DIVERSIFIED BET-HEDGING AS A REPRODUCTIVE STRATEGY OF SOME EPHEMERAL POOL ANOSTRACANS (BRANCHIOPODA)

A B S T R A C T In Ca li fo rn ia. most ano s traca n cru stace ans inhabi t e pheme ral we tlands, s uc h as ve rna l pool s, mountain mead ow s, a nd desert playa s wi th bi phasic wet/dr y cyc les . Th e sh rimps hatch and mat ure during the aqua tic pha se a nd deposit dor m ant cysts which remain in th e so il th rou gh the dry phase. In so me habitats, du e to the variabl e nature of local rainfall patte rns, po ol s at ti mes fill o nly partia lly a nd dry qui c kly befor e the s hrim p a re ab le to mature a nd re prod uce . S pec ies in such unpred ict abl e e nviro nme nts prod uce c yst s th at do not a ll hatch w he n firs t hydrated . A po rtion rem a in dorm ant and hat ch in later poo l filli ngs . We in vest igated the hatching patter n of two ne wl y described s pecies of an ost raca ns found in un predictabl e vernal pool s in southe rn California, U.S.A., Branch in ecta sandiegonensis Fu gate and Streptoceph alus woott oni Eng et a l. Hatching rates for the se s pecies wer e amo ng the lo west yet rep o rted for anos traca ns . In itial hatch fro m the so il cy st ban k wa s 6 % for B. sandiegonensis and 0. 18% for S. wootton i a nd c um ulative to tal s we re 28 % an d 2 .8 %, resp ecti ve ly, over 3 hydr at ion s . Fur ther more, hatch­ ing var ia bility was dem onstrated fo r the c ysts of si ng le pai rs. T his life-histo r y pattern confor ms to the 3 major predi cti ons of mod el s o f diversified r isk-spreading o r bet-hedgi ng s trateg ies as de velo ped for annu al plants . By definition, ephe mera l wetlands, s uc h c ies, and thu s no offspring result fro m the as vernal po ol s, des ert playas, and mountain populations experiencin g these fal se s tarts. meadows, ex hibit periodic fillin g a nd dry­ Organisms living und er these unpredict­ in g (or fre ezing) . The biphasic, aqu ati c/ter­ able circumstan ces mu st share a th ird c har­ restrial nature o f the habitat requires its bi­ ac te ris tic ; the y mu st have developed so me o tic community to be ada pted to tw o very way to a void ex tirpa tio n in the f ace o f pre­ different environmenta l regimes (Be lk and mature drying events. Such species are ex­ Cole, 1975 ; Wiggins et al., 1980 ; Bren­ pected to exhibit life -h istory strategi es that donck and Per scone, 199 3). Consequently, inc lude some so rt of reproducti ve bet-hedg­ suc h organ ism s often sha re certain life-hi s­ ing res embling I risk -sp reading stra te gies, tory characters. Fir st , inhabitants wh ich are where the reproducti ve output of an indi­ obligate ly aq uatic for so me sta ge o f their vidual is sp re ad over tim e, as documented life must develop and reproduce fairly in annua l pl ants of arid environments. This qui ckly be for e the pool dries and the ter­ stra teg y has bee n modeled extensivel y by res trial o r nonaqueou s phase begins . Se c­ Cohen (1 966 , 1967, 1968 ), modifi ed by ond , the se o rganisms mu st have a means of M acDonald and Watkinson (1981 ), Bulmer sur viving the nonaqueou s period, ge ne rally (1984), Brown and Venable (19 86 ), and as dormant eggs (cysts) o r seeds. Ven able and Brown ( 1988), and re viewed However, the predict abil ity a nd freq uen­ and di scu ssed by Se ge r and Brockman cy of thi s wet/dry cycle varies am on g hab­ (1 987 ), Philippi and Sege r ( 1989), and Ka­ itats. Spring snow melt may predictably fill lisz and McP eek (1992), a mo ng oth ers. mountain meadows, but desert playas may Anostracan crustaceans are one of the most go years between fiIlin gs. Between the se co ns picuous fauna l el e ments of ephemeral e xtremes a re the ve rna l po ols o f the grass­ pools ( Be lk, 1975; Dod son, 1987 ; Eng et lands and cha parr a l o f southern C alifornia. al., 1990; King et al., 1996). Th ey hatch These habitats are typi cally charact erized soo n after th e pools fill , and develop and by variabl e precipitation patterns . Often the reproduce re la tive ly qu ickly, producing aq ua tic pha se lasts for o nly a few days desiccation resist ant cysts which are left in ( Ba ude r, 1987a, b). In so me cases, thi s is the dr yin g mud to hatch in future fillings. less time than is ne ed ed for development Anostracan cys ts recovered from soi l sam­ and reproduction of mo st vernal pool spe- ples, from pool s that fill predictably, ex hibit