Name that Species: Hemigrapsus crenulatus

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I came across these beauties while on a routine sampling foray in the Avon-Heathcote estuary today. This morning, the estuary was bright and warm and bustling with abundant crab activity, but these two crabs stood out in particular for their behavior. They were stacked together, right on the surface of the mud, and didn’t move away as I invaded their space to take some (blurry) photos.

Okay, okay– I know what this looks like. But in fact, these crabs aren’t engaged in copulation. Instead, they’re displaying a behavior called mate guarding. What we’ve stumbled across here is sexual competition in action. See, breeding female crabs are at a premium in the estuary. When a male comes across one, he wants to make sure it’s his sperm that fertilizes her eggs. But this can be tricky with female crabs, because copulation and egg-fertilization happen at different times. After mating, female crab stores sperm in an organ called a spermatheca until she actually deposits the eggs. The period between mating and egg-deposition can last a few days. During this time, the female can mate with several males— and that decreases the chance that the original mate’s sperm will fertilize her eggs.

So to successfully sire a clutch of crabs, it’s not enough for a male to mate with a female– he also has to make sure that other males don’t get that chance. The solution? Mate guarding. When a male finds a breeding female, he physically shields her from the intruding presence of other males. In this crab species, the male guards the female from before she’s receptive all the way through to egg-deposition.

Or rather, he tries to. Incoming males try to interfere with this mate monopolization, attacking the pair in the hopes of scaring off the guard and getting a chance to mate with the female himself. If the new male is bigger than the incumbent, he has a pretty good chance of success. The fights can get quite rowdy, often  resulting in scarring and lost legs for all parties involved. It’s a rough world out there in the estuary.
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Onto the ID: These two fine crabs are of the species Hemigrapsus crenulatus, or papaka huruhuru, the hairy-handed mud crab. The name comes from the hairs that line their back legs…

…and, one can’t help but realize, makes an excellent epithet to hurl at jealous lovers. (You’re nothing but a hairy-handed mud crab! and so on.)

Until next time,

E

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