FACTORS AFFECTING THE DISTRIBUTION AND CO-OCCURRENCE OF TWO SOUTHERN CALIFORNIAN ANOSTRACANS (BRANCHIOPODA), BRANCHINECTA SANDIEGONENSIS AND STREPTOCEPHALUS WOOTTONI

Abstract

We address the rol e of temp erature and maturation rate in limiting the distribution and co -o c­ currence of 2 ephemeral pool branchiopods, Branchinecta sandiegon ensis and Streptocephalus wool/ani (Anostraca), in southern California. Branchinecta sandiegon ensis oc curs in pools of variable depth (fro m 30 e m) and duration, while Streptocephalus wool/ani is found only in deeper pool s (>30 ern) o f longer duration. These 2 species co-occur in a few pools, but their adults are never observed simultaneously. To bett er understand these patt erns, field­ collected cysts of both species wer e hatched at an array of co nstant and 12-h fluctuating tem ­ peratures. Maturation rates wer e compared in aq uar ia at room temperature (-20 - 2 2°C) and by field ob servation. B oth spec ies hatched best at cooler temperatures ( lOOC and fluctuating S­ IYC), but S. WOOl/an i was more eurythermal. Both were inhibited at higher temperatures unless these temperatures were included in a fluctuating regime. After hatching , B. sandiegon ensis did not mature at soc. In laboratory and field ob servations, B. sandiegonensis matured quickly ( 1­ 2 week s) at moderate temperatures and died before S. WOOl/ani rea ched maturity. These results indi cate that temperature play s a role in re stri cting the distribution of the se species to the coast, where temperatures are favorable. Slower maturation rate helps to explain the ab sence of S. woottoni from shallow , potentially shor t-Jived pools and why the 2 spec ies co-occur but do not co exi st as adults in deeper pool s.