I saw a very special fish on the reef yesterday. And it’s not even a goby.

It’s Pholidichthys leucotaenia, the convict fish. It’s sometimes called a convict benny or a worm goby, but it’s neither a blenny nor goby, but in its own fish family. It looks like a goby in the front, but like a tiny eel in the back. These fishes are about 2-5 cm short when they are juveniles, and they swarm in active schools over the reef without changing the school location much. There are typically several hundred fishes in one such school, often of the same size.

In fact, I did not see a convict fish (school) only once lately. The coral reefs of Moalboal are full of them. Their schools typically line the steep reef walls we have here, up to a few meters off the corals, about 15 – 30 m deep. I haven’t seen them much in the really shallow parts of the reef.

I am suspecting a slight psychological effect in my recent frequent sightings of the convict fish. I might just notice them more than previously, due to recent discussions with fellow convict fish admirers, Ned and Anna DeLoach. It’s an effect any naturalist knows: if you read and think about one animal, your senses will be more sharpened to notice it in the wild.

Now, what is so special about the convict fish?

While the juveniles are swarming and feeding on plankton, the adult is living in a burrow which it digs in coral rubble. The adult never leaves this burrow, it seems! Instead, it is being fed by the juveniles, which it swallows as a whole – and later spits out again in one piece, after they vomited out their stomach contents (see Burrow distribution and diel behavior of the coral reef fish Pholidichthys leucotaenia, Clark et al., 2006). So, there is a division of labor between the generations.

2 years ago I wrote a paper which speculated about the causes for a peculiar absence in nature – the absence of eusocial fishes. Eusocial means “individuals are so specialized that only some reproduce”. It’s the type of social specialization you find in ants and termites: only one or a few animals called “queens” lay eggs. The others work for the benefit of the colony – a division of labor between the generations. Just like in the convict fish!

Now, eusociality was thought not to exist in fishes. In my paper I argue that this is due to the physics and large-scale ecology of the ocean. Basically, sibling fish are dispersed too widely to be able to cooperate later in life. But the convict fish might prove me wrong! Maybe it is in the early stages of eusociality, with the sole adult being a fish “queen”? This is what fascinates Anna, Ned and me about the convict fish. And, as a good scientist, I of course like to be proven wrong (by solid studies, in interesting ways).

So. If Joe Average Diver swims across the reef, he will see many beautiful colors and shapes, and enjoy himself thoroughly. I am happy for Joe. But when people like Ned, Anna or me swim over the reef, we see these shapes and colors, but we also see evolution, mimicry, symbiosis, parasitism, and maybe even eusociality. We see an amazing biodiversity, and how the animals making up this diversity interact and how they sort themselves into narrow ecological niches defined by depth, bottom composition and time of day. Looking at the reef has become an intellectual pleasure in addition to an aesthetic pleasure. When we dive, we also savor the intriguing theoretical questions which come up with all these special animals in the sea.

As a reader of this blog, you are probably aware that I am teaching marine biology courses for interested laypeople at Savedra. The aforementioned seeing of evolution, biodiversity and ecology is what I hope to teach!