This is part III on hermit crabs. Hermit Crab Basics is here, and hermit crab reproduction is here.
Tonight I’d like to focus on hermit crab symbionts. Symbiosis is a relationship between unrelated species of animals. There are several different types, ranging from parasitism, where one species benefits and the other is harmed, to mutualism, where both species benefit from the relationship. With hermit crabs, symbiotic relationships tend to be mutualistic, meaning that although the relationship isn’t neccessary for the the survival of either species, it is nonetheless beneficial to both parties. I’d like to take a look at three symbiotic examples here.
Snail Fur
As we saw in past diaries, hermit crabs use the abandoned shells of dead snails as their primary form of protection. The body of the crab is highly specialized to adapt to this univalve shell, and as the crab grows it must find larger and larger shells to move in to. What’s amazing to me is that in all three symbiotic examples I’ll be giving, the symbionts exist on the snail shell while it is inhabited by the hermit crab, but never when the snail itself is alive.
The first example is a hydrozoan called snail fur. Hydrozoans are cnidarians, related to jellyfish and sea anemones. Snail fur grows as a colony of animals armed with stinging cells on their tentacles. They are predatory, stunning and feeding on zooplankton that drifts by the hermit crab’s shell. The polyps themselves are tiny, and the colony appears as a soft fur-like covering over the host’s shell.
The snail fur colony not only gains from a convenient substrate to attach to, but the movement of the crab provides ample food by both stirring up sediment as the crab moves, as well as transportation for the colony, moving it to different feeding zones where zooplankton is more plentiful.
Flat Slipper Limpet
Slipper shells are snails. As I described in this diary they normally grow in stacks, with an equal number of males and females in each colony. The flat slipper limpet, as the name suggests, has a shell that is extremely thin, sometimes just a fraction of an inch in thickness. Although they may grow on rocks and other hard substrates, such as on the undersides of horseshoe crab exoskeletons, the preferred habitat is on the inside opening of snail shells inhabited by hermit crabs.
Like other slipper limpets, the flat slipper shell is a protandric hermaphrodite, meaning all individuals are born male and turn into females as they age. Because they are not motile, remaining in place once they’ve settled on a home base, this behavior maintains an even number of males and females in one location. This ensures that the animals are able to reproduce without having to venture out and search for a mate during the reproductive season.
Sea Anemone
Like their relatives the jellyfish, sea anemone have feeding tentacles covered by stinging cells called nematocysts. Anemones will grow on nearly any hard substrate, sticking to the surface with the basal disk and growing much like a plant. Although they are capable of movement, most anemones will remain in one place throughout their lives.
Some species of hermit crabs encourage anemones to move onto their shells. This will benefit the crab by providing a poisonous covering over the body. Like the snail fur colony, the anemone gets a free ride from a mobile creature which carries them to new feeding grounds. Also, as the hermit crab feeds, bits of food and sediment will be kicked up into the water column and be available for the anemone to feed on.
As mentioned in earlier diaries, hermits need to move into larger snail shells as they grow. What’s amazing about the anemone-hosting species is they will actually encourage their anemones to change shells with them. After the hermit switches shells it will approach the old shell and coax the anemone into releasing its grip on the shell. It will then gently grab the base of the anemone and transfer the animal to its new shell.
Describing the three interspecies relationships above, two of these can be categorized as mutualistic and one of these is commensalistic. In mutualism both of the animals benefit from the relationship. In the hermit crab/snail fur and hermit crab/anemone relationships both benefit. The crab gains added protection from the stinging cells and the cnidarians gain greater access to food sources.
The hermit crab/slipper limpet relationship can be better described as commensalistic. In commensalism one member benefits while the other is unaffected (neither harmed nor helped). While the filter feeding slipper limpet gains access to greater food resources, the hermit crab does not appear to be helped by the association. Although it is not harmed by the relationship either.
Other diaries in this series can be found here.
