The trouble with Googling hermit crabs is that they are popular pets. When I Googled "hermit crab" suppliers, the list included 110,000 sites. And most pet hermits are land species. They don't breed well in captivity, because the young have to hatch and grow in water; they move to the land as they mature. Few pet owners can manage the complicated schedule and arrangements necessary. When I finally realized this, and specified "marine hermit crab mating", I found what I was looking for.
The questions I was asking were: How do I know the sex of my hermits? Do they need to molt before they mate? Are my hermits displaying normal mating behaviour? And, suggested by Laurie; Will they be successful in spite of being in a tank? (Which I am careful to keep with a species, water, and temperature mix that matches their home environment, but still ... a big human peering through the wall all the time? Electric lights? Is that too stressful?)
Grainy hand hermit, Centennial Beach. Already worried.
Sexing a hermit crab is easy. Sort of. On the last set of legs, the female has two small holes in the joints; the gonophores, where the male will insert his sperm. The male has none. The only problem is to catch the female out of the shell far enough and long enough and in the right position to be able to see this. A rare event.
Never mind; I know an easier way. I'll get to that.
I found an article in the Revista Brasileira de Zoologia, by Alexander Turra*. (2005) He studied the mating strategies of 4 species of intertidal hermit crabs on the Atlantic coast of Brazil. One of those species was of the same family as my two, Paguridae, and the same genus, Pagurus. All four of them acted in ways similar to mine, although he noted that the displays were family-related characteristics.
He started out with a collection of around 800 hermits from an intertidal beach and mud flats, and housed them in plastic pools with sand and mud from their home base, in running seawater. More or less the same procedure as I'm using, although I sure don't want 800! He fed them fresh fish and mussel meat; mine get dried fish, seaweeds, and occasionally a bit of mussel or nudibranch.
Then he watched them around the clock. When a couple formed, everything they did was recorded. Let's compare his Pagurids to mine:
1. - Pagurus criniticornis (the Brazilian hermit, from now on called "Crini"): the male grabbed a smaller female and turned her to face him.
- Rex (Pagurus hirsutiusculus) grabbed his little gf and held her facing outward:
- Boy Blue (P. granosimanus) held his smaller girl like Crini did, face to face.
2. - Crini picked at the female's pincers and legs with his pincers.
- I didn't see Rex or Boy Blue do this.
3. - Crini held his female's shell with his pincers or legs.
- Rex used only his smaller pincer for holding her, and the legs for adjusting her position.
- Boy Blue used both pincers or a pair of legs; sometimes, because he was busy eating, he just kept one leg deep inside her shell.
4. - Crini sometimes dragged his girl over the bottom.
So did Rex, but not Boy Blue.
This I thought was very interesting: in some invertebrates, the female resists mating, and with reason; it puts her in a precarious position, and uses much of her strength. The female hermit seems not to mind; watch:
5. - When Crini's mate was ready, she
"... signaled to males they were ready for copulation by touching male eyestalks with their antennules."- I didn't see this in my hermits; I wasn't watching closely enough.
The pair eased out of their shells and put their gonophores in contact. Copulation lasted from 10 to 36 seconds, and was repeated up to 4 times. No wonder I missed this; that was quick!
Then the female slid back into her shell. Crini continued guarding her, holding her shell or her pincers.
6. - Afterwards, Crini's gf stayed quiet, sometimes buried in the sand, for several hours.
- Rex's mates wandered away. Or maybe he abandoned them; hard to tell, because he dragged them around a lot.
- Boy Blue's girl (she needs a name, since I can recognize her; how about Bo Peep**? "Bo", for short.) sleeps. At least, she stays in the shell and doesn't move for hours. I caught her and Boy together this morning; now she's sleeping again.)
7. - Some of Crini's mates molted; some didn't.
So things line up; I think I can be confident that this is mating behaviour, that the smaller hermits are females, and that, no, she didn't need to molt first.
If everything worked out, the females will be holding eggs alongside their abdomen in the shell. Later, she will release them, they will hatch, and the tank will suddenly have almost microscopic babies swimming about. I may miss these, and if there's an increase in the number of Hairy hermits (Rex's babies, that would be), I wouldn't be sure, since there are quite a few little ones already. I will notice any replicas of Boy and Bo.
If. Laurie's question still needs to be answered.***
I'll let you know.
Young Grainy hand. Not one of mine; she's still on Centennial Beach.
*Turra Alexander. Reproductive behavior of intertidal hermit crabs (Decapoda, Anomura) in southeastern Brazil. Rev. Bras. Zool. [serial on the Internet]. 2005 June [cited 2010 Mar 09] ; 22(2): 313-319. Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0101-81752005000200003&lng=en. doi: 10.1590/S0101-81752005000200003.
** To go with Boy Blue.
*** Alexander Turra's hermits were successful parents: two Pagurus species' eggs hatched at 19 days. (Turra, 2007) I'll be on the lookout in a couple of weeks, then.