Wednesday, May 15, 2013

Cold Crabs Conquer Puberty!

Cold crabs terminally molt at smaller sizes than warm crabs!


OK, let's start over. You know how snow crabs have a final, or terminal molt? Instead of molting every year and growing, snow crabs will molt up to a certain (as yet undetermined) point, then stop molting. The terminal molt is marked by their claws growing larger than the rest of their body's growth, meaning that, if they were humans, all of a sudden you'd see this guy:

He's never going to get any bigger. He's terminally molted.

The tricky thing with snow crabs is that they'll terminally molt at all sorts of different sizes, so while some may max out at 2 inches carapace width, others can grow to over 4 inches (and beyond). Because of their terminal molt, some snow crabs will never grow large enough to be legally harvested for the commercial fishery (the size limit is 78 mm or ~3 inches). For example, below is a plot of snow crabs surveyed in the Bering Sea last summer (2012), with a bright orange line marking the legal size limit.

 you can tell the large clawed, terminally molted males
from the small clawed (still have at least one more in 'em) males
based on the proportion of their claw ("chela") height to their carapace width

For the males that are designated "small clawed", they're not worried being to the left of that orange line: they will molt at least once more and potentially grow larger than 78 mm, thus grow into the fishery! But the "large clawed" males aren't getting any bigger than what you see, so all those dark circles to the left of the orange line are snow crabs that can mate with the ladies but won't be on your dinner plate any time soon ever. So what's the worry? We've discussed this idea before that the smaller terminally molted males may end up reproducing more than the larger guys simply because they won't be picked up by the fishery and will have more time to sow their crabby oats on the Bering Sea floor. If size at terminal molt is a heritable trait, this could lead to more snow crabs at small sizes, which would obviously be bad for the fishery.

But where does temperature come in?!?!? Keep your pants on, I'm getting there! The colder the temperature, the smaller the males seem to be when they have their final molt (see Dawe et al., 2012). In higher latitudes, like the North Atlantic and North Pacific, this means a greater proportion of males that won't enter the fishery. That's definitely bad for fishermen.

HOWEVER, (picture Marisa Tomei saying that)

... if ocean temperatures continue to warm as they are already, more males will terminally molt at larger sizes! We just don't know how the increased temperatures will affect other aspects of their metabolism, so it's up in the air about whether or not global climate change is a bonus for male snow crab size. I guess we'll find out!

Read more from this post and from this paper:
Dawe, E. G., D. Mullowney, M. Moriyasu, and E. Wade. 2012. Effect of temperature on size-at-terminal molt and molting frequency in snow crab Chionoecetes opilio from two Canadian Atlantic ecosystems. Marine Ecology Progress Series 469: 279–296.

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