Physiological traits associated with a reproductive - resting stage in

We investigated physiological traits associated with a reproductive-resting stage in adult female harpacticoid copepods, Coullana canadensis (Willey). Our hypothesis was this stage represents a life-history strategy that could increase ®tness by improving winter survivorship and future reproductive success in spring. To test if physiological rates are suppressed in this stage, we compared gut-cell morphology, nitrogen excretion rates, enzyme activities, and phytoplankton grazing rates of reproductive-resting and reproductive females reared in the laboratory under high food conditions. Copepods came from laboratory cultures originating from individuals collected in Maine and Maryland, USA in 1990. Reproductive-resting females had lower physiological rates, and the surface area of gut-cells was reduced compared to reproductive females. Distinct morphological di€erences in the distribution of lipids between reproductive and reproductive-resting females were observed under light and electron microscopy, the latter having a di€use accumulation of lipid in the area normally occupied by the ovaries. Di€erences in lipid composition were also found. Reproductive copepods had a signi®cantly higher ratio (%) of polar lipids to total lipid, and a lower proportion of triacylglycerols compared to reproductive-resting copepods. These laboratory ®ndings are consistent with the hypothesis that the reproductive-resting stage in C. canadensis is an adaptive response to increase winter survival. Introduction Dormancy is an important life-history strategy which allows organisms to survive periods of such environmental stresses as low food resources, high predation pressure, or low oxygen (e.g., Hairston et al. 1983; Hand 1991; Santer and Lampert 1995). Tauber et al. (1986; also see Hand 1991) point out that dormancy is any hypometabolic state, including both quiescence and diapause. The main di€erence between the two is that quiescence is a reversible state directly caused by unfavorable conditions, while diapause is a neurohormonally mediated state responsive to a number of environmental cues (e.g., temperature and daylength; Hairston and Kearns 1995; but see Williams-Howze and Coull 1992) that usually precede unfavorable conditions. In diapause, the hypometabolic response prevails when favorable environmental conditions for population growth return prior to appropriate ``emergence'' cues (Tauber et al. 1986). We tested the hypothesis that physiological characteristics of the reproductive-resting stage of Coullana canadensis could increase winter survivorship (Lonsdale et al. 1993). This estuarine copepod has a broad geographic distribution ranging from Nova Scotia (Willey 1923) to at least South Carolina (Lonsdale et al. 1988) on the east coast of North America. Previous studies indicated populations can vary in several life-history traits (e.g. egg and adult body sizes), and the di€erences re ̄ect genetic variation among populations (e.g., Lonsdale and Levinton 1985a, b). Of special interest to the present study was the previous ®nding that under certain conditions of water temperature and daylength, developing females switch from progressing to active reproduction to a reproductive-resting stage, even under high food conditions (Lonsdale et al. 1993). The latter females characteristically do not develop oocytes but accumulate lipid in the oviducts and other body cavities (Fig. 1a to c). The environmental cues necessary to induce a reproductive-resting stage also vary among populations. Marine Biology (1998) 131: 123±131 Ó Springer-Verlag 1998 Communicated by J.P. Grassle, New Brunswick D.J. Lonsdale (&) á R.P. Hassett á F.C. Dobbs á J. Yen Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York 11794-5000, USA Present addresses: Department of Biological Sciences, Ohio University, Athens, Ohio 45701, USA Department of Oceanography, Old Dominion University, Norfolk, Virginia 23529-0276, USA For example, at 15 °C, >90% of females collected in Maine became reproductive-resting females on a 14 h light:10 h dark cycle, while females from Maryland required a 12 h light:12 h dark cycle (Lonsdale et al. 1993). Interpopulation crosses indicated that the daylength necessary to trigger the reproductive-resting response was under genetic control. Variation in daylength period that triggers the reproductive-resting response likely rēects latitudinal di€erences in the population growth season. It is probable that C. canadensis in Maryland reproduces further into the fall compared to those in Maine. If reproductive-resting females are a major source of naupliar recruitment in the spring, as Lonsdale et al. (1993) speculated, then this stage of C. canadensis likely represents an overwintering strategy. Under laboratory conditions favorable for population growth, we investigated whether reproductive-resting females have lower physiological rates and increased long-term storage reserves compared with reproductive females. Such physiological changes would likely increase winter survivorship in temperate estuaries. Materials and methods Field collection and batch culturing of copepods Nauplii and copepodites of Coullana canadensis (Willey) were collected with a 64-lm mesh net at two sites; the Saco River, Maine (ME population; collected in June/July 1990) and the Patuxent River, Maryland, USA (MD population; April 1990). Copepods were sorted under a dissecting microscope and placed in 2-liter ̄asks containing 20-lm-®ltered ambient seawater, enriched with laboratory-cultured Isochrysis galbana (ISO) and Thalassiosira pseudonana (3H) at about 2.5 ́ 10 cells ml. Copepods were batch cultured by population at 15& S and 15 °C or 21 °C under a 14 light:10 h dark cycle. Under these conditions, reproductiveresting females develop for the ME population but generally not for the MD population (Lonsdale et al. 1993). About three times a week, copepods were fed, and once a month, approximately 50% of the seawater was replaced with fresh, 20-lm-sieved and autoclaved seawater at 15& S. At this time, copepods from separate batch cultures from the same source population were routinely mixed to minimize genetic heterogeneity among cultures. We classi®ed copepods as ``reproductive'' if oocytes were present in the oviducts and as ``reproductive-resting'' if no oocytes were visible and they showed lipid accumulation in the body under a dissecting microscope (Lonsdale et al. 1993).