Monday, May 27, 2013

How low can they go?

You didn't think I forgot about answering the questions from our crab chat did you? Nope! The next question to answer is:

Just how deep in the ocean do king crabs and snow crabs live?

To answer this I looked at the crab survey data from 2012. This is only looking at Bering Sea crabs in summer, so it's a little limited, but I figured it was a great place to start. Here's what I learned:

King crabs really stay out of each others' way! Every sampling location that found red king crabs was absent of blue king crabs. Also, on average, blue kings were found deeper in the Bering Sea than red kings; the average blue king crab depth was 87.6 meters (287.5 feet) and the average red king crab depth was 51.7 meters (169.7 feet). This was interesting to me because the bycatch of blue king crabs in red king crab fisheries has closed down red king fisheries in the past (due to low blue king crab abundance), but it would seem that for the summer of 2012 that might not have been a concern.

Heading deeper into the Bering Sea, I found the Chionoecetes crabs! Both Chionoecetes bairdi and Chionoecetes opilio liked an average depth around 91 meters (91.5 m and 91.4 m, respectively), which is close to 300 feet. Unlike the king crabs, these Chionoecetes cousins hung out together quite a bit, which isn't too shocking since they're known to hybridize.


The thing that caught my eye with hybrid Tanner crabs was that their average depth was slightly shallower than the two "pure blooded" crabs (88.1 meters). If I didn't have a real job, I would look into the sex distribution of male and female bairdi and opilio crabs and compare that to the distribution of hybrids to finally answer who's mating with whom. Anyone else want to check it out!?!? Let me know. (Seriously. I'm totally interested.)

average depths (in meters) for the locations where red king crab,
blue king crab, Chionoecetes bairdi, C. opilio, or their hybrids were present

The other thing that caught my eye is that hybrid crab presence was NOT dependent on both "pure" crabs being there (during the survey, that is). Of the 157 sites that had hybrids, 25 sites were missing on of the original species. Plus, there were 70 sites that had both bairdi and opilio crabs but no hybrids. (Fun fact: those sites had a deeper average at 107.4 meters.)

So, that's how low king crabs and snow crabs can go! For perspective, if the Bering Sea's depth was in line with a football field, red kings would make it just past the 50 yard line while blue kings and hybrid Tanners would be in the red zone. But bairdis and opilios would go ALL. THE. WAY!!!

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.

Wednesday, May 8, 2013

Sea Week is Crabtastic!

We've been busy busy busy over here at Snow Crab Love! An HPLC adventure, a quick trip to Illinois, and Sea Week at NOAA have dominated our schedule! Sea Week is actually 3 weeks of marine animal fun hosted by NOAA. Elementary school children come out to the Ted Stevens Marine Research Institute and learn about fish, whales, marine mammals, birds, and invertebrates. They have a scavenger hunt for animals in our 4 aquaria, walk through a whale's mouth and feed it a "herring" (stick a paper herring onto the stomach of the whale display), learn about global currents on a giant map that they can walk on, and learn how to safely handle some small friends that they can find in the rocky intertidal. If you aren't a Juneau elementary student and missed Sea Week, here's your chance to see some invertebrate friends:

this orange little guy caught my eye and stole my heart
(same as last year!)

a hermit crab perched on some very old barnacle shells

we have a lot of Dungeness crabs, but this fellow was particularly friendly

a decorator crab showing off its beautiful sponge accessories

be still my heart:
we have a real live heart crab in an aquarium! I've finally seen one!!

the educators were able to show kids how sea stars use their tube feet
to manipulate their prey as this guy opened up a blue mussel

here's an urchin chowing down on a little sea star!

The students really seem to enjoy seeing all the fish, playing on the map, and touching the sea cucumbers, scallops, and crabs,and they get to learn about why it's important to keep our oceans clean! Does your community have any fun ocean-themed outreach programs for the kiddos?