Tanner Crab and Snow Crab(Chionoecetes bairdi and Chionoecetes opilio)
Whenever I mention tanner crabs to our guests, they return a
questioning look. “What is a tanner crab?” “I’ve never heard of a tanner crab.”
If you enjoy eating crab, you’ve undoubtedly consumed tanner crab at a
restaurant, but the menu probably listed the delicacy as “Alaska Snow Crab.”
In the 1960s and 70s, when the king crab fishery exploded,
commercial fishermen considered the smaller tanner crabs pests worth nothing. A
decade later, though, when the king crab fishery failed in many areas, savvy industry
marketers began advertising tanner crabs as snow crabs, and suddenly, their
value soared as demand grew.
To make the tanner crab – snow crab situation more
complicated, fishermen call Chionoecetes bairdi by the common name, “tanner
crab,” but they refer to Chionoecetes opilio as “snow crab.” To further
confuse things, where the two species’ ranges overlap, they can interbreed,
producing offspring bearing characteristics of both parents. For this article,
I will refer to Chionoecetes bairdi as tanner crab and Chionoecetes
opilio as snow crab.
Tanner crabs (Chionoecetes bairdi) and snow crabs (Chionoecetes
opilio) are considered short-tailed or “true” crabs. A tanner crab’s body
is a chitinous carapace with a small abdominal flap. A male’s flap is
triangular, while a female has a broad, round abdominal flap. A tanner crab has
five pairs of legs and the first pair sports pincers. By the time it reaches
adulthood between the ages of seven to eleven years, a tanner crab weighs from
two to four pounds (0.91 to 1.81 kg).
[image error]Tanner Crab
Unlike king crabs, tanner crabs do not continue to molt (shed their old shell and grow a new one) throughout their lives. Once they reach sexual maturity, both males and females undergo a terminal molt, after which they will never again shed their shell. A female tanner crab mates for the first time during her terminal molt. She releases pheromones to attract a male and remains receptive for 21 days. The male crab clasps the female and inserts his sperm into her. Laboratory observations suggest this clasping embrace can last as long as 14 to 151 hours.
After her first mating session, biologists think a female
tanner crab produces another four clutches of eggs before dying. During
subsequent mating sessions, the female has a hard shell, and in the absence of
a male, she can produce an egg clutch with sperm she stored from a previous
mating. A female tanner deposits between 85,000 to 424,00 eggs in a clutch. She
extrudes the eggs within 48 hours of fertilization onto her abdominal flap,
where they incubate for a year.
The eggs hatch the following spring from April to June, and
hatching usually coincides with the peak of the spring plankton bloom, providing
ample food for the larvae. At first, the larvae are free-swimming, and they
molt many times as they grow. The swimming phase lasts about 63 to 66 days, and
then the larvae settle to the bottom. The young crabs continue to molt and grow
for several years. Females reach maturity at approximately five years of age,
while males mature at six years. Tanner crabs can live 14 years.
Biologists do not fully understand the migration patterns of
tanner crabs, but they know the sexes remain separated during most of the year
and move into the same areas only during the mating season.
Tanner crabs eat a wide variety of organisms, including
worms, clams, mussels, snails, crabs, and other crustaceans. They are preyed
upon by fish, sea otters, and humans.
Tanner crabs are susceptible to an illness called Bitter Crab Disease, caused by a specialized dinoflagellate from the genus Hermatodinium. As its name suggests, crabs infected by Hermatodinium taste bitter, and the meat appears chalky. The disease is often fatal, and dying crabs release spores which infect nearby crabs.
[image error]Snow Crab (NOAA)
Snow crabs are smaller than their tanner crab cousins and reach a maximum of only one to three lbs. (.5 to 1.35 kg). Females carry up to 100,000 eggs, and biologists estimate snow crab can live up to twenty years. Snow crabs and tanner crabs have similar life cycles.
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Robin Barefield is the author of four Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter, and Karluk Bones. You are invited to watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska, and listen to her podcast, Murder and Mystery in the Last Frontier.
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