Friday, March 29, 2013

hermit crabs do the twist

After this post about when hermit crabs become, well, hermits and hide their soft abdomens in shells, Sherry asked:

Do hermit crab abdomens always spiral in the same direction?

I'm not gonna lie: I was stumped. The logical person in me thought: not all snail shells spiral in the same direction, so not all hermit crab abdomens spiral in the same direction, right? Shells can spiral to the right (dextral) or to the left (sinistral), and, just to really ruin your day blow your mind, some gastropods will display BOTH spiral directions within the same species!

Sinistral (on the left) and dextral (on the right) shells from
Amphidromus perversus snails (aka super cute land snails)

Then I thought, that may be so, but that doesn't mean that hermit crabs will necessarily inhabit every type of shell ever produced by nature, RIGHT? What I found is that, while hermit crabs prefer dextral gastropod shells, they can and will inhabit sinistral shells. But this seems to be a last-option-for-housing kind of situation, as the occurrence of crabs in the left-spiraling shells is so rare.

to the left, to the left!
everything about me in a shell to the left!

So are the hermit crabs that dwell in sinistral shells curling to the left? NO! Larval hermit crabs have symmetrical abdomens, but as they grow and develop, the abdomen begins to curl. It TURNS out (ha!) that the abdomen curls in a spiral towards the right for hermit crabs! (I'm afraid to say that this is true for all hermit crabs, but so far my quick literature review hasn't told me otherwise.) 

this hermit crab is doing its Tilda Swinton impersonation

I hope that cleared it up for you, Sherry! (Because the shell is clear...) Hey-oh!

Want some light reading?
Chapple, W. D. 1977. Central and peripheral origins of asymmetry in the abdominal motor system of the hermit crab. Annals of the New York Academy of Sciences 299: 43 - 58.

Kosuge, T., and M. Imafuku. 1997. Records of hermit crabs that live in sinistral shells. Crustaceana 70: 380 - 384.

Tuesday, March 26, 2013

Someone get the cocktail sauce!

We've got A LOT of prawns to eat!

prawn-filled beach in Coronel, Chile

Just kidding. A tide of death has swept Chilean beaches, covering them with thousands of dead prawns and hundreds of dead crabs. Local fishermen are quick to point their fingers at the power plants nearby, which use seawater as a coolant, but an actual cause is not yet known. Researchers are looking into water temperatures and oxygen levels: there may have been an area of hypoxia that was too large for the critters to escape (hypoxia = low oxygen = hard to breath!).

poor little crustaceans

This isn't the first time we've seen a wave of dead crabs wash ashore. Remember the megalopae in Phuket? They may have washed ashore due to hypoxic conditions. And the devil crabs on the Thanet Coast, UK? They may have suffered from a sudden drop in water temperature (although scientists also initially considered bitter crab disease as the culprit). Hopefully Chilean researchers will get to the bottom of their prawn/crab die-off for, if nothing else, the peace of mind of the fishermen. Do you have any theories?

Friday, March 22, 2013

Out of Town!

Hey there! I've been a bit busy with work, but don't worry, I didn't forget about Snow Crab Love! I was out of town for a bit to attend a meeting in Seattle (for a Gulf of Alaska project which is also funded by North Pacific Research Board! Hollah!), so while I was there I made sure to eat lots of crustaceans!

you know you want to throw a fish around!

What was my most invertebrate-filled meal? That would have to be the bowl of Dungeness crab (Metacarcinus magister) and oyster (I don't know which species) chowder with half of an Oregon bay shrimp (Pandalus jordani) sandwich! I went with some friends to Pike Place Chowder in Post Alley (no, we did not put gum on the Gum Wall. I can't believe I missed it!) and was not disappointed. The chowder was warm and delicious - perfect for a sunny spring day in Seattle - and the sandwich was loaded with little pink shrimp!

soup and a shrimpy sammie!

I've shared before that I don't normally eat shrimp because of the overwhelming bycatch that is often caught in trawl fisheries, but the Oregon bay shrimp fishery has its act together. The fishery runs from April 1 - October 31, which avoids the reproductive season of December through March (yay for making shrimp babies!). Best of all is that they use bycatch reduction devices (BRD) which allow fish to escape the net while shrimp are caught!

video

See how that lingcod and canary rockfish were able to just swim out and on their way instead of being smooshed into the codend with all the shrimp? Totally awesome! The Oregon Department of Fish and Wildlife have a couple other videos of how BRDs work which are worth checking out here. What fishermen end up with is lots and lots of shrimp with few fish to sort out rather than a deckload of fish with a few shrimp scattered around!

catch vs bycatch:
just two small bins of non-targeted fish compared to all those baskets of shrimp!
(this is probably the happiest picture I've seen in terms of shrimp fishery bycatch -
the rest can be just heartbreaking)

This fishery is so good that the Monterey Bay Aquarium's Seafood Watch program gave it the "Best Choice" label and the Blue Ocean Institute gave its bycatch a score of 3.75 out of 4! That, my friends, means happy eating for this gal!

Friday, March 8, 2013

teeny tiny shells

This weekend is the Southeast Alaska Regional Science Fair! The student I mentored did a project looking at shell selection behavior of hermit crabs under different chemical cues (more on that later). This project got me thinking: when exactly do hermit crabs begin using gastropod shells? Is is right when they settle out of the water column onto the benthos or do they have to reach a certain size before they can hoist a shell around?

First let's learn a little about hermit crab development. When they hatch, they are free swimming zoea. They go through a number of zoeal instars, meaning they'll molt and grow, but will still have basically the same form and function.

little larval Anapagurus laevis

After a certain number of instars, the zoea will molt into a megalopae stage. Megalopae will swim around until they find a suitable substrate to settle. Once they settle, they can then molt and grow into a juvenile hermit crab, when they'll finally look like mini-versions of the hermit crabs we know and love!

almost there, little buddy!

So when does the protective gastropod shell come into play? You may have guessed it already: during the megalopae stage! As they settle, the little guys are looking not only for good habitat structure and food availability, but also for shells or shell fragments. If no shells are available to protect their minuscule bodies, hermit crab megalopae can delay morphing into the juvenile stage. But they don't want to dilly-dally, since shell-less hermit crabs at either stage suffer higher mortality rates than those with shells.

Now, what do I mean by "little"? Check out this guy:


the large claw on this Pagurus bernhardus is about 1 mm long

For comparison, pick up a nickel. A nickel is almost 2 mm thick, so this hermit crab megalopa is probably around the same size as your nickel is thick! That's how tiny hermit crabs are when they start looking for shells, or more likely at these sizes, shell fragments to protect their soft abdomens!

Where's the science? 
Roberts, M. H. Jr. 1971. Larval development of Pagurus longicarpus Say reared in the laboratory. IV. Aspects of the Ecology of the megalopa. Biological Bulletin 141: 162 - 166.

Oba, T., and S. Goshima. 2004. Temporal and spatial settlement patterns of sympatric hermit crabs and the influence of shell resource availability. Marine Biology 144: 871 - 879.

Harms, J. 1992. Larval development and delayed metamorphosis in the hermit crab Clibanarius erythropus (Latreille) (Crustacea, Diogenidae). Journal of Experimental Marine Biology and Ecology 156: 151 - 160.