Today’s creature feature is a lovely video of the Three-band Anemonefish (Amphiprion tricinctus) which was taken in Kwajalein’s sandy inner lagoon at a depth of 40 feet. This incredible red Haddon’s Carpet Anemone (Stichodactyla haddoni) was documented over the course of three years and was always observed with a robust gathering of A. tricinctus among its tentacles.
This species is one of the more enigmatic members of the diverse clarkii group and presently known only from the Marshall Islands in the Central Pacific. Some particularly noteworthy diagnostic traits for this fish include: 1) the dark caudal fin. 2) the dingy brown base coloration of adults. 3) the prominent third stripe, present just before the caudal fin.
The Marshall Islands sit at the easternmost distributional edge of the larger clarkii group, and this likely helps to explain the unique appearance of A. tricinctus. Isolation tends to cause weird things to happen during the course of speciation, as with the many strange endemic taxa at places like the Marquesas in the South Pacific or St Helena in the Atlantic.
But another possible explanation might relate to the documented instances of mixed-species pairs between A. tricinctus and A. chrysopterus. In a less remote location, the genetic anomalies brought about by such interspecies mating would likely disappear into the larger gene pool of the resident parent populations. However, a place like the Marshall Islands, positioned on the geographic periphery for the clarkii group, likely has a proportionally small population present… and this population likely has limited contact with its nearest relatives in the Caroline and Mariana Islands, making it more susceptible to genetic introgression brought about by hybridization.
So how did A. tricinctus ultimately acquire its sui generis appearance? Perhaps both of these hypotheses are correct to a certain extent. It’s certainly not hard to imagine how a small, isolated, hybridized population might end up looking a bit different from your average clarkii. But obviously there is much that we don’t know about this peculiar fish. For an excellent discussion of this fish, be sure to check out the many photographs and life history notes at UnderwaterKwaj.