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 (), SANDIEGONENSIS AND 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 (), 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California has a diverse array of ephem­ R. Gonzalez, unpublished).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 . 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 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 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. ing. Simulated poo l wnter is deionized wat er that is ru n through field-collected poo l soil in a 105- f.l.rn sieve and the bord er areas of Baja California, to ad d necessa ry ions to the water. This water was then Mexico (E ng et al., J 990; Simovich and run throu gh a second, clean 105-f.l.m sieve to ensure Fuga te, 1992; Brown et al., 1993). Strep ­ that 30 cm) of lon ger duration, so me of o ther o rga nisms . Ne w ly hatched larvae wer e placed ind ividu all y in whi ch have been artific ially deepened with cups co ntaining l50 -ml sim ula ted pool water and in­ berms (i.e., cattle tanks and road emban k­ cubated at the same series of temperatures as in the HATHA W A Y A ND SIM OVIC H: DI STR IOUJ'ION OF A NO STR ACANS I N SOUTH ERN CAL IrORN IA 67 I

5 10 15 20 25 5-15 10-2015-25

Fig. 1. Percent age of hatching in a single hydration (mean S E) for five constant and three fluctu atin g temperature regimes. hatching ex pe riments. Twenty-one to 35 shrimp were In addi tion , we collected life-h istory da ta in' the field used per temperature treatment. Sh rimp were fed daily by frequently checking several natural pools nearby a slurry of baker's yeast in exc es s of dem and , in ad­ wh ich co ntaine d B. sandiegonensis onl y or both spe­ diti on to the small org anis ms in the simu lated pool cies for de velopment al stages of the shrimp present. water (rotifers, bacteria, etc .). To re move exc ess yeast, Pool s containing on ly S. wootoni we re not clo se molts, and exc rement, the sed ime nt at the bottom of enoug h for frequent field visits. the cup was remove d daily. Fre sh, sim ulated pool wa­ Th e number of day s to ma turity wer e co mpa red be­ ter wa s then ad ded to maintain wat er volume. Devel­ tween temperatures for B. sandiegonensis usin g ANO ­ opme ntal progress was noted daily. Maturity was de­ VA, P = 0 .05. Wh ere sig nifica nt treatment level dif­ fined as the presence of eggs in the o visac for fem ales ferences were indicated, multiple comparisons among and the presence of sperm in the testes for males. individ ual mea ns wer e made, using the Sc he ffe F- tes t, These method s worked we ll for B. sandiego nensis. and the least significant values are presen ted (Ott, Larvae of Stre p tocephalus woottoni, how ever, showed 1977 ). poo r surv ival under these co nditions. Therefore, direct com parisons of maturat io n be tween species at different RESULTS temperatures und er the same reg ime (single shrimp per cup) were not possibl e. In o rde r to make ge nera l com­ pariso ns of maturation ra te, large scale hyd rations were Hatching in Relation to Temperature performed, usin g cysts an d soil from mo nospe cific Branch inecta sandiegonensis hatched poo ls of both species and from a poo l wh ere the two co-occur. For these, 40 -1 aqu aria were filled with 30 I best at co oler temperatures, with a signifi ­ of 10°C, deion ized wa ter. Unsieved so il co ntaini ng cant temperature effect (F = 5.81 , P < cys ts was add ed. Th e aq uaria we re kept at room tem­ 0.0005 ) (F ig . I). The ro-c treatment perature (-20 -22°C) with co nstant light (GE wide showed significantly high er hatching per­ spec trum F40 PLlAQ) and ae ration. The aqua ria were centage than all but the treatment observed daily, and the appearance of larv ae, matura­ s- Isoe ~ tion, and longevi ty were noted. Wh ile not controlled (F 2.62, P < 0.05). No hatching oc curred o in term s of den sity and d iet , ma king quantifiabl e co m­ at 2S e nor at ] S- 2S°C. parisons and statistical analysis diffi cult, these meth ods Th e hatching response of S. woottoni was allow qualitati ve compariso ns. Fu rthermore, while no t similar to that seen in B. sa ndiegonens is, as elaborate as the cultur ing appara tus developed by also with a sig nificant temperature effect (F Brendonck et at. (1991 ), this technique worked well, beca use the hyd rated soil provided algae and micro­ = 6.72 , P < 0 .000 2). The hatch was highest organ isms for foo d. at lOoe and at 5- j soe with the Jalter tern ­ 672 JOURN AL OF CRUSTACEA N BI OLO GY. VOL. 16. NO.4. 1996

30

25 T

:r:: 20 ~ ji o Branchinecta sandiegonensis f2 15 (/) [2] Streptocephalus wootton; ~ 0 >« 10

T 5

0 5 10 15 20 25 5-15 10-20 15-25

N= 4027 116 52 15 2 11 2 o 6 61 79 8 23 0 51

TEMPERAlURE De Fig. 2. Numb er of day s from hydration to hatching (mean:!: SE ) at various temperatures. SE is not reported for S woottoni at 15° and 200 e because only two cysts hatched at these temperature treatments.

perature treatment showing significantly Maturation Rate higher hatch rates than at the constant 15, did not survive 20, and 25°e treatments (F 2: 2.89, P < to maturity well enough in individual cups 0.05). In contrast with B. sandiegonensis, to allow for statistical comparisons of mat­ some hatch was seen at 15-25°e and at uration rate or longevity between species 25°C. under these conditions. There was a significant temperature and For B. sandiegonensis, maturation rate species effect on average days to hatch be­ varied significantly with temperature treat­ tween B. sandiegonensis and S. woottoni (F ment (F = 24.89, P < 0.0001). In individ­ , = 43 .90, P < 0.0001). The average number ual cups, all shrimp died prior to maturation of days to hatch was significantly lower for at 5°C. Branchinecta sandiegonensis devel­ B. sandiegon ensis than for S. woouoni (F oped to maturity in ~ I week at tempera­ 2: 3.24, P < 0.05) (Fig. 2) in all but the tures above l O'C, and developed signifi­ ~ ] I' 10-20oe temperature treatment (which cantly slower, in 7 days, at ] ooe (F 2: most closely resembles natural pool fluctu ­ 3.01, P <0.05) (Fig. 3). Temperature also significantly affected longevity (F = 10.] 9, I ation observed in the field) . Branchinecta P < 0.0001). Shrimp longevity was greatest sandiegonensis hatched after approximately at the 10, 20, and 10-20oe temperature eight day s at 5°e, while hatching occurred i treatments (34.4 2: 7.0, 29 .7 2: 6.4, and in approximately three to five days at other 31.] 2: 14.5 days, respectively) with lon­ constant and flu ctuating temperatures. gevity at lOoe significantly greater than at Streptocephalus woottoni hatched in ap­ 15°e (F 2: 2.3, P <0.05), and longevity at proximately 20-25 days at 5, 15, and 25°e, 10, 20, and ]0-200e signifi cantly greater but in less than 12 days at 10, 20, 5-]5, than at 2SOe and ]5-25°e (F 2: 2.5 , J0-20, and ] 5-25°e (only two cysts P<0.05). These trends were also evident in hatched at constant ] 5°C and 200e) . field observations (fluctuating ambient tem­ HA T HAWA Y AND SI MOVICH: D IS TR IllUT ION O F A NOSTRACANS IN SOlrTHERN CALIFORNI A 673

40 ­ T T T 30 - ~: iii:

"!.:' o M ATUR ATION "'T'" 20 - g T @! ::::; : :::::: :::: :: ~: :::: T !21 L.ONGEVITY == ~i~~~~j ~~~i~~ T ;I. ;::=. I Ii. I- 10 - 14.: :.: [iif ...... z: I- r:;: i:::: ; r= :;:; I::;::: f~: i I ~t i il'~1 ~!} : ::~:' ill.::' ..i!;1 !11!li I~~ ~!i!!t o I I 5 10 15 20 25 5-15 10-201 5-25 (% survival to maturity = 0 21 33 29 24 13 22 28)

TEMPERATURE DC

Fig. 3. Num ber of days from hatch ing to maturity and longevity (mean :!: SE) for Branchinecta sa ndiegonensis.

peratures) and large-scal e hydration s ( ~2 0 ­ the range of temperatures inc luded SoC , 22°C) , where shrimp matured in 10-20 and whi ch was lethal when held co nstant. 7-10 day s, respectively (Table 1). Longer Temperature fluctu ation between 10°C maturation rates in the field are presumably and 20°C appears to be typ ical of temper­ a response to natural temperature fluctua­ ature regimes observ ed in the field during tions of pools to below 10°e. Ov erall, de ­ spring months. Temperatures in the field spite delayed maturations at 10°C, increased were recorded from as low as 9°C (it is like­ longevity resulted in a comparable number ly that temperatures dropped somewhat of reproductive days (days after maturity) lower; no temperature data were tak en be­ as at 20°C and 10-20°e. fore 0800) to as high as 30.8°C with records Temperature fluctuation in the laboratory of fluctuations up to 16°C withi n a single also had sig nificant effects by all owing a day. maturation rate intermediate between that In large-scale lab oratory hydrati ons of the higher and lower co nstant tempera­ ( ~2 0 -22°C) both species sur vived well. tures. Temperature fluctuation also allowed Larvae began to app ear within two days. development to maturation to occur when Adults of B. sandiegonensis appeared 7-10

Table 1. A comparison of approxi mate de velopmental ra tes of B ran chinect a sa nd iegonensis and Strep tocephulus woottoni based 0 11 laboratory mass-rearin g in aquaria (-20- 22°C) and field observa tion.

B. sandiegonens is S. wo ott oni n. sa nd iegonen sis S. wootton i Matu rity 7-10 days 7 weeks 10-20 days 8 wee ks Longevity 28- 30 days 2.5 months 4-6 wee ks 4 mo nths"

~ s. W OO f/ Oil ; were still prevent :l~ the pool d ried . 674 JOU RNAL OF CRUSTACEAN BI O LOG Y . Y OL o 16. NO . 4. 1996

days aft er hy dration and died by day 2 8­ with the inhibi tion o f germination increas­ 30, wher e as adu lts o f S. woottoni matured ing with temperature. afte r about 7 weeks (3 weeks after the dea th Streptocephalus woottoni also hatched of B. sandiegonensisy (Table I) . T hese re­ under cool to modera te te mperature s ( 10­ su lts wer e ess ent ially identical in the mono­ 20° C), but was able to hatch at warmer te m­ specific and d ispeci fic hy dra tions a nd par­ peratures (2.S°C) as we ll, especially if the all el ed field o bservations. temperature fluctuated . T h is species is Pools in the field vary in depth and tem­ fo und only in deeper pool s (> 30 ern), and perature. In a variety of pools observed, may not be as adversely affected by ha tch­ adults of B. sandiegonensis were present in ing from pool fillings in warmer months as less than two weeks aft er filling , but were would be the case in sh o rt duratio n/shallow gone after about 4 -6 weeks. Adults of S. pools. woottoni (fo und only in deeper pools) did U nfortuna te ly, S. wootto ni d id not do not app ear until ap proximate ly two months we ll un der laborat ory condi tions in individ­ afte r pools filled, al though juveniles were ual cups on a d ie t of yeast, algae, a nd small prese nt ea rlier. Streptocepha lus woottoni organis ms from hydrated soils. S tre pto­ pe rsisted as adults for severa l months o r un­ cephal ids are known to ea t a variety of or­ til the pools dried. ganisms incl uding algae, ne mat odes, ro ti­ fers , and o ther crus taceans (Merte ns et al. , D ISCUSS ION 1990; Brendonck et al., 1991; D e Wa lsche Branchine cta sandiego nensis do best at et al., 1991 ; Mitcheli, 1991; Courteau et al., cool to mode rate tempe ratures ( I0-20°C). 1992 ; B re ndonck, 1993a, b; Ali and Bren­ T his species ha tches but do es not survive do nck, 1995). It is possible that the cups did a t cool temper atures (SOC). War m temper­ no t contain h ig h eno ug h densities of p rey a tures appear to inh ibit hatching, while fluc­ or d id not a llow for some larval foraging tuating temperature s can enhance hatc hin g behaviors, beca use shrimp rai sed in mass and de ve lopment. These temperature re­ cultures and trans ferred to cups a t maturity quirements may part ia lly exp lain the distri ­ survive and reproduce (unpublished data). bution of B. sandiegon ensis in coastal ar­ Maturation ra tes were thu s compared using eas, w hich ha ve moderate cli mates. In the lar ge scale hy drations a nd fie ld ob ser va­ inland areas of southern California, moun­ tions. Al though this does no t allow fo r tain and desert pools may be too cool meaningful sta tis tical comparisons , the lab­ « SoC) or too wa rm (> 30°C) for thi s sp e­ orator y data mimic the fie ld da ta a nd there ­ cies to exist over ex tended periods of time. by some trends are discern ible. Streptoceph­ Fu rthe rmo re, hatching under cooler condi­ alus woottoni de vel o ped re lat ivel y slowly tions may confine hatching re sponse to fill­ and matu red in ~ 7 weeks a nd survived ings followi ng wi nter a nd early sp ring ~ 2 .S months in the laborator y or as lon g as rai ns . Ec los io n may be inhibi ted a fter s um­ wa ter re maine d in the natural pool s in the mer o r warm spring rai ns , when it is un­ fie ld. This lengt hy ti me to matu ri ty presum­ likely that pools w ill remain lo ng e nough ab ly restricts th is sp ecies fro m oc cu pying for the shri mp to mature. s ha llow pools that often last o nly several Te mperature preferen ces that match the days to a few weeks. seasonal timi ng of pool inu ndatio n a llo w In comparison to ot her southern Califor­ bo th species to beg in their life cycles soo n nian species , S. woottoni has a fairly w ide after pools fill . Belk an d Nelson (199 S) temperature tolerance. Howe ve r, it do es not fo und high tempe rature (30°C) inhib ited reach as high as that of the desert-dwelling Bran ch inecta lindahli Pac kar d , 1883, from Thamnocep halus platyurus P ac ka rd, 187 9, h atching, even if cysts were su bsequently which can hatch at 32°C and survive at transferred to favo rab le hatching te mpera­ 40°C (Belk, 1977) nor as lo w as Brachi­ tures (lSOC) . In additio n, E. T. Bauder (un­ nec ta mackini D e xter, 1956, which can pu blished data) not surprisingly found sim­ swim under ice (p ersona l observation). ilar temperature restrict ions in the germi­ With findings similar to o urs, Rz6ska na tio n of plants wh ich occur in the sa me ( 196 1) concluded that c limate is the maj or pools inhabited by these coastal shrimp. co ntrolling factor for the distribut io n of fau­ She found greatest ge rm ination at lOoC , na in tropical ep he meral po nds. Li kew ise, HA THAWA Y AND SIMOVIC H DISTRI[JUTION OFA NO STR ACA NS IN SO UTHERN C ALI FORNIA 675

Horne (1967, 1971), Belk (1977, 1984), and suggested that pool chemistry plays an im­ Belk and Nelson (1995) have suggested that portant, if not overriding role in ano stracan temperature is a primary environmental fac­ distributions (C oo pey, 1946; Hartland­ tor in determining anostracan zoogeography Rowe, 1966; Horne, 1967, 1971; Broch, and seasonal occurrence. In Belk's (1977) 1969; McCarraher, 1970; Geddes, 1973 ; studies of several species of North Ameri­ Belk, 1977 ; Alonso, 1990). Concurrent can anostracans, he found some species to physiological studies on these species in­ have restrictive hatching temperature ran g­ dicated that , while the coastal pools in es, while others did well over a broader which B. sandiegonensis and S. woottoni range. He attributed many of the distribu­ are found exhibit a relatively limited range tion patterns of anostracans in Arizona to of water chemistry, the shrimps can survive these thermal hatching responses. a somewhat larger range. Nonetheless, Our results indicate that differences in these species were found to be restricted to the hatching temperature requirements of B. fairly dilute pools and their range was not sandiegonensis and S. woottoni are not re­ as high as that tolerated by inland relatives sponsible for their offset appearance as such as B. mackini and B. lindahli (Gon­ adults in mixed-species pools, nor is num­ zalez et al., 1996 ). ber of days required to hatch a factor. Mat­ Our data indicate that temperature plays uration rate, however, may explain this pat­ an important role in the spatial and tem­ tern . Branchinecta sandiegonensis grows poral appearance of B. sandiegonensis and rapidly and reproduces very qui ckly, per­ S. woottoni. Hatching temperature require­ mitting it to live in shallow, short-lived ments which allow or inhibit hatching of pools as well as larger ones. The distinctly cysts appear to restrict these species to the longer maturation rate of S. woottoni pre­ coast and exclude them from nearby moun­ sumably restricts this species to deeper, lon ­ tain and desert pools, where temperatures ger-lasting pools. In pools where the two may be too cool or too warm. We found co-occur, they may hatch within a few days that within their distributional ran ge , the of one another and overlap briefly as larvae rate of development likely determines hab­ and eventually as adult (B. sand iegonensis j itat and patterns of co-occurrence. Branchi­ and subadult (S. woottoni). However, B . necta sandieg onensis, which develops sandiegonensis can reproduce and disap­ quickly, can exploit pools of a variety of pear from the pool before S. woottoni depths and duration, while S. woottoni, tak­ reaches maturity. It is unknown whether the ing longer to mature, is restricted to deeper two species compete for the same food or pools that last longer. For this reason, mat­ how the availability of different foods uration rate is als o responsible for offsetting through the life of the pool may affect their the appearance of the species as adults in developmental rates. pools where they co-occur. Therefore, tem­ Differences in developmental rate have perature restrictions and maturation rate are been implicated in studies of other ephem­ important in determining appropriate habi­ eral system s wh ere more than one species tat and should be considered in manage­ of branchiopods co-occur, yet show limited ment programs and impact assessments, es­ to no overlap as adults (Moore, 1963; Re ­ pecially in regard to pool restoration, crea­ tellack and Cl ifford, 1980; Donald, 1983; tion, or watershed modifications (M . A . Si­ Belk, 1991 ; Mura, 1991 ; Thiery, 1991 ). movich, M . Boudrias, and R. Gonzalez, The most comparable study (Belk, 1991) unpublished). For example, land dev elop­ showed a similar pattern for two other spe­ ment surrounding pools can restrict or en­ cies of the same gen era . In Belk's study, hance watersheds, thereby changing pool Branchinecta packardi Pearse, 1912, and characteristics (depth/temperature/lon­ Str eptocephalus texanus Packard, 1871, co­ gevity) and potentially altering ch ar­ exist in some pools. However, S. texanus acteristics outside the requirements of the has a slower developmental rate and is ex­ shrimps (M . A. Simovich, J. L. King, and cluded from the shallow, shorter-lived pools R. C. Brusca, unpublished). where B. packardi can exist. ACKNOWLEDGEME NTS In addition to thermal requirements and WI::. than k Cris Esparza, Julie Kline, Jeff' Glaspy, developmental rates, other research has Heather Lamberson, John Kornrney er, C hris Aeria, 676 JOURN AL OF CRUSTA CE AN BIOLOGY . VOL. 16. NO. 4. 1996

Don Gennero, and Kim Caruso for technical assi s­ egon .-Transactio ns of the American Microscopi cal tance. We also extend our grea test appreciation to Ja­ Societ y 65: 338-345. mie King, Denton Belk, Michael Wells, Michel Bou d­ Coutteau, P., L. Bren don ck, P. Lav ens, and P. So rge­ rias, Richard Gon zale z, Michael Fugate, and Clay Sas­ loos. 1992. The use of manipulated baker's yeast as saman for their invaluable sugge stions and editorial an algal substitute for the laboratory culture of An­ critique of this pape r. Thi s wo rk was supported in part ostraca.-Hydrobi ologia 234 : 25-32. by grants from the Department of Defense, the Nature Dab orn, G. R. 1978 . Dist ribut ion and biology of some Con servancy , and the University of San Diego. nearctic tundra pool phyll opods.-Verhand lungen der intern ationalen Vereinigung fur rheoretisch e und angew andte Limnologie 20: 2442 - 245 1. LITERATURE CITED De Walsche, C; J. Mertens. and H. J. Dum ont. 199 1. 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