Archaster typicus

Chan, Yong Kit Samuel, Toh, Tai Chong & Huang, Danwei, 2018, Distinct Size and Distribution Patterns of the Sand-sifting Sea Star, Archaster typicus, in an Urbanised Marine Environment, Zoological Studies 57 (28), pp. 1-11 : 4-5

Clustering in Archaster typicus in Singapore

Across all 44 transects, most NNI values were below 1 (n = 40; 90.9%), highlighting a clustered distribution, with only 4 transects (9.1%) having NNI values above 1 ( Fig. 3 ). The NNI values depicted greater clustering at PSL - with most values lower than 0.3 - and weaker clustering at HAN - with most values around 0.6 ( Fig. 3 ). Clustering did not seem to vary according to month, with no clear pattern through time ( Fig. 3 ).

Ontogenetic shifts and mating seasonality in Singapore

There was a clear absence of sea stars within the Pulau Hantu mangrove during all sampling trips. The number of sea stars within the seagrass patches were also very low (n = 10) compared to within sandy shoals (n = 2294). Though there were no significant differences between sea star size on sandy shoals and seagrasses, sea stars were still slightly larger at sandy shoals (∆ = + 5.86 mm; p = 0.0602) ( Table 2). Additionally, the smallest individuals on seagrasses were 28 mm compared to the 33 mm ones on the shoals. No individuals smaller than this arm length had been observed at other habitats in Singapore.

Counts of mating sea stars did not seem to indicate mating seasonality, although mating proportions seemed to increase in December and January, with sporadic high mating proportions in individual transects across sites and months with no clear trend such as in one transect in HAN in

October and one transect each in PSL and SJI in August.

Sediment analysis

Sediment fractions across months did not vary considerably, though the sediment profile of the different sites differed, being a possible explanation for the differences in sea stars across sites ( Fig. 4 ). In general, most of the sediments found consisted mostly of medium to coarse sediments, with fine sediments least common in PSL and very coarse sediments least common in SJI ( Fig. 4 ).

HAN PSL SJI

Figures

Fig. 3. Nearest Neighbour Index (NNI) values of transects across months showing generally clustered populations of A. typicus at the transect level. Red dashed line (NNI = 1) indicates a random distribution, with values above 1 indicating dispersion and values below 1 indicating clustering (HAN: Pulau Hantu, PSL: Pulau Subar Laut, SJI: St John’s Island).

Fig. 4. Sediment proportions across sites and between months showing greater proportions of coarse material at PSL (HAN: Pulau Hantu, PSL: Pulau Subar Laut, SJI: St John’s Island). © 2018 Academia Sinica, Taiwan