JOURNAL O f' CRUSTAC EA N BIOL OGY . 16(4) : 669 - 677, 1996 FACTORS AFFECTING THE DISTRIBUTION AND CO-OCCURRENCE OF TWO SOUTHERN CALIFORNIAN ANOSTRACANS (BRANCHIOPODA), BRANCHINECTA SANDIEGONENSIS AND STREPTOCEPHALUS WOO1TONI Stacie A. Hathaway and Marie A. Simovicn ABSTRACT We address the role 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 occurs in pools of variable depth (fro m < S em to > 30 em) and duration, while Streptocephalus wool/ani is found only in deeper pool s (>30 ern) of 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 constant and 12-h fluctuating tem ­ peratures. Maturation rates were compared in aquaria at room temperature (-20- 22°C) and by field observation. Both species 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 observations, 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spec ies co-occur but do not coexist as adults in deeper pool s. California has a diverse array of ephem­ R. Gonzalez, unpublished). When multiple eral wetlands which accommodate the high­ species co-occur within pools, their appear­ est diversity of anostracans of any area in ance is often temporally separated, presum­ North America. Anostracans can be found ably because of differences in hatching in high mountain meadows, desert playas, temperature or developmental rates (Don­ and in both coastal and central valley vernal ald, 1983; Mura, 1991; Thiery, 1991). pools (Eng el al., 1990 ; Simovich and Fu­ Those that coexist temporally may be spa­ gate, 1992; Fugate, 1993). These various tially separated, presumably by differences habitats fill with winter rains, spring snow in food or pool microhabitats (Daborn, melt, or summer monsoons, and occur on a 1978; Vekhov, 1988; Hamer and Appleton, variety of soils (see, for review, Eng et al., 1991b; Thiery, 199]). Unfortunately, few 1990). studies have evaluated the relative impor­ Researchers world-wide have considered tance of the various factors involved in de­ several factors important in determining the termining the occurrence patterns of anos­ distribution of anostracans. Temperature tracans. and water chemistry are most often empha­ Despite the pre sence of anostracans in sized (Coopey, 1946; Prophet, 1963; Hart­ numerous Californian habitats, these organ­ land-Rowe, 1966; Horne, 1967, 1971 ; isms are not well known. For example, six Moore, 1967 ; Belk, 1977 , 1984; Geddes, of the 22 Californian species have been de­ 1983 ; Alonso, 1990; Belk and Nelson, scribed only in the past five years (Eng et 1995), but the significance of pool size and al., 1990; Fugate, 1993; Thiery and Fugate, length of inundation for accommodating I 995). However, at the same time that new species wi th certain developmental rates species are being found in ephemeral wet­ and niche requirements has also been rec­ lands, these habitats are disappearing, Cur­ ognized (Wiggins et al., 1980; Belk, 1991 ; rent estimates of losses are 90% throughout Hamer and Appleton, 1991£1, b; Thiery, California (Stone, 1990) and 93-97% for 1991 ; Meintjes et al., 1994 ; Seaman et al., San Diego County (Bauder, 1986, 1987); 1995; M. A. Simovich, M . Boudrias, and the majority of these wetlands have been ----- --_. 669 670 JOURNAL OF CRUSTACEAN 1l10LOGY . VOL I c> , NO 4. 1996 lost In the last 20 year s (Bauder, 1986; E, ments). The two spec ies are known to T. Bauder and H. A. Weir, unpublished ). co-occur in a few deep pools. How eve r, Losses of this magnitude are particularly where they do co-occur, they have not been threatening to species that are narrow en­ observed to coexist as adults. Adults of B. demi cs. Eight anos tracan species are en­ sa ndiegonensis are found earlier in the sea ­ demic to California, and some are restri cted so n than S. woottoni (Simovich and Fu gate, to ju st a few areas, Th e combination of en­ J 992). Neither species is found in the near­ demism and hab itat destruction has resulted by desert or mountain areas. in three of the anos tracan species in Ca li­ fornia being listed in the Federal Register M ATERIALS AND M ETHODS as endangered, one as threatened, and an­ Hatching in Relation to Tempe ra ture.s-A n order to as­ other has been proposed for listing (Fed eral ses s the effect of temperature on hatching, single so il Regi ster, 1988, 1993, 1994a, b). Plans for sa mples from poo ls with mo nospecific popu latio ns the protection and management of these were washed, usin g deionized water through stacked 500-f.l.m and 150-f.l.m sieves, and exami ned under a spec ies require informa tion about the fac­ dissecting microscope. Individu al cys ts were remo ved tors determining thei r distribution, such as from the so il using fine insec t force ps. Intact cy sts physiological tol erance s, life histories, gen­ were divided into 32 aliquo ts o f 100 cy sts each and era l ecology, and population stru cture. Lit­ immediately placed into cups conta ining 150 ml of deionized water. Four cups were inc uba ted under I of tle information of this kind is available for several temp erature treatments, including 5 co nstant the more recently described species. (5 , 10, 15, 20, and 25°C) and 3 12-h fluctu ating (5­ In this study, we investigate hatching I S, 10-20, and 15-2S°C) reg imes. Fluctuating reg imes temperature requ irem ents and the matura­ were acheived by tran sferring cups from the low or tion rate s of two recentl y described, so uth­ hig h temperatures every 12 h. Light wa s consta nt us­ ing pla nt/aq uarium lig hts (GE w ide spectrum F40 ern Cal ifornian anostr acans, Branchi necta PUAQ). Cys ts were checked twice a da y and hatch ed sa ndiegonensis Fugate, 1993, and Strep to­ lar vae were rem o ved . Deioni zed wa ter was added as cephalus woo ttoni Eng et al., 1990 . The re­ necessary to compe nsa te for eva po ratio n. All cups sults are discussed in reference to how the se were checked for a min imum of 21 days or to 7 days past the las t hatch, whichever period was lon ger. factors may affect the distributions and pat­ The use of deionized wa ter for hatching experiments terns of co-occurrence of these species. ke eps water chemistry co nsis tent and allows compar­ iso n to previous experiments on other sp ecies (e.g., Background.-Bran chinecta sandiegone n­ Moor e, 1967 ; Belk, 1977) . Its use is not , howev er, co n­ sis has been pro posed for status as an en­ ducive to lar val sur vival ; larvae sur vive o nly a few dan gered species (Fede ral Register, 1994a). ho urs und er these hypo tonic co nd itions (pe rsonal ob­ This species has been found in pools o nly se rvation). Th e number o f cysts hatched o ut of 100 ( % hatch ) on the me sas of western San Diego County, and number of days to hatch we re compared between U.S.A., and into Baj a California, Mexico temperatures for B. sandiegonensis and S. wool/an i us­ (Simovich and Fu gate, 1992; Brown et al., ing ANaYA , P = 0.05 . Wh ere significa nt treatmen t 1993; Fugate, 1993). It can be found in a level differences were indicated , multiple comparison s amo ng individual means we re made using the Scheffe range of pool depth (from <5 cm to > 30 F -test (Ott, 1977). cm) and duration, and is present soon after Matura tion Rate.-In order to assess the effect of tem­ the pool s first fill with winter rain s. In this perature on maturation, de ionized wa ter wa s run area , shallow 'pools are ge nerally of shorte r throu gh so il from San Diego County kno wn to contain duration than deeper pool s. cysts o f both .species, soil co ntai ning cy sts of B. san ­ S treptocepha lu s wootto ni is fed erally diegonensis only, and soil know n to co ntain cy sts of S. wool/ani only, in a series of SOO-f.l. m and 150- f.l. rn listed as endan gered (Federa l Regi st e r, sie ves . Th e so il remaining in the ISO-f.l.m sieve wa s 1993). This species has been found in o nly ex am ined for cys ts. Cy sts were hydrated in simula ted a handful of pools in Riverside, San Di ego, pool water at lOoC, and chec ked twice daily for hatch­ and Orange Counties, California, U .S.A.