Fish Journal Club: Gobies and Crabs

You probably know by now that I am very interested in the symbiosis between gobies and alpheid shrimp. An interesting new paper just came out which describes a similar symbiosis, between gobies and crabs:

Occasional utilization of crustacean burrows by the estuarine goby Mugilogobius abei by Yumi Henmi, Yuya Okad and Gyo Itani.

The authors investigate the life of a goby, Mugilogobius abei, which, according to FishBase, “inhabits sandy and muddy bottoms of estuaries and coastal freshwater streams not far from the stream mouths”. The fish enters the burrows of one shrimp, and one crab. It does not engage in a true mutualistic symbiosis with either, but rather freeloads and uses the burrows without doing anything for the crustacean in return.

This is the goby in question (photo by 杨朙暵 via Fishbase):

Curiously, the crab sharing its burrows with the goby is named Macrophthalmus banzai, seemingly after the infamous Japanese “BANZAI” battle cry. I wonder if these crabs used to participate in death-defying bayonet charges against American machine gun nests?

Back to the gobies; The authors studied them both in fish tanks in the lab as well as in two different field sites in Japan. What they found was that the goby uses the burrows, but less so than mutualist gobies (“shrimp gobies”). Some gobies didn’t even enter crustacean burrows in fish tanks, and the time spent in the burrows was brief. They also reported that in tanks: “M. abei was observed to be expelled from the burrow by the aggressive behavior of the shrimp, which used its chelipeds.”

In the field, the gobies used crustacean burrows more when other types of structure usable as hiding places (dead leafs, ect.) were not available. These alternative hiding places were more common in one field site, and there the gobies used the burrows less.

Very interesting: in this early (?), facultative phase of a goby – crustacean relationship, the crustaceans are rude to gobies and the gobies only use their burrows to hide if no other options are available to them.

Eggs for Lunch

I also came across this video, which shows two gobies, a crested goby (Lophogobius cyprinoides) and frillfin goby (Bathygobius soporator),  going for the eggs of a mud crab in a brackish water aquarium:

Hence..

I think the paper and the video go together quite well. Small benthic crustaceans and gobies share many of the same environments, and will repeatedly encounter each other every single day. Crustaceans dig burrows, and gobies like to hide in dark places – they are predestined for life together. But is this interaction always friendly? No, crustacean eggs are just too nutritious. The goby does not live with the crustacean out of platonic altruism. This also raises the question, what was the initial motivation for the gobies to join the crustaceans? To use their burrow, to feed on their eggs, or both? Did an initially egg-predatory interaction develop into a mutualism?

Any kind of symbiosis should be gauged carefully how much of it is parasitism/egg predation, and how much of it is truly mutualism.