They grow up so fast… when they’re cold

Here's the paper I wanted to share with you but then got distracted on Australia Day:

You’d think that, given their name, snow crabs would like the cold. And they do: 97.8% were associated with eastern Bering Sea bottom temperatures between -2.0 and 4.0° C (Otto, 1998). Once they get to 7° C, their metabolism starts kicking in too much and they expend more than they can consume (their energy budget goes in the red; see Foyle et al., 1989).

That being said, a new paper came out showing opies terminally molt at younger instars when in colder temperatures. AnnDorte Burmeister and Bernard Sainte-Marie (a rock star in the snow crab world) recently looked at the geographic variation in size at terminal molt of snow crabs off the west coast of Greenland. They found that in both male and female snow crabs, size at terminal molt is positively correlated with temperature. They explained this by snow crabs possibly terminally molting at lower instar numbers (maturing faster) rather than growing less during each molt. Growing the same amount in different temperatures kinda rocked my socks, but Burmeister and Sainte-Marie did see similar sizes at instars between two sampled bays despite temperature differences!

carapace widths of terminally-molted male and female snow crabs

along western Greenland: larger adults are found at higher temperatures

(this pattern is also seen in eastern Bering Sea females = black diamonds)

While this study did not see a relationship between size and latitude, the idea of decreasing size with decreasing temperature can be related to latitude through the ‘converse Jame’s cline’. The Bergmann/Jame’s clines describe a positive relationship between maximum size of animals and latitude. In sandy beach isopods Excirolana hirsuticauda, larger growth was observed at the southernmost study site:

isopod growth in two Chilean locations

(read more here)

The converse Jame’s cline is therefore seen in the eastern Bering Sea, where mature female snow crabs are smaller the farther north you go (I’ve seen this myself, but you can also check out Zheng et al., 2001).

Burmeister and Sainte-Marie concluded their study with implications for the fishery: seeing the relationship between temperature and terminal molt, increasing ocean temperatures can directly affect snow crab stocks by increasing size at maturity. For females, this increase will mean greater fecundity due to ability to hold more eggs in their clutches. But for males, it will mean more will be vulnerable to fishing pressure because more will molt into the fishery rather than terminally molt at sub-legal sizes.

crabbing in Disko Bay, Greenland

Read it:
Burmeister, A., and B. Sainte-Marie. 2010. Pattern and causes of a temperature-dependent gradient of size at terminal moult in snow crab (Chionoecetes opilio) along West Greenland. Polar Biology 33: 775-788.