Effect of Eyestalk-Ablation on Circulating Ecdysteroids in Hemolymph of Snow Crabs, Chionoecetes opilio: Physiological Evidence for a Terminal Molt1

SYNOPSIS.

Bering Sea snow crabs (Chionoecetes opilio) are a commercially important crab harvested in the Bering Sea. Optimal management of this species requires an understanding of the biology of this crab that is currently incomplete. Fisheries managers apply a continuous growth model in their management of snow crab, which assumes that male crabs increase in size throughout their lifespan. Male snow crabs undergo a morphometric molt that leads to a disproportionate increase in chelae size and it is still debated whether this molt is associated with a terminal molt. This study was conducted to determine whether adult male C. opilio are anecdysic. Using current knowledge of the hormonal regulation of crustacean growth, snow crab physiology was manipulated to induce an increase in molting hormones (ecdysteroids). Since female snow crabs are known to undergo a terminal molt after attaining reproductive maturity, we compared ecdysteroid levels in eyestalk-ablated terminally molted females, small-clawed males and large-clawed males. Snow crabs were collected from the Bering Sea and maintained in circulating seawater at approximately 6°C. Animals were either eyestalk-ablated or left intact. Ecdysteroid levels in hemolymph were quantified using an enzyme-linked immunosorbant assay (ELISA). Circulating ecdysteroids were significantly higher in small-clawed male crabs when compared to large-clawed males or terminally molted females. Eyestalk-ablation increased circulating ecdysteroids in small-clawed males, but had no significant effect on circulating ecdysteroids in large-clawed males or in terminally molted females.

INTRODUCTION

Snow crab, Chionoecetes opilio, are commercially important crabs living at high latitudes that are harvested on the Atlantic as well as the Pacific coasts. In the Eastern Bering Sea alone, over one hundred thousand metric tons of male snow crabs were harvested each year for over a decade until 1994 when the fishery collapsed and was declared over fished (Otto, 1998). Since only adult male crabs having a carapace width greater than 102 mm are commercially harvested (legal size is 95 mm), the understanding of growth physiology is important to the successful management of this species.

Currently, snow crab management employs a growth model that assumes indeterminate or continuous growth of crabs (Zheng et al., 2002). It is unclear and disputed as to whether C. opilio are capable of continued growth or undergo a terminal molt after which external growth does not occur. The first thorough study to describe the terminal molt in C. opilio used both a field and a laboratory component to investigate growth (Conan and Comeau, 1986). Their results were disputed on the basis that the timing for investigating the occurrence of premolt in adult males (July to December) was not optimum (Donaldson and Johnson, 1988). A better design for the trawl survey would be to sample from December to March during which time snow crabs are known to enter premolt. Most studies pertaining to growth have been conducted using the Atlantic and Japanese stocks, and these support the terminal molt hypothesis (Conan and Comeau, 1986; Moriyasu and Mallet, 1986; Hebert et al, 2002). A more recent publication fueled the controversy over a terminal molt by describing the molting of functionally mature male Tanner crab, C. bairdi, which are a congener species (Paul and Paul, 1995).

Male C. opilio, like other crabs in the family Majidae, undergo a morphometric molt during which the chelae grow disproportionate to the carapace resulting in a "large clawed" male. This molt is not associated with the onset of reproductive maturity since males with smaller chelae are capable of synthesizing sperm (Watson, 1970; Sainte-Marie and Hazel, 1992; Sainte-Marie et al., 1995). Instead it is thought that this molt is associated with functional sexual maturity, a stage at which males will compete with other males and are able to mate with females; therefore this molt is commonly referred to as the morphometric molt to functional maturity and these crabs are termed adults (O'Halloran and O'Dor, 1988). In contrast, small-clawed reproductive males are termed adolescent. Adolescent and immature crabs are known to molt until they reach adulthood. A complete understanding of growth in C. opilio is critical for managing this species as the legal harvesting of large adult males is artificially selecting for smaller sublegal adult males under the terminal molt hypothesis, and may result in further population declines in snow crab.

Hormones regulate crustacean molting and regeneration and this regulation is well described in reviews (Skinner, 1985; Chang et al, 1993; Lachaise et al, 1993; Reddy and Ramamurthi, 1999). In brief, ecdysteroids promote molting and are synthesized by endocrine y-organs (Chang and O'Connor, 1977; Keller and Schmid, 1979; Watson and Spaziani, 1985). Y-organs are under the inhibitory regulation of an eyestalk neuropeptide known as molt-inhibiting hormone (MIH) and removal of eyestalks results in an increased secretion of ecdysteroids (Webster, 1986; Chang et al., 1993) and an acceleration of the molt cycle (Passano, 1965; Snyder and Chang, 1986). In crustaceans that undergo a terminal molt, there is evidence that secretion of ecdysteroids diminishes (Hinsch, 1972; Laufer et al., 2002) due perhaps to the degeneration of the y-organ (Carlisle, 1957). Ultrastructural studies of the isopod Sphaeroma serratum, showed that y-organs degenerate upon undergoing the terminal pubertal molt (Charmantier and Trilles, 1979). The activity of the y-organ is known to vary during the molt cycle (for a thorough review; see Lachaise et al, 1993), and eyestalk-ablation was shown to increase y-organ cell volume, which correlated with y-organ activity (Simione and Hoffman, 1975).