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).

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).

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).

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.

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.

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.


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).


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).