje Jans, the Netherlands, 19-5-2018. 

At that time I saw some half-done egg ribbons (fig. 6 & 7) deposited by the Thorny doris', but only afterwards at home, I saw they were chasing and crashing into one another. Foreplay to mate (fig. 5 & 6)!

fig. 6  Foreplay of Thorny doris, Acanthodoris pilosa (NL: Egelslak), crashing into each other: Neeltje Jans, the Netherlands, 19-5-2018. 

Eventually the two largest specimens were copulating (fig. 7). As usual with the right side turned to one another, because that is where their reproductive organ is situated. If I had no TG-5 I had not known of the chase and copulating Doris'. And of course no nice photo’s of this sea slug that looks like a fluffy toy.


fig. 7  In the end: copulating Thorny doris, Acanthodoris pilosa (NL: Egelslak). Neeltje Jans, the Netherlands, 19-5-2018. 

More about the Thorny doris

There are several websites and books with good descriptions of the Thorny doris. See Literature and websites at the end of the post. The most extensive description is given at the website of OPK Opistobranquis (see Pontes et al. 2014). Another website I like to mention is Morddyn (at Flickr.com a.k.a. I.F. Smith). First I advise you to have a look at their pages (click on the text in blue). Done? Hereafter follows the text based on their websites, in which I have incorporated some remarks and additions.

NB. For my Dutch readers: there is also an extensive description in 'Schelpdieren van het Nederlandse Noordzeegebie' (De Bruyne et al., 2013). This excellent book is still for sale, at this moment for a discount price, so be quick to buy one.


fig. 8  Fully stretched Thorny doris, Acanthodoris pilosa (NL: Egelslak), its foot protruding beyond the mantle. Westbout, the Netherlands, 29-11-2014 (in vitro). NB. figures 8, 9, 10 and 12 regard the same specimen (adult of 20 mm).

Description
The mantle covers the body, shaped like a dome. The body colour is variable and could be white, pale grey, yellow, orange, brown, purple or black. The light coloured specimens, especially the juveniles, may have darker spots or freckles (fig 8-10, 12; adult specimen) and their internal organs can be seen by transparency. The mantle is heavily spiculated with long, thin, radially arranged spicules; these are often visible in the mantle when viewed from below (fig. 9). The spicules do not interlock, so the body is soft and yielding. It is also covered in soft, tall, thin conical tubercles that give it the characteristic fluffy look (several figures). These tubercles could be contracted, making identification more difficult.


fig 9  Photographed upside down showing the head, mouth and spicules of the Thorny doris, Acanthodoris pilosa (NL: Egelslak). Westbout, the Netherlands, 29-11-2014 (in vitro).

The foot is translucent white to cream coloured (fig. 10), occasionally with dark spots on the sole; the upper side is freckled as the mantle. The foot is both anteriorly and posteriorly rounded and it protrudes beyond the mantle when the animal moves (fig. 8). Locomotion by monotaxic retrograde waves on sole.

fig. 10  Photographed upside down you can see the foot and mouth, but also a rhinophore and the gills of the Thorny doris, Acanthodoris pilosa (NL: Egelslak). Westbout, the Netherlands, 29-11-2014 (in vitro). 

The head has a narrow oral veil that forms two broad, flat oral tentacles at the sides (fig. 10). It has long retractable rhinophores, rising from a low sheath with tuberculated rim (fig. 11). The base of the rhinophores is smooth, the upper part with 10-24 lamellae (numbers increase with specimen size) and a nipple on the tip. They share the same colour of the mantle, but the lamellae could be frequently yellowish and occasionally bright orange. The rhinophores usually bent back and outwards.

fig. 11  The conspicuous large rhinophores and gills of the Thorny doris, Acanthodoris pillosa (NL: Egelslak). Porthkerris, United Kingdom, 1-9-2014.

There are up to nine relatively big tripinnate gills (fig. 11) of the same colour of the mantle - often with white axis and some white on the leaves - that are located around the anus in the back of the dorsum. The gills could be retracted independently under the mantle if the animal is disturbed; they do not retract in unison into a pocket. The body usually measures about 30 mm long, but may reach almost 70 mm. It reaches maturity between 7 and 10 mm long.

fig 12  Thorny doris, Acanthodoris pilosa (NL: Egelslak). Westbout, the Netherlands, 29-11-2014 (in vitro).

Distribution
The Thorny doris is recorded on mid- and lower-shore and sublittorally to about 170 m.

There are reports from the Arctic Ocean (Faeroes and Iceland) down to Morocco, it is common around the British Islands and on both sides of the North Sea (including the Netherlands). It is considered 'dubia' in the Mediterranean. Considered common in Palermo, Italy by Philippi (1836) (as Doris stellata Gmelin in Linnaeus, 1791 fide Thompson and Brown, 1984), in fact this species has only been reported in the Mediterranean by Forbes (1844) for the Aegean Sea (Cattaneo-Vietti & Chemello, 1987). Remark: the species is listed in Trainito & Doneddu, 2014 (Nudibranchi del Mediterraneo), but not as doubtful.
It is present on both Northern American shores, from Greenland to Virginia in the Atlantic and from the Aleutian Islands to Morro Bay, California in the Pacific.
Another addition also with a remark: Australia is mentioned, but ‘needs confirmation’ (Vicente, 2008). It is not mentioned in Burn, 2015 (nudibranchs of Australia).

Reproduction
Thorny doris are simultaneous hermaphrodite. Spawning occurs in all months with a maximum in spring, when adults are most common, with a secondary peak in autumn. The spawn is an undulating ribbon of white eggs (fig. 13) of 64-76 microns in diameter, up to about 150.000 eggs, forming a two turns spiral, attached to the substrate by one edge. Often several egg ribbons are laid in close proximity. Planktonic veliger larvae live for about ten days at 10ºC before metamorphosis.  
The egg ribbons resemble those of the Barnacle-eating onchidoris, Onchidoris bilamellata (NL: Rosse sterslak)(fig. 16), but those of the Thorny doris are smaller and less high.

fig. 13  Egg ribbons of Thorny doris, Acanthodoris pillosa (NL: Egelslak) attached to a broken and weathered stipe of Cuvie, Laminaria hyperborea (NL: Groot vingerwier). Porthkerris, United Kingdom, 1-9-2014.

Food
It usually feeds on encrusting bryozoans like Alcyonidium hirsutum (NL: Ruwe zeevinger), Sea chervil, A. diaphanum (NL: Doorschijnende zeevinger)(fig. 14), A. gelatinosum (NL: Gladde zeevinger), A. polyoum, Flustrellidra hispida and the encrusting (usual) variety of the Hairy sea-mat, Electra pilosa (NL: Harig kantmosdiertje)(fig. 15).

fig. 14  Sea chervil, Alcyonidium diaphanum (NL: Doorschijnende zeevinger). Thorny doris feed on this Bryozoan in the sublittoral. Porthkerris, United Kingdom, 1-6-2018.

fig. 15  Hairy sea-mat, Electra pilosa (NL: Harig kantmosdiertje on Laminaria species. Isleornsay, United Kingdom, 29-8-2007.

Name(s) 
I came across the following vernacular (English) names: Thorny doris, White hedgehog sea slug, Hairy spiny doris and in the USA: Papillose horned dorid. I like the name Hedgehog slug (just as - translated - its vernacular name in Dutch), but a land slug has already snatched this name: Arion intermedius. And I resent four word-names (Linnaeus is turning in its grave) and ‘white’ in White hedgehog sea slug?

Its scientific name derives from: genus: Acanthodoris from Greek 'akantha' = thorn, spine and 'Doris', a sea nymph in Greek mythology, wife of Nereus, nymph of the waters and mother of Nereids and species: pilosa: derives from Latin 'pilosus' = hairy, shaggy, covered with hair.

Similar species (in Dutch waters)
In Dutch waters the Thorny dorid may be confused with three other dorid species, however I think they are obviously different: the Barnacle-eating onchidoris, Onchidoris bilamellata (NL: Rosse sterslak)(fig. 16), the Sea lemon, Doris pseudoargus (NL: Citroenslak)(fig. 17) and the White jorunna, Jorunna tomentosa (NL: Satijnslak)(fig. 18).


fig. 16  Barnacle-eating onchidoris, Onchidoris bilamellata (NL: Rosse sterslak). Neeltje Jans, the Netherlands, 10-1-2015 (in vitro).

fig. 17  Sea lemon, Doris pdeudoargus (NL: Citroenslak). Westbout, the Netherlands, 7-2-2015 (in vitro).


fig. 18  Jorunna tomentosa (NL: Satijnslak). Zeelandbrug, the Netherlands, 29-6-2016.


fig. 19  For comparison one last photo of the Thorny doris, Acanthodoris pillosa (NL: Egelslak). Heerenkeet-Flaauwers, the Netherlands, 27-4-2013 (in vitro).

Some more about the Olympus TG-5

Is it the best camera you can get for the kind of environment I described (think also about shallow rockpools) in combination with small marine life? I think so. If you think it to be more easy to take good shots with a TG-5 than with my full frame camera’s: it is not. The TG-5 is much lighter, smaller (not always an advantage), has image stabilization and a f2.0 lens. But there is - isn’t there always? - a catch: its small sensor. It is unbelievable how much ‘information’ Olympus captures on such a tiny 12 million pixel sensor in RAW. But when adjusting the photo in Lightroom and/or in Photoshop, you soon realise the limitations of the TG-5 RAW files, especially in comparison with the files of a full frame sensor: noise! The exposure of the photo you take with a TG-5 has to be spot on, because the more you adjust, the more noise kicks in. Furthermore: I tested it at a snorkeling trip a few days ago and I found it hard to handle: there were never so many photo's ready for the dustbin. But maybe with a bit more practice...

The pro’s:
  • Light sensitive f2.0, 4x zoomlens from 25 mm wide angle to 100 mm tele (both in full frame equivalent)
  • In Microscope Modes up to 7:1 magnification (my excellent Canon MP-E 65mm f/2.8 Macro stops at 5:1!)(fig. 20)
  • AF-point adjustable (thanks to Ron Offermans for showing me how!)
  • Image Stabilisation
  • RAW Capture
  • Incredible depth of field because of the small sensor (the smaller the sensor, the larger the depth of field)
  • Waterproof to 15 m (with a separate underwater housing up to 45 m)
  • Low-priced for what you get
The con’s:
  • Small sensor: the bokeh (the background blur) is not very nice and obviously: sharpening makes it worse
  • Small sensor: limited in adjusting photo’s because of noise (even at photo's shot in 100 ISO)
  • Limited manual possibilities (aperture is fake)
  • No tilting screen 

fig. 20

More about camera’s, lenses etc. 

For diving and snorkeling I used a Canon 5DMKII untill september 2016; nowadays I use a Canon 5DSr. Both in Ikelite underwater housings. Most of the times I use a Sigma 50mm f/2.8 EX Macro (1:1), now and then a Canon EF 100mm f/2.8L Macro IS USM and rarely a Canon MP-E 65mm f/2.8 1-5x Macro. All three lenses combined with a flat port. For larger objects or underwater landscapes I use a Sigma AF 24mm f/1.8 EX DG (FX) and Canon’s EF 8-15mm f/4 USM L Fisheye in combination with a 8 inch dome port. For lighting I use two Ikelite strobes: a DS160 and a DS161.

For in vitro photography I use both forementioned camera’s and Canon’s MP-E 65mm f/2.8 1-5x Macro, sometimes combined with intermediate rings and a 1.4x converter. When an object is larger than 24 x 36 mm I use Canon’s EF 100mm f/2.8L Macro IS USM. For lighting I use a Canon Macro Twin Lite MT-24EX.


fig. 21  The author with his usual camera gear in 2010. Photo: Ruud Versijde.

Acknowledgement

I thank all the authors for their shared information and especially the authors of OPK Opistobranquis: M. Pontes, M. Ballesteros & E. Madrenas for their permission to use the text of their website and I.F. Smith (Morddyn).

Note

The Strandwerkgroep Waterweg-Noord is a group of amateur and professional marine biologists and other enthusiasts. By turning stones, rock pooling, beachcombing, snorkeling and diving we study and record all kind of marine life, especially of the Oosterschelde, Westerschelde and lake Grevelingen in the Netherlands.

Literature and weblinks

  • Behrens, D.W. & A. Hermosillo, 2005. Eastern Pacific Nudibranchs, A Guide to the Opistobranchs from Alaska to Central America. ISBN 0930118367.
  • Blauwtipje.nl.
  • Bruyne, R. de et al., 2013. Schelpdieren van het Nederlandse Noordzeegebied. ISBN 9789052108216.
  • Burn, R., 2015. Nudibranchs and related molluscs. ISBN 9780980381382.
  • Conchological Society of Great Britain and Ireland, The
  • Encyclopedia of Marine Life of Britain and Ireland / Habitas.org.uk (Picton, B.E. & C.C. Morrow).
  • McDonald, G.R. & J.W. Nybakken, 1980. Guide to the Nudibranchs of California. ISBN 0915826089.
  • Morddyn at Flickr.com (I.F. Smith). 
  • OPK Opistobranquis (Mediterranean and Iberian heterobranch molluscs): see Pontes et al.
  • Picton, B.E. & C.C. Morrow, 1994. A Field Guide to the Nudibranchs of the British Isles. ISBN 1898162050.
  • Pontes, M., M. Ballesteros & E. Madrenas (2012-2018) "Acanthodoris pilosa" in OPK-Opistobranquis, Published: 19/09/2014, Accessed: 09/07/2018. 
  • Sea Slug Forum Australian Museum. 
  • Slugsite.us (M.D. Miller).
  • Smith, I.F. - Morddyn at Flickr.com.
  • Snail's Odyssey, A - Nudibranchs & relatives (Tom Carefoot): spicules
  • Swennen, C. & R. Dekker, 1987. De Nederlandse Zeenaaktslakken. Wetenschappelijke Mededelingen KNNV, nr. 183. ISBN 905011010X.
  • Thompson, T.E. & G.H. Brown, 1976. British Opisthobranch Molluscs. ISBN 0126893500.
  • Trainito, E. & M. Doneddu, 2014. Nudibranchi del Mediterraneo. ISBN 9788865204801.
  • Vicente, N., 2008. 100 & une limaces de mer, Guide d'indentifaction des Mollusques Opisthobranches d'atlantique et de Méditerranée. ISBN 9782741703532.
  • World Register of Marine Species (WoRMS).