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As we saw last week in the diary Hermit Crab Basics, hermit crabs have abandoned a crustacean's typical total exoskeleton body coverage in exchange for the security of living inside an old univalve shell. The advantage of this strategy is that it can shed the portion of the exoskeleton covering the front part of its body without having to endure that dangerous "soft-shell" stage other crabs have to deal with. As I mentioned, one disadvantage is the crab must periodically find new, larger shells as it grows. As we shall see here, there are other obstacles this animals must contend with as well.

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As a group, crabs tend to produce vast numbers of young in order to maintain their populations. Although the young are not cared for by the parent, being free floating planktonic creatures for several days or weeks before settling to the bottom, the eggs typically are. The female does this by holding and protecting the fertilized eggs beneath a structure known as an abdominal flap, or "apron". This is the V-shaped "hatch" found on the underside of the crab, as pictured above. (Males also have an abdominal flap, but theirs is much narrower since it does not need to hold thousands of eggs.) Here is a photo of the eggs held beneath this flap.

Not having this posterior portion of the exoskeleton, hermit crabs do not have abdominal flaps. Still, the eggs are well protected in spite of the adults lacking this appendage. But first let’s look at the mating process.

Crabs, in general, tend to exhibit sexual dimorphism, with the females growing to a larger size than the males. The reason for this is simple, the larger the female is the more eggs she can hold beneath her apron. Hermit crabs are an exception, with males tending to grow larger than females. Also, one of the male’s claws is unusually large. This claw is used in territorial displays to drive competing males from its habitat. It is also used to claim a female.

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Male hermit crab, with the
smaller female in tow.

When a male chooses his mate, he grabs the female’s shell (that is, the borrowed snail shell the female is inhabiting), with this large claw and may spend days hauling the poor thing around with him. The terrified female spends most of this time hiding as deep inside the shell as she can go. Males may try to steal each others mates by grabbing the opposite side of the shell opening and pulling her away.

During this charming mating ritual the eggs of the female are starting to mature. These eggs are laid by the female through a duct deep inside the shell called a gonopore, and as they are laid they are glued to the soft abdomen of the mother. Pheromones released by egg production intensify the territorial behavior of the male, making it more and more difficult for competitors to snatch her away. The male fertilizes the eggs by handing over sperm packets to the female, who takes them and tucks them inside the shell with her claws. Once the eggs are fertilized the male releases the female and disappears to find another mate.

Click here for a link that shows a female hermit crab brooding her eggs.

Once the eggs begin to hatch, the female extends her body out of the shell as far as possible and, using her claws, fans the hatching larvae into the surrounding water. The larvae are planktonic and spend up to two weeks metamorphosing into the adult form. Once they settle to the bottom they must quickly find a tiny, empty snail shell to live in or they will perish.

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Planktonic larval stage.

Most hermit crabs live in shallow water, sometimes so close to shore that they are in literally inches of water. Temperate species are very tolerant of temperature variations, but even a hardy animal like the hermit crab cannot survive shallow water conditions during New England winters.

As soon as the water gets cold in late fall, local hermit crabs will move into slightly deeper water and burrow down under the sand, entering a dormant state known as crab torpor. Colonies containing thousands of crabs will disappear overnight, hibernating beneath the substrate until spring arrives.

Crabs in torpor do not grow, so the winter dormancy does not need to be interrupted by shell searching. However, once they emerge from their dormancy one of the first acts is a calorie binge followed by a spring shedding. Their main foods, such as algae and carrion, won’t be very plentiful this early in the year, so hermits will spend most of the day sifting the sand with their mouthparts, searching for any organic materials that have settled there over the winter.

Other diaries in this series can be found here.

Originally posted to Mark H on Thu Mar 06, 2008 at 05:05 PM PST.

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