Life in coastal pebble sediment: Unique interstitial organism community and selective feeding on meiobenthos by interstitial fishes (Luciogobius: Gobiidae)

Interstitial areas of coastal pebble sediment in the Japanese Archipelago are inhabited by extremely elongated gobies of the genus Luciogobius, which are characterized by an increased number of vertebrae and reduction of scales, eyes, and fins. To explore the little-known interstitial life of Luciogobius gobies, we investigated the diets of two interstitial Luciogobius species, L. elongatus and L. grandis, and the interstitial organism communities of the gobies’ microhabitats in an exposed gravelly coast in Shirahama, southern Japan. The interstitial organism community in pebbly sediment was dominated by minute arthropods such as harpacticoids, isopods, and ostracods, presenting a marked contrast to the communities in sandy sediments, which are dominated by nematodes and turbellarians. The gut contents of the two goby species were composed exclusively of interstitial organisms, especially harpacticoides and isopods. Although each prey assemblage was roughly similar to the interstitial organism community in the corresponding microhabitat, marked preferences for harpacticoids and flabelliferan isopods were detected in L. elongatus and L. grandis, respectively. Irrespective of their intense feeding of harpacticoids, rare catches of large isopods were suggested to be nutritionally important for the gobies. These results suggest that the Luciogobius gobies are the first known fishes that depend exclusively on interstitial organisms, and that selective feeding upon meiobenthos may facilitate the coexistence of several interstitial goby species in pebbly sediment.