21 November 2014

John dory: curious and cosmopolitan fish

Zeus faber, Porthkerris, UK (montage)

I was diving at Porthkerris (Cornwall, United Kingdom), getting cold and running out of air. Just when I decided to swim back to the beach, I saw from the corner of my eye something whitish swimming along with me. When I turned my head I saw my first John dory, Zeus faber (NL: zonnevis)(specimen 1). I had already heard they were seen by divers and I would love to see this peculiar and exotic looking fish. But I had decided not to search for them, because it would take a lot of time (needle in a haystack) and their were so many other beautiful things to see.

Zeus faber, Porthkerris, UK (specimen 1)

At first we were looking at each other and when I slowed finning the John dory kept pace. It was about 30 cm long. I started taking photo’s. I had to try to come as close as can be, because I had a fisheye lens fixed on my camera. So after a few photo’s I decided to slowly advance to the John dory. As I expected it swam away, but then it eased. I had another try; this repeated for a while and at last I could come as close as 50 cm.


Zeus faber, Porthkerris, UK (specimen 2)

I was amazed: this John dory was not afraid at all and curious, because if it had decided to disappear it would have been easy. They are quite compressed and swimming through the underwater forest of cuvie, Laminaria hyperborea (NL: groot vingerwier), as you can see in the photo above and below, it would have (dis)appeared as a loose stalk of this seaweed.

What avoked this curiosity? At first it could not have been the reflection of itself in the domeport of my underwater equipment - a suggestion often made - because it swam alongside. With a lot of anthropomorphizing and judging by its 'haughty' looks (due to its big mouth with steeply-angled jaws) it could have thought: what is this stranger doing on my property? I noticed it chasing other fish away. I just don’t know and it doesn’t matter. I had a perfect encounter for almost 10 minutes and some nice photo’s.

Zeus faber, Porthkerris, UK (specimen 1)

As you have read the story so far and my former posts, you may wonder if I have forgotten the promised posts about Botanical Beach and other items of Vancouver Island's fauna. I haven't but for now I am switching to the fauna and flora of the UK and especially of Porthkerris. In one of my next posts I will describe and show this excellent diving spot (and dive centre) in the southwest tip of England.

I had 16 dives at Porthkerris in the period of 31 August to 13 September; 7 last year and 9 this year. The described encounter with the John dory was my penultimate dive last year and I was happy to meet them again this year. And not just one but at least 7 (I photographed 4 specimens; my dive buddy Ruud 3). Just as last year they seemed to appear out of nowhere. They were of the same size as the one I saw last year and a few were smaller.

Zeus faber, Porthkerris, UK (specimen 1)

My first John dory (specimen 1) looked quite pale, hence my remark 'something whitish swimming along'. But John dories can be very colourfull. Especially specimen 3 with its golden glow (photo below). Their is discussion regarding the origin of its common name, but in French 'jaune dorée' means yellow gold. Not far-fetched.

If you have a good look at my photo's you can see that the pattern on their sides is unique. Just like an enlarged thumbprint. I wondered if I had seen the same specimens on different days and years. I compared all the photographed fish, eight in total, and they were all different specimens (hence the reason for making notice of 'specimen 1' etc.). For comparison you must have photo's of the same side, because the pattern on both sides of a specimen is not identical!


Zeus faber, Porthkerris, UK (specimen 3)

But what if they can change their pattern? A lot of fish species like blennies, gobies and for instance plaice, are able to change, recede or intensify their colour and their colour pattern to blend in with the environment. John dories are able to adapt their colour (saturation) to their surroundings: I saw the pale(r) ones in open water or hovering above seaweeds; the more colourfull specimens were swimming in front of rocks or in darker spots. But that is not the same as changing pattern.

I came upon a remarkable story by Dan Bolt - 'Babbacombe's cleaning stations' - about a cleaning station serviced by Leach’s spider crabs, Inachus phalangium (NL: gladde sponspootkrab) which John dories visit to get rid of small parasites (not the ones you can see with your naked eye). He writes: 'Spending so much time in one small area it becomes easy to recognise individual animals because of the unique markings these fish have on their body.'

Inachus phalangium, Pointe de Trévignon, Brittany, France

I asked Dan if he knew more about changing pattern. He replied: 'In short, no I am not sure that they can not change their pattern. The observation period for the cleaning behaviour was relatively short; about 8 weeks in all, and certainly in that time-scale I could recognise individuals by their patterning. But observing only for 8 weeks does not proove that they do not change their patterns throughout the year. Certainly they can make their colour fade or more bold, but sadly I can't give you a definitive answer about changing patterns.'

Zeus faber, Porthkerris, UK (specimen 4)

You must have noticed the dark spot with its light margin in the centre of each flank. It is supposed to have the effect of an eye i.e. to confuse its prey and predators. In legend these are St Peter’s fingerprints, left when he took a coin from the fish’s mouth to pay up his tax. Hence its popular name in a lot of countries referring to St Peter: Sankt Petersfisch in German, Saint-Pierre in French, St. Pietersvis in Dutch etc. It could also explain the name John dory: St Peter is the gatekeeper in heaven, in Italian 'janitore'.



Distribution map of Zeus faber. Except for the America's the John dory is a cosmopolitan. It is remarkable that it thrives in cold and tropical waters.

John dories are solitary fish. They live in a depth of a few meters deep to 200 meters (Lythgoe, J. & G., 1976). Edgar (1997): 'depth 1-170 m; it occurs most abundantly in deep offshore waters, although individuals occasionally enter shallow water estuaries' (describing species of the Australian coasts). Heemstra (2004): 'adults in 50-400 m' (species of South-Africa), so even 200 m deeper.
According to most authors John dories can be found in sand-, reef- and kelp-habitats or a mix of these and are often catched by trawlers above sandy bottoms.

Wheeler (1975): 'A very similar species, Zeus japonicus, is found in the Indo-Pacific. The two scarcely differ morphologically.'

Zeus faber, Porthkerris, UK (specimen 3)

 As you can see, they have a laterally compressed shape with a long dorsal fin - composed of long, strong fin spines - and oversized pelvic fins. As said, it makes them hard to spot in kelp forests. You can often observe them tilted at all sorts of angles and even upside down (Naylor, 2011)! Have a look here at a short video.


Sharlin cruiser (left © archangel72367, right © babylon5.wikia.com)

 As a big fan of the science fiction series Babylon 5, they remind me of a 'sharlin cruiser': laterally compressed with oversized wings. Who is to know the artist wasn't inspired bij John dories?


Zeus faber, Porthkerris, UK (specimen 3)

 Muus (1966) reports an average length of 25-30 cm and at that size it has a weight of 2 kg, but up to 60 cm and 8 kg; Allen (1999) up to 75 cm and Debelius (1998) and Heemstra (2004) even 90 cm! A bit scary at this size coming out of the blue… Heemstra (2004): 'females grow larger than males.'
According to Janssen (1979) who has studied John dories in the Netherlands from 1960 to 1977, they are 15 to 28 cm after two years and 29 to 42 cm in their third to fourth year. Heemstra (2004) reports a lifespan of 15 years for females and 13 years for males.

Duncker (1960) estimates first sexual maturation of males at 25–28 cm, and females at 34–38 cm. They spawn in summer at the south coast of England at a depth of less than 100 m. In the Mediterranean, in warmer waters, they spawn in spring (Muus, 1966). The southern North Sea is to shallow as a breeding ground (Nijssen & De Groot, 1987). The eggs are pelagic (Muus, 1966). 

Zeus faber, Porthkerris, UK (specimen 3)

John dories are clumsy swimmers. So it has to depend on stealth and its big protrusile mouth (have a look here), which enables it to suck in a large volume of water with unsuspecting prey. Unfortunately I have never seen them catching prey.

It seems fish is their main diet, but Nijssen & De Groot (1987) mention that, apart from fish, crustaceans and cephalopods have been found in their stomach.

Predated species of fish in northwest Europe according to Muus (1966), Naylor (2011) and Nijssen & De Groot (1987) are herring, Clupea harengus (NL: haring), pilchard, Sardina pilchardus (NL: sardien), sprat, Sprattus sprattus (NL: sprot), sand smelt, Atherina presbyter (NL: koornaarvis), horse mackerel, Trachurus trachurus (NL: horsmakreel), sand eel, Ammodytes species (NL: zandspiering), gobies, Gobiidae (NL: grondelsoorten) and more specific two spot gobies, Gobiusculus flavescens (NL: Ruthenspars grondel).

Gobiusculus flavescens, Porthkerris, UK

Two spot gobies were very abundant at Porthkerris as you can see in the photo above. Here, I guess, this species is John dories staple food. 

Gobiusculus flavescens, St John's Point, Ireland

Two spot gobies are about 6 cm long, but often much smaller. They are, contrary to most other gobies species, not bottom dwelling but demersal fish. Only when the water is getting cold, they will rest on the bottom.

Gobiusculus flavescens, 't Koepeltje, Grevelingen, The Netherlands

At first view two spot gobies look like 'innocent' guppies. But have a look at their teeth!


Zeus faber, Porthkerris, UK (specimen 2)

A John dory with some two spotted gobies and, at the top, pollack, Pollachius pollachius (NL: pollak). Pollack are also preying on two spotted gobies.

Caligus species on Zeus faber, Porthkerris, UK (specimen 1)

Almost every John dory at Porthkerris was infected by sealice, one of the many Caligus species. I asked marine biologist 'universalis' Marco Faasse of eCoast Marine Research, Belgium, if he could identify the specie(s). Marco: 'Two species of Caligus are known on John dories: Caligus elongatus Nordmann and Caligus zei. Maybe both species are visible on your photo, because I see two rather different parasites. However, I won't rule out it is the same species, only a different sex.'


unknown flatworm(?) species on Zeus faber, Porthkerris, UK (specimen 4)

Specimen 4 was not infected bij Caligus but other, smaller parasites. Marco: 'I presume these are parasitic flatworms of the class of Monogenea. I  have found the flatworm Udonella caligorum on Caligus elongatus on the lumpsucker, Cyclopterus lumpus (NL: snotolf). This flatworm settles itself on sealice, but it is feeding on the mucus secreted by the skin of the fish. See article in Het Zeepaard 2005(4). The specimens on your photo could be a related species. However, searching for flatworms (there is an incredible amount of species of worms) related to Zeus faber I could only find worms living in its digestive system.'


unknown flatworm(?) species on Zeus faber, Porthkerris, UK (specimen 4)

I want to thank Dan Bolt for his reply, Marco Faasse for providing information about the parasites and Godfried van Moorsel for supplying an article about changing colour in rockpool gobies.  


Literature:
  • Debelius, H., 1998. Vissengids Middellandse Zee en Atlantische Oceaan.
  • Duncker, G. 1960. Die Fische der Nordmark. De Gruyter & Co., Hamburg.
  • Edgar, G.J., 1997. Australian Marine Life. 
  • Faasse, M., 2005. De snotolf een dierentuin: over Caligus elongatus (Copepoda) en Udonella caligorum (Platyhelminthes, Monogenea). Het Zeepaard 65(4): 123-127.
  • Heemstra, P. & E., 2004. Coastal fishes of Southern Africa. 
  • Janssen, G.M., 1979. The occurence of Zeus faber in the coastal waters of the Netherlands. Bull. Zool. Mus. Univ. Amsterdam, 6 (20): 153-158.
  • Lythgoe, J. & G., 1976. Vissen van de Europese kustwateren en de Middellandse Zee. 
  • Muus, B.J., 1966. Zeevissengids.
  • Naylor, P., 2011. Great British marine animals. 
  • Nijssen, H. & S.J. de Groot, 1987. De vissen van Nederland. 
  • Wheeler, A., 1975. Fishes of the world.
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