Why did this shrimp kill its tank mate?

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Dave Wolfenden advises a reader on why a Blood shrimp may have made a meal of a smaller tank mate.
 

Q. My mother has a 90 l/20 gal reef tank that was set up a couple of weeks before Christmas. It contains live rock, a couple of snails, a few hardy coral frags, a Blood shrimp, Cleaner shrimp and an Emerald crab. There are no fish.

Yesterday she saw the Blood shrimp — the bigger of the two shrimp — chase and kill the Cleaner, which it then started to eat. The Cleaner didn’t appear to be ill at all. Both shrimp were added together about three weeks ago.

Is this a regular occurrence — or is it possible they’re not getting enough to eat? The Blood shrimp is out a lot, considering they’re meant to be quite shy — is this because it’s hungry? There are no fish food leftovers for the shrimp because the tank doesn’t have any fish. Instead, they’re fed a few drops of a liquid food called Gamma Nutraplus every other day.

There were remnants of the usual brown algae bloom when the shrimp were first introduced, but that’s gone now. Please could you advise — and will the crab be safe?

N. Young, email

A. Blood shrimp, Lysmata debelius, are generally peaceful and easy-going inverts. They tend more towards scavenging than the cleaning-obsessed Ambon shrimp, L. amboinensis, and generally get on fine with other ornamental shrimp with the exception of, for example, Banded shrimp, Stenopus hispidus, and certain pistol shrimp.     

Lysmata have very small claws that make them less equipped than these other big-clawed species to dismember their tank mates. There are occasional reports of Blood shrimp attacking other crustaceans, but it’s not clear why this happens. It may be a territorial dispute, but as I say, most of the time these little shrimp will get on just fine.

It could be an issue with feeding (perhaps the Blood shrimp was simply ravenous), and I suspect that the feed that was being supplied wasn’t sufficient — these shrimp have a surprisingly large appetite. Many aquarists report that Blood shrimp are shy, but some individuals are very outgoing, so it’s difficult to say whether this one was particularly visible due to it being hungry. In any case, I would definitely recommend feeding the shrimp on some small pieces of Mysis or other frozen foods, or even pellets are often eagerly accepted (and this applies to the Emerald crab as well, incidentally).

The Emerald crab shouldn’t be at any risk from the shrimp — they’re much tougher than the spindly, delicate cleaners. However, all crustaceans are vulnerable at moulting. It’s for this reason they assume cryptic behaviour around this time, hiding out until the shell hardens up. The crab should look after itself, but ensure there are plenty of rocky nooks and crannies into which it can hunker down when it’s due to moult. I do, in fact, wonder if the Cleaner was entering a moulting phase, and the Blood shrimp seized the opportunity to have a feast (moulting crustaceans give off pheromones that can be detected by others); it’s a possibility, but hard to say.

On balance, I’d up the feed and avoid adding another Cleaner in view of the Blood shrimp’s track record. Adding another Blood shrimp (with caution) shouldn’t pose any problems, providing they’re well fed.

Dave Wolfenden