Spatial distribution, abundance, seasonality and environmental relationship of amphioxus in subtropical Hong Kong waters

Amphioxus are a small group of cephalochordates inhabiting shallow-water coastal areas in both tropical and temperate oceans. In Hong Kong, only one study of the species composition and distribution of amphioxus was conducted in the 2000s. The present study aimed to provide up-to-date information on the diversity, seasonal abundance and distribution of amphioxus and identify environmental factors that might play a role in structuring their distribution. Benthic grab samples were collected from 23 sites spanning the eastern, southern, and southwestern waters of Hong Kong in 2019 (dry season) and 2020 (wet season). Samples from 13 sites yielded at least one individual of amphioxus, including several sites that were not covered in the previous study. A total of 1845 specimens belonging to three species of Branchiostoma were recorded, including B. belcheri, B. japonicum and B. malayanum. Among the 13 sites, amphioxus density varied from 14 to 618 individuals m−2 in the dry season and 4 to 322 individuals m−2 in the wet season. A high density of amphioxus (> 200 individuals m−2) was found at Po Toi O, Pak Lap Wan and Tai Long Wan. The three species of amphioxus typically co-inhabited these sites, with B. belcheri, B. japonicum, and B. malayanum accounting for 72.63%, 16.26%, and 11.11% of the total abundance, respectively. The three species all preferred shallow-water sites with coarse sediment and low organic matter content. Comparison of the population size structure between the two seasons revealed higher percentages of juveniles in the dry season, indicating reproductive seasonality in these species.