UBC Theses and Dissertations
The adaptive significance of coloniality and harem polygyny in the sand tilefish, Malacanthus plumieri Baird, Troy Alan
The social and reproductive biology of the sand tilefish, Malacanthus plumieri (Malacanthidae), was studied in the field at Glover's Reef, Belize, Central America. Discrete colonies of tilefish were highly clumped in some sandy habitats with coral rubble used by tilefish in burrow construction. Both females and males each occupied one "home burrow" as a refuge from predator attacks. Tilefish were most abundant in a channel through the fringing reef. Smaller numbers of fish occupied slopes adjacent to isolated patch reefs (reef slopes) inside the lagoon. Neither limitation of habitat nor social transfer of foraging cues appears to explain colony formation in M. plumieri, because unoccupied habitats were abundant and fish forage solitarily. The proximity of unoccupied to occupied habitats also suggests that colonies do not result because larvae are transported only to some patches. Rather, juveniles appear to settle preferentially near adults, perhaps because proximity to conspecifics reduces predation risk. Contagious reactions to predators and experimental disturbance support this hypothesis. Tilefish disappeared more frequently in the channel than in the reef slope, perhaps because channel fish incurred higher predation rates. Tilefish exhibit a harem polygynous mating system. Home range overlap among fish of the same sex was low. Females and males defended exclusive use of most of their home ranges against all conspecifics except mates. Areas defended by males overlapped the territories of up to six females. Males maintained dominance over mates by aggression. Females spawned as frequently as every day, with the male whose territory encompassed their territories. Histological and behavioral evidence indicate that M. plumieri is capable of functional protogynous hermaphroditism. Removal tests and observations on foraging indicated that females defend burrows and feeding spaces. Male removals revealed that females mate with whichever male occupies their feeding territory, and do not position territories solely to be near mates. Female removals confirmed that intrasexual competition restricts use of space by some females. Colony formation suggests, however, that competition costs are not so high as to prevent female occupation of adjacent territories. Instead, joining harems may promote spawning opportunities for females that are restricted to burrows for predator avoidance. Removal tests indicated that males position their territories to defend female territories and acquire mates. However, males did not prevent mates from moving to other harems when females were removed. Males also did not abandon their territories when mates were removed. A dichotomy between resource and female defense does not apply to tilefish or to other group-living fishes where females are site-restricted and egg production occurs year-round. Differences in tilefish density suggest that reef slope and channel habitats may differ in quality. Channel females spawned twice as frequently as reef slope females, but daily batch fecundity and net yearly mating success were similar in the two habitats. By contrast, net yearly male mating success was higher in the channel because harems were larger. Adults did not move between reef slope and channel habitats. Therefore, the dense concentration of adult M. plumieri in the channel, appears to be a consequence of larval transport by prevailing currents. Supplemental feeding increased female fecundity and growth in both habitats. Different responses to feeding by reef slope and channel females is consistent with the hypothesis that reproductive tactics are conditional upon the type of predation risk that fish experience in local habitats.