Population structure of six sympatric species of Trapezia associated with the hermatypic coral Pociliopora damicornis with a hypothesis of mechanisms promoting their coexistence

Seasonal changes in species composition and size frequency distributions of six sympatric coexisting species of Trapezia associated with the hermatypic coral Pociliopora damicornis were investigated to analyze their population structure, mechanisms promoting coexistence, and the organizing processes of the symbiont community. All Trapezia species showed a long recruitment season, but they could be divided into two groups according to variations in their size frequency distribution pattern throughout the year: Young T. ferruginea and T. guttata occurred all year round, while young T. cymodoce, T. areolata, T. digitalis, and T. sp. were absent part of the time (a few months). Recruitment of T. cymodoce and T. areolata occurred from April to August and from February to March, respectively. Young T. digitalis were abundant only in July. Even among larger colonies, those harboring only one or two Trapezia species were rather common. This suggests that agonistic interactions occur among these crab species. Although it is also true that host colonies harboring several species are sometimes found. Since no conspicuous hierarchy in inter-specific agonistic interactions has been demonstrated (Tsuchiya and Yonaha, 1992), new crab recruits of any species can invade colonies lacking strong competitors.Although these crab species require very similar resources, i. e. pocilloporid corals as their habitat and mucus produced by host corals as food, their coexistence on larger single colonies is promoted by several mechanisms: 1) different recruitment seasons, 2) weak hierarchy of agonistic interactions among the six species, and 3) microhabitat segregation between adults and young on a colony.