Sunday, July 7, 2013

Payton Hermanson- Week Two

As promised last week, this week's post will be about the life history of the Dungeness crab.
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The crabs mate in the spring time, the males locating the females by scent prior to the females' molt. During the molt, in which the female sheds their hard outer shell or exoskeleton, the male places his sperm into pouches, called spermathecae, where they remain viable until the female extrudes her eggs, which are then fertilized. This storing process may take many months, with the female retaining the spermatophores until the fall of the same year. (In Oregon, eggs are extruded between October and March.) After extrusion, the female carries the fertilized eggs on the ventral, or underside, of her body for 60 to 120 days as the eggs develop.

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The eggs hatch into a stage known as a prezoea, from which it quickly molts into a zoea. The zoea life stage consists of five larval forms and appears little like the adult crab. Zoeae are free swimming, adapted for life in the pelagic (open) ocean, where they spend several months among the plankton (small drifting plants and animals) before being swept inland once more. There, the zoeae molt into megalopae (singular, megalopa or megalops), which are adapted to find a place to settle, then molt into juvenile crabs. It is at the megalopal stage that we are finding the Dungeness crabs snared in the light trap.

Adult crabs reach sexual maturity between two and four years of age, and can live for eight to thirteen years. Adult crab lifespan is heavily determined by the fishing industry, with most males of catchable size (essentially all individuals with a carapace, or top shell, width of greater than 160 mm) being caught within the first two months of the fishing season opening. This presents a very unique opportunity for researchers such as Dr. Shanks to have a highly accurate measure of a specific population size.

Despite the intensive fishing, Dungeness crab populations have not been adversely affected by the fishing industry as other organisms have, with the Washington to California range of Dungeness crab maintaining a "Best Choice" for eating rating courtesy of the Seafood Watch program, which evaluates the sustainability and health of marketable species.

Information and images obtained from the December 1989 National Wildlife Research Center Dungeness crab species profile, the December 2007 Monterey Bay Aquarium Seafood Watch Report, and the published papers of Shanks et al.

Next week: Lab updates and field work!

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