Intraspecific competition controls spatial distribution and social organisation of the coral-dwelling goby Gobiodon histrio

The role of competition in regulating reef fish populations has been controversial. Here, we test the effect of intraspecific competition on the spatial distribution and social organisation of Gobiodon histrio, an obligate coral-dwelling goby that inhabits the branching coral Acropora nasuta. At Lizard Island (Great Barrier Reef), social groups of G. histrio consisted of either a single individual or a breeding pair and occupied 98% of A. nasuta colonies, indicating that corals are a limiting resource. Furthermore, there was a positive relationship between fish size and coral size, with small, single G. histrio mostly occupying small corals (<15 to 20 cm diameter) and larger paired fish occupying large corals (>15 to 20 cm diameter). A manipulative experiment involving small and large corals demonstrated that this positive relationship between fish size and habitat size was due to sizebased competition for large corals. Small and large G. histrio exhibited a similar preference for large corals (using large corals in approximately 80% of trials), but small fish were usually excluded from large corals in the presence of a larger fish. Small fish were more likely to use large corals in the presence of a single adult (38% of trials) than in the presence of an adult pair (8% of trials), suggesting that small fish may be able to enter a large coral following the loss of one individual in a breeding pair. The presence of a threshold coral size for the formation of breeding pairs means that an individual’s ability to compete for large corals will influence its reproductive success.