Homing and site fidelity of the symbiotically luminous coral reef fish, Siphamia tubifer

Background/Question/Methods

The cardinalfish, Siphamia tubifer (Perciformes: Apogonidae) inhabits coral reefs throughout the Indo-West Pacific, seeking protection among the spines of sea urchins during the day. At our study site in Okinawa, Japan, S. tubifer aggregates in groups among the spines of the long-spined sea urchin, Diadema setosum, and the banded sea urchin, Echinothrix calamaris. The fish is unusual among apogonids for its formation of a bioluminescent symbiosis with a light-emitting bacterium, which it harbors in a ventral light organ connected to the intestine. Observations of S. tubifer in the wild reveal that the fish begins emitting luminescence and leaves an urchin to forage at dusk and returns to an urchin at dawn. Observations of the fish in aquaria indicate that S. tubifer releases feces rich in its symbiotic bacteria the following morning at an urchin. In this study, we used field transects together with laboratory choice experiments, and mark and recapture methods to define the preference of S. tubifer for a host urchin species, the fish’s homing ability, and its fidelity to a particular urchin at a reef.

Results/Conclusions

A distinct preference for associating with D. setosum (P<0.001) was evident, especially among larger fish (>25 mm standard length, P<0.01). Choice experiments in the laboratory revealed the ability of S. tubifer to recognize and orient to both a host urchin and conspecifics (P<0.05), but the fish did not exhibit a preference for associating with familiar conspecifics or familiar urchins. Between 43-50% and 26-37% of tagged S. tubifer returned daily to the same urchin after three days and one week, respectively. Tagged fish also returned to their original reef when displaced up to two kilometers. Within two days, 23-43% and 27-33% of tagged individuals returned from distances of one and two kilometers. The homing and site fidelity exhibited by S. tubifer implies the fish uses site-specific environmental cues to home. This behavior, along with the daily release of symbionts at the urchin, might shape the genetic structure of both host and symbiont populations by enhancing symbiont acquisition by S. tubifer larvae, which we hypothesize recruit to their natal reefs. To investigate this possibility, we are defining the population genetic structure of the host fish and its symbiont using single nucleotide polymorphisms detected by restriction site-associated DNA sequencing (RAD-Seq).