An almost-blind killer and its watchman

Symbiosis (from Greek, meaning living together) is almost a rule rather than an exception among the inhabitants of coral reefs. I have previously discussed the relationship between anemones and anemone- or clownfish (https://blogs.unbc.ca/reeftank/2013/06/14/everybodys-favourite-nemo/), and this time I will describe a very odd couple, the pistol shrimp and the shrimp goby. Unfortunately they are not visible in the UNBC reef tank anymore, after having taken up residence close to the front for a long time, but the association is too interesting to ignore.

There are over 600 described species of pistol shrimp in 45 genera. The species in the reef tank is the Randall’s pistol shrimp, Alpheus randalli Banner and Banner (Family Alpheidae), and it has partnered with an orange spotted shrimp goby, Amblyeleotris guttata (Fowler) (Family Gobiidae). The genus Alpheus alone has more than 280 described species (Encyclopedia of Life http://eol.org/pages/4264098/overview) making it the most species rich genus of all Decapod shrimps. Approximately 20 of these species associate with over 100 species of goby in seven different genera. Some of these gobies will hunker down with any willing shrimp partner, while some are specialists and will only live with a particular species of shrimp. Given their rather cryptic lifestyle, and the fact that the association has only been known since the late 1950’s, and studied in more detail since the 1970’s, I think it is safe to assume that there is still much to learn about these fascinating crustaceans and their fish partners.

How do you get a pair for your a reef tank? Well, you can sometimes buy a shrimp-goby pair from the live fish store, but this is not necessary. In the case of the UNBC tank, the goby was added almost a year before the shrimp, which was simply let go into the tank without consideration for where the goby was. The shrimp disappeared from view for a long time, but when the goby showed up in front of a burrow eventually, it had paired up with the shrimp and they have been together ever since.
What roles do the two partners have, i.e., why do they live together? Well, the goby is the watchman, keeping an eye out for predators and food. This is of great benefit to the shrimp, as it is nearly blind. When you watch a pair, you will find that the shrimp almost always keeps an antenna in contact with the fish when it is outside of its burrow. The shrimp in turn serves to construct the burrow, and it takes its job extremely seriously. When outside, the shrimp is constantly moving rocks and sand, sometimes carefully placing them around the opening and sometimes moving it away from the burrow. The chores appear endless, because you rarely, if ever, see the shrimp idle. Thus, this association is an example of mutualism, where both partners benefit.

Randall’s pistol shrimp and its orange spotted shrimp goby partner outside their burrow in the UNBC reef tank.Photo: D. Parkes.

Why the name pistol shrimp though? Well, a close look at the claws of this little shrimp reveals that they are asymmetrical. One is shaped in a rather odd way. The shrimp can open this claw, and then snap it together with great speed, making a sharp sound, hence the name. In a reef tank containing pistol shrimp, you can sometimes hear the sound as a click. But the sound is not generated by the closing claw. The force created by the claw, produces a jet of water that is so fast that a cavitation bubble is formed Versluis et al. 2000). When that bubble collapses, it produces the loud sound, which can approach 200dB (Au and Banks 1998). Furthermore, the collapse of the bubble generates heat, reaching temperatures of several thousand degrees (Lohse et al. 2001). So what in the world is the purpose of this? Appropriately, the shrimp uses the claw to “shoot” prey. Directing the snap at a prey animal will stun it, allowing the shrimp to capture other crustaceans or fish for food (You can view the process at https://www.youtube.com/watch?v=XC6I8iPiHT8) . For that reason, you have to expect some losses if you chose to keep a pistol shrimp. To date the toll has been quite modest in the UNBC reef tank (perhaps a few hermit crabs), and given that the pair remained in full view for at least a year, I consider the risk worth it.

References

Au, W.W.L. and K. Banks. 1998. The acoustics of the snapping shrimp Synalpheus parneomeris in Kaneohe Bay. The Journal of the Acoustical Society of America 103: 41-47.

Lohse, D., B. Schmitz and M. Versluis. 2001. Snapping shrimp make flashing bubbles . Nature 413: 477-478

Versluis, M., B. Schmitz, A. von der Heydt and D. Lohse. 2000. How snapping shrimp snap: through cavitating bubbles. Science 289: 2114-2117

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