Tonight I watched one of the large hermit crabs steal a partially-digested snail from the anemone's mouth, then dash away before his larger companions could snatch it from him. He ran, at top speed, the full length of the aquarium. Backwards.
I couldn't do that. Not without stumbling, not unless I had eyes in the back of my head.
In the morning, the hermits are quiescent, some apparently sleeping (with their eyes open) in the corners, some stacked on top of each other like sleepy kittens, some meditatively grooming their antennae, some picking at a morsel of seaweed or a barnacle. I drop the day's ration of shrimp pellets into the other end of the tank. The scene changes, even before the pellets have sunk to the tank floor; all the hermits are running, scrambling out from the bottom of a hermit pile, dropping from the seaweed. The crab appears from his hidey-hole behind the abalone shell, but he's already out-done by the hermits; they have collected their breakfast, and are hauling it out of reach. He has to mug a slower one for his meal.
How do they do that? They couldn't see the food; they barely had time to smell it, let alone identify and locate it.
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| Grainy hand hermit, waving assorted antennae and mouthparts. |
Their senses are different than ours. Not better, not worse; just different. Their eyes are compound, on stalks, and arranged so that they look both forwards and backwards. One may be looking at me through the glass wall, but at the same time, he's watching his back, in case of a visit by the crab. Handy trait!
As for smell and taste, the hermit has all the bases covered. He has four antennae; the smaller pair, the antennules, is a sensitive "smeller". Besides that, his legs are covered with sensory hairs; he can smell things just by stepping on them. And this covers a deficiency in his sight. Although he can see backwards, he can't bend those stalks down, so he can't see under his own feet. Not a problem when you can smell the ground under you!
A hermit can identify, by smell, a
useful snail shell buried in the sand; he can pick out the species that he likes best, also by smell or taste. The hairs on his legs and pincers are multipurpose sensors that also distinguish temperature, touch, and water current.
Do they hear? Yes; those four antennae do that. How well, I couldn't find out. (Should I play some music to mine, and see if they respond?)
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| Zooming in, to show facial detail. |
And then there are those long, long antennae. In this hermit, they're orangey-red. They are feelers, the equivalent to our fingers. The shorter pair in between the eyes are called antennules. The hermit
flicks them continuously, waving and spreading the hairs on the flagella (that blue "flag"), capturing scent and taste from the surrounding water.
In the photo above, two of the three pairs of mouthparts are visible. These are always difficult to see, since they're constantly moving. There is a lower pair, green with orange, hairy tips; these manipulate the food and move it into the mouth. Just above these are a set of long spines, also hairy. I can't see what's behind them.
I don't know if all these mouthparts are sensory organs. I would think so, but haven't found any real info.
The idea of hair in the mouth makes me shudder, but then, I'm not a hermit crab.
