Phototaxis & co.


I got home late tonight and fed the critters after the lights were off. Most of the shrimp seem to eat whether it’s light or dark… a few are nocturnal, but none seem to be exclusively diurnal.

As I was feeding the new ‘Borneo Orange’ shrimp, I noticed that the tank was full of hovering larvae. Since my last attempt at raising these guys was a failure, I immediately dropped what I was doing and prepared to hoover up the hoverers. Yamato zoes are strongly attracted to light, and the orange zoes look just the same, so I set up a spotlight to illuminate a corner of the tank in order to corral them.

When I returned 15 minutes later, there were no zoes in the flashlight beam. Furthermore, the cloud of larvae elsewhere in the tank had vanished. I poked around a bit and discovered that a few were clinging to the sides and bottoms of the tank, so I began, tediously, to suck them into a bucket, one by one.

It took 10 minutes or so for my benumbed brain to process this new information, and then I figured out what was happening and switched off all the lights. A few minutes later the cloud of zoes was back, hovering in the water as before. So. Borneo Orange zoes aren’t attracted to light, and, in fact, actively dive for cover in the light and only swim around when it’s dark. Sensible behavior if you’re tiny and delicious. But then, being equally tiny and delicious, why are Yamato larvae attracted to light? I’ve always presumed that, inasmuch as a Yamato zoe’s main job is to wind up in the ocean, they need to swim towards the light in order to maximize their swept-awayness and avoid getting derailed under a rock or in an eddy. Would that mean that Orange zoes aren’t supposed to drift into the ocean?

Of course, it could be that this doesn’t tell me anything about where the zoes want to wind up, and instead reflects something about the predator population or turbidity of their particular patch of salt marsh. This whole project seems increasingly silly in light of the fact that somewhere out there is an Indonesian fisherman who knows where these shrimp are being caught and most likely has a telephone and could tip me off about most of my variables. I should probably stop treating these animals like a black box and try harder to find out about their natural habitat.

In the meantime, I have a new crowd of larvae swimming around in my array of plankton tanks. And, a new variable to try: photoperiod. Yamato zoes die in the dark, so I’ve been leaving the lights on 24/7. But if the orange zoes hide from the light in their freshwater phase, perhaps even after their downstream metamorphosis they can only eat in the dark. If this batch doesn’t make it, I’ll try dropping down to 12 hours of light and see if that makes a difference.

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