Clark's anemonefish, Amphiprion clarkii, local distribution, group composition, sex change, agonistic behaviour and growth : a study of anemonefish under natural and captive environments

Abstract

Anemonefishes, Amphiprion spp., are relatively well-studied, hermaphroditic, group-living reef fishes under both field and laboratory conditions, although relationships among group composition, social behaviour, sex change and growth of anemeonefishes are still unclear. Using Clark’s anemonefish, Amphiprion clarkii, as a model species, these topics were variously studied through laboratory experiments and field surveys. Amphiprion clarkii was collected from Hong Kong waters and kept in captivity to form 6 different group (or ‘set’) types FMJ, FM, FJ, MJ, JJ and J (F, M and J indicate female, male and juvenile respectively) for up to 4-months and their aggressive interactions, growth and sex were monitored. Field observations on social behaviour were compared to captive observations and social structure.

Although A. clarkii is common in Southeast Asia, its distribution and group composition in Hong Kong are poorly understood. Hence, these were studied through field surveys in 2015 and 2016 in Hong Kong eastern waters to better understand distribution, host anemone species and habitats of A. clarkii in Hong Kong.

The results of laboratory experiments suggest that sexual development suppression (submissive juveniles in MJ and JJ sets) and induction (submissive juveniles in FJ sets and dominant juveniles in JJ sets) may occur, according to these group compositions. The results suggest that dominant individuals do not regulate the sexual development and growth of submissive individuals through significant changes in agonistic behaviour frequency. Agonistic behaviours were comparable between captive and wild social groups.

The three most common social group types (sets) found in the wild were single juveniles, female-male pairs and single males which accounted for 54-57%, 12-21% and 9-14% of all sets surveyed respectively. Other group types (FJ, MJ, FMJ and multi-juvenile groups) accounted for 16-20% of social groups encountered; multiple juveniles were not encountered with an adult pair. Adult females were significantly fewer than adult males, with the observed adult female-male ratio of 3:5. The observed group compositions and the imbalanced sex ratio could be explained by several factors such as high anemone density environment, sex change system and group formation dynamics linked to choices made by both juveniles and adults.

Amphiprion clarkii were found to be widely distributed and abundant in eastern Hong Kong waters and were recorded in 42 out of 53 surveyed sites. More than 100 individual fish were recorded in 2 sites. Several factors could explain the abundance and distribution of this species, including host anemone availability, recruitment variability and interspecific competition. Two species of host anemone, Entacmaea quadricolor, and Macrodactyla doreensis, were recorded with E. quadricolor being the most common. At each of two surveyed sites, Basalt Island and South Ninepins, there was more than a hundred of E. quadricolor, suggesting that the moderately exposed and boulder-dominant habitats at these locations are favourable for this host anemone. In addition, two other anemone-symbiotic fishes, Dascyllus trimaculatus and Apogonichthyoides niger, and two anemone-symbiotic shrimps were recorded. The distribution and habitat data can act as a baseline for future studies of Hong Kong A. clarkii populations.