Morphology of mandibles and food size in two fairy shrimps (Branchiopoda: Anostraca) from Thailand

Abstract

We analyzed the asymmetrical molar surfaces of two fairy shrimps, Streptocephalus sirindhornaeSanoamuang, Murugan, Weekers & Dumont, 2000and Branchinella thailandensisSanoamuang, Saengphan & Murugan, 2002. The peripheral region of the left and right mandibles in both species has several rows of rectangular teeth. Each row has 5-8 small stubby denticles in S. sirindhornae and 6-14 numerous, small, sharp denticles in B. thailandensis. The posteriodorsal region of the left mandible bears one row of 11 large, stout teeth and one row of six large, stout teeth, respectively. The posteriodorsal region of the right mandible in S. sirindhornae has three rows of teeth, whereas in B. thailandensis it has two rows plus a tuft of setae. The intersetular distance of filter-feeding limb endites in B. thailandensis is wider (1.37 ± 0.27 μm) than that of S. sirindhornae (0.38 ± 0.02 μm). These morphological differences affect feeding behaviour. Feeding experiments showed that B. thailandensis can consume larger starch grains than S. sirindhornae. The largest percentages of ingested food size in both immature and mature shrimps are in the 5-30 μm range. Because B. thailandensis tends to mature far earlier than co-occurring S. sirindhornae, we hypothesise that B. thailandensis adults may consume S. sirindhornae nauplii. No evidence of cannibalism was found in the two species.