Saturday, April 30, 2011

Predators of the Touch Tank

            We had a casualty in the touch tank this morning.   Sometime between around 9pm last night, and 10am this morning, one of the starfish in the touch tank ran afoul of a crimson anemone.   The anemone wasn’t even one of the large ones; in fact, it’s only about half the size of the sea star in question.   To be fair, the sea star wasn’t doing all that well to begin with; one of the aquarists had moved it into the anemone pool from the sea star tank because the larger sea stars seemed to be picking on it.   Possibly if it had been in better shape to begin with, it would have been able to extricate itself.   The first think I knew about it was when I uncovered the tank in the morning – and two starfish arms were sticking out of the anemone’s mouth.   I radioed the aquarist (he was amused that I used the euphemistic phrase ‘health check’, for what was pretty obviously a half-eaten animal); we were both of the opinion that at this point, it was really too late to do anything but let the anemone finish his meal.   The sea star was, unfortunately, still alive; for the next three hours I got to watch its tube feet flailing around as it was sucked all the way into the anemone.   

            Welcome to the touch tank, red in tooth and claw.   Although it isn’t always obvious to our guests, many of the touch tank animals are actually predators – for all that they are only a few inches long.  Also, most of them were collected from the wild.   Before coming to the aquarium, presumably these animals were surviving as predators just fine.  In the touch tank, our anemones no longer have to kill and eat other animals to survive, because we feed them what they need.   But that doesn’t mean they stop being predators.   That’s what they are.   

            With some animals, we even showcase their predatory talents for the edification of our guests.    Go ahead, touch the crimson anemone.    

              Do you feel the tentacle sticking to you?   It’s a little like touching a piece of scotch tape.   Believe it or not, that stickiness is actually the anemone’s stinging cells at work.   That little guy is trying to kill you and eat you.   Because humans are comparatively gigantic, and have very thick, multi-layered skin, the anemone’s chemical armament is woefully inadequate.   All they can do is stick.

            Regardless of their failure rate, the crimson anemones keep trying.  They’ll still be sticky for each one of the few hundred aquarium visitors that are going to touch them tomorrow.   They’re predators, and that’s what they do.   

The crimson anemone digesting his meal...

            Currently, the carnivorous anemone is swelled up to about three times its normal size.   I am guessing he has eaten enough food to last him for the next six months or so.   It’s interesting, but if the anemone had gotten ahold of the sea star a few hours earlier, or pulled him in a little quicker, we might never have known what had happened.   Someone would probably have noticed that one of the anemones had tripled in size overnight – but it might have taken a day or two for anyone to notice that we hadn’t seen the green starfish in a couple of days…

            Speaking of which, we are also missing two hermit crabs.  One was confirmed eaten by an anemone; all we found was part of a claw.   (The anemones are pretty serious predators.)   No idea what happened to the other one.   It might have been picked off by an anemone, or possibly it was done in by one of the larger crabs.   Of course, it could also be that the missing crabs expired of natural causes, and their tank mates simply took advantage of the free meal.  The touch tank hermit crabs, while uncannily passive towards people poking their shell, can be very aggressive towards other animals.   They steal food from the anemones, they snip the tube feet off of the sea cucumbers, and pull the opalescent bits off of the opalescent nudibranch.   One crab who used to live in the tank would occasionally snip arms off of the sunflower sea stars when he was feeling hungry.   

            But the hermit crabs seem to reserve their crabbiest behaviors especially for other hermit crabs.  When crabs molt their exoskeleton, it takes a day or so for the crab’s new exoskeleton to harden up (ever heard of soft shell crab?).   Some crabs take advantage of this temporary disability to rend the newly-molted crabs limb from limb.     It’s a recurrent problem with raising crab.   When crab first hatch, they are very small (about the size of the period at the end of this sentence), and very numerous – so to feed the crabs, you have to find food that is even smaller and even more numerous to add to their water.   When they’re still in their larvae stage, it can be difficult to get the crabs to eat anything at all.   Then they get larger, and they start eating each other.   The attrition rate can be high.   

            Given how crabbily our hermit crabs behave, I am continually surprised by the number of kids that visit the touch tank who used to have one as a pet.  I dimly recall my parents buying me a hermit crab as a child, being by turns interested and terrified of it, and that it died a few months after we bought it.   This also seems to be the experience of most of the small children who visit the touch tank.   I have yet to meet any kid who’s managed to keep a pet hermit crab alive for any length of time.   I sometimes wonder if this isn’t some sort of deliberate parental plan – that the first death of a pet that their child experiences will be a pet that isn’t all that loveable in the first place.          

            Since it can be almost impossible to raise some of these animals to adulthood in the safety of an aquarium tank, I have a lot of respect for all those crab living out in the wild.   For most crabs, just growing to the size of a penny is a major accomplishment considering that thousands of their siblings did not make it that far.   Have you ever seen the Discovery Channel show Deadliest Catch?   Some of those crab fishermen will make their fortune by the number of king crabs they catch each winter.   They’re the lottery winners in an industry that is statistically more deadly than almost any other occupation there is.   Now, think about all of those big king crab getting pulled out of crab pots and stuffed into the hold.   They were lottery winners, too.   Right up to the moment they wandered into the crab pot.

King Crab

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