Species and size matter: An experimental study of microhabitat use under the influence of competitive interactions in intertidal gobiids

Patterns of microhabitat use and biotic interactions were examined under experimental conditions using three intertidal gobies, Bathygobius fuscus, Chaenogobius annularis and C. gulosus. A series of microhabitat choice experiment was conducted to document microhabitat utilisation, spatial use overlap and occurrence of agonistic behaviour in single-individual and mixed-individual setups using two size-categories of the three species. Patterns of habitat utilisation as observed under single-individual conditions were significantly modified in the presence of another individual, depending on species identity, body size and time of day. Chaenogobius species showed marked changes especially when paired with a large individual, occurring more frequently outside the shelter and using those substrates that were infrequently used by the paired individual. In contrast, B. fuscus did not show marked changes. Observed (realised) overlap in spatial use under mixed-individual conditions was significantly smaller than the expected overlap based on single-individual conditions (i.e. without any biotic interactions). Marked departures from expected patterns were detected for intra- and inter-specific combinations of Chaenogobius species: there were effects of biotic interactions. C. gulosus rarely showed direct aggression, in contrast to other species, though it exerted an apparent influence on the microhabitat use of a paired species/individual. The occurrence of aggression was different among species pairs and times of day. Body size differences affected the outcome of a contest, but not the occurrence of behaviour itself. The pattern also varied depending upon species combinations and times of day. Fish activity was also influenced by the presence of a paired individual in some cases. The present study revealed that species identity and body sizes are important elements of biotic interactions that affect the patterns of habitat utilisation in these fishes. Such diverse and subtle manners of interaction and habitat use may contribute to species coexistence in tidepool fish assemblages.