Tanner crabs, Chionoecetes bairdi
Much of my career was spent studying Tanner crabs, Chionoecetes bairdi, in Alaska. Here are some photos and information about Tanner crabs. All photos by Brad Stevens.
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​Male crabs are typically about 25% larger than female crabs. Mating occurs in competitive situations where size is an advantage. In addition, the majority of males mating with females are hard-shell (see below), which means they must wait at least a year after their terminal, pubertal molt before they can mate successfully (Stevens et al., 2003)
Males have a typical triangular abdomen, whereas females have a rounded abdomen that serves to protect their egg masses.
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Tanner (and snow) crabs undergo a terminal molt, called the pubertal molt, at which they achieve maturity. During this molt, the chelae undergo positive allometric growth, and are then considered “morphometrically mature” (MM). The chela height (ChH) of immature crabs is equal to about 15% of the carapace width (CW), whereas in MM crabs, the ratio of ChH to CW ranges from 20 to 30% (Stevens et al, 2003). This photo shows the claws of a mature (top) and immature (bottom) crab.
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Because they don’t molt after reaching maturity, shells of Tanner crabs become degraded over time due to fouling, scrapes, injuries and bacterial degradation. This allows biologists to classify them into rough categories, in order of increasing age since their last molt. This photo shows four stages of shell aging, classified (from top to bottom) as New-shell, Hard-shell, Old-shell, and Very-old-shell. Isotopic aging (using Thorium) has shown that new shell crabs have molted within the last few months, Hardshells within the last 1-2 years, and old shells within the last 3-6 years.
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Prior to their maturity molt, female crabs are considered “pubescent”. Their shells may be covered with fouling organisms and barnacles. A few weeks before ecdysis, they probably start to release metabolic byproducts of molting hormones in their urine, which can be detected by the males as a sort of “perfume” or pheromone (this has been demonstrated in other crab species, but we are just speculating that it also occurs in Tanner/snow crabs). Males can detect females that are in pre-molt condition, and will grasp them with their chelae (shown here). This pre-mating embrace may last up to two weeks, until the female molts (shown here). He then helps her out of her old, hard shell, and mating occurs in her soft, new-shell condition.
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In areas of the Bering Sea where Tanner and snow crabs overlap, hybridization of crabs (Tanner x snow) may occur. Some of these are fecund, and their offspring may mate with other crabs. On rare occasions we find hermaphroditic crabs (shown here). Their abdomens are rectangular, somewhere between the triangle of a male, and the rounded female abdomen. The majority of these hermaphrodites are hybrid Tanner x snow crabs. Dissection of a hermaphroditic crab shows that it has both ovarian (orange) and testicular (white) tissues.
