Seasonal variation in the nutritional vulnerability of first-stage larval porcelain crab, Petrolisthes laevigatus (Anomura: Porcellanidae) in southern Chile

Abstract

In temperate marine environments, meroplanktonic larvae face seasonal variations in temperature and food availability. In a laboratory study conducted under seasonally varying temperature conditions, we evaluated the nutritional vulnerability in different cohorts (August, September, October, November, December, and January) of first-stage larvae of a porcellanid crab, Petrolisthes laevigatus. Besides continuously fed and starved control groups (FC, SC), the experimental treatments comprised: (1) differential periods of initial starvation and subsequent feeding (point-of-no-return, PNR: minimum time of initial starvation preventing recovery after later feeding); (2) initial feeding and subsequent starvation (point-of-reserve-saturation, PRS: minimum time of initial feeding allowing for food-independent development through the rest of the moulting cycle); (3) limited daily access to prey (feeding pulses, 4 and 6 h/day). The development time of FC larvae ranged from 10.6 ± 0.4 days (January) to 21.9 ± 2.6 days (August), without significant variation in mortality among cohorts. Under continuous absence of food (SC), first-stage larvae never survived to the zoea II. Short-term feeding pulses had significantly different effects on development time of different cohorts, but not on survival. The PNR was lowest in larvae produced near the end of spring (November). However, when the PNR was expressed as a percentage of the development time in FC larvae, no significant differences were found among cohorts (relative PNR: 32.8–42.5%). The relative PRS was reached after 60.6–73.5% of the development time in FC larvae. In spite of strong seasonal (presumably temperature-induced) differences in the absolute PNR and PRS values of different cohorts, the relative values were generally similar. This suggests that successive cohorts of early larvae can develop with similar success, unless severe winter conditions or oceanographic fluctuations cause a significant decline in short-term prey availability or plankton productivity. Due to long development duration at low temperatures, the larvae produced in winter and early spring have greater potential for dispersal, favoring a genetic exchange between distant populations and colonization of new habitats. Summer larvae, by contrast, may be more important for maintaining local populations. An index of nutritional vulnerability (NVI = PRS 50/PNR 50) is proposed in this paper to compare larvae with differential developmental times. In most larvae studied so far, the NVI is < 1, indicating some degree of independence of food. In larvae of P. laevigatus an index > 1 reflects a relatively high degree of dependence on planktonic food, especially in winter larvae.