A Fish Isopod Photographed by a Free Diver - Observation of the Week, 12/14/20

Our Observation of the Week is this Fish Isopod (likely Genus Anilocra), seen off of Greece by @vasilis\_stergios!

“The sea is where I really feel at home, unburdened by gravity, exploring a space with such biodiversity that you can always count on spotting something new every time you look below the surface,” says Vasilis Stergios. And he spotted this bright red isopod while free diving in the Saronic Gulf, where he says “[we] get to witness an abundance of planktonic life, from the common Leucothea multicornis ctenophore, to the rarely seen moray eel larva!” But it took a few tries with his camera.

On one of these dives, I spotted this free-swimming Anilocra isopod at almost zero depth, probably looking for a host fish to attach to. Must have taken at least 50 shots with my compact Olympus TG-4 in microscope mode to produce this one almost clear shot of a tiny, less than 1cm long, constantly moving creature, with a clear focus on the compound lenses of its eyes. Thankfully this was a day with zero wind or current, otherwise I wouldn’t have even bothered to try in the first place.

As Vasilis mentioned, isopods in this family (Cymothoidae) generally parasitize fish. Once they find a host, they will attach and feed on the fish’s blood. Many are seek out specific host species as adults, and often attach to certain parts of the host’s body (eg gills, behind the eyes, etc). Fascinatingly, they are protandrous hermaphrodites - juveniles start out as males, but will become female if no other females are around. Fish parasitized by these isopods will often visit shrimp that will remove and feed on the isopods.

Vasilis (above) grew up hiking the mountains and snorkeling and fishing by the sea, and tells me he likes to spend “every moment of my spare time away from work.”

He first found iNat via the Smithsonian’s Instagram account. “What really surpassed my expectations when I started using the site, is the image identification algorithm, which I have been using ever since as an invaluable tool on the field,” he says.

Being able to quickly pinpoint genus, or even species of any plant or animal I encounter, has really widened my focal range! While I originally would take shots of my favorite plants or animals, I now take time to photograph and identify any creature that draws my attention (rarely or first seen being my number one criteria), in an attempt to aid in recording the diversity of Greek flora and fauna.

- Speaking of Instagram, you can check out Vasilis’s Instagram account here!

- Vasilis posted the first observation of an Oceania armata hydroid to iNat a few weeks ago, it’s quite stunning.

- This is not the first Observation of the Week that documented a fish isopod. 

Posted on December 14, 2020 10:39 PM by tiwane tiwane


Thats an awesome photograph! Congrats on the great find.

Posted by cesarcastillo over 2 years ago (Flag)

What a beautiful find and a gorgeous photo. Congrats! :)

Posted by twan3253 over 2 years ago (Flag)

Fantastic photograph! Great work.

Posted by lonnyholmes over 2 years ago (Flag)

Thank you @tiwane and fellow naturalists for this incredible opportunity, all your kind words and support! It's amazing to be part of this active community and be able to add to this ever growing map of life!!!

Posted by vasilis_stergios over 2 years ago (Flag)

Great job for a great country!

Posted by valentino_traversa over 2 years ago (Flag)

That is a fantastic shot! Kudos to such a cool find.

Posted by walkingstick2 over 2 years ago (Flag)

Splendid shot!!

Posted by gregoiredubois over 2 years ago (Flag)

Fantastic photo! Thank you : )

Posted by claudia_ma over 2 years ago (Flag)

Many cymothoids hide in the surface plankton during the day and descend to find a host during the night. Juveniles are very difficult to identify. There are 2 species of Anilocra in your area. Wonderful photograph.

Posted by ernesthwilliams over 2 years ago (Flag)


Posted by imladris over 2 years ago (Flag)

Thank you @ernesthwilliams for the additional info on cymothoid behaviour! A fellow freediver just informed me that the numbers of fish spotted with isopod parasites in Southwestern Crete keep increasing by the year. Does that point to a decrease in their natural enemies or a change in the water temp/composition? What species predate on Anilocra isopods?

Posted by vasilis_stergios over 2 years ago (Flag)

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