Today’s animal is another funnily named fish. I don’t really understand a lot of the common names for animals, but I sure can appreciate them. So although I don’t think this one’s name is as fantastic as the plainfin midshipman (I mean, what name could be?), it’s still a pretty silly name for an animal.

The mangrove killifish is a small fish found in and around the Caribbean. The mangrove part of the fish’s name is pretty easy; it lives in ponds by mangroves, and parallels the range of these trees very closely. The killifish part is a little harder. Apparently killifish is a name for a a whole group of fish with similar characteristics. After a bit of digging it turns out killifish is derived from the Dutch word ‘kilde’, which means a small creek or puddle. This makes sense, because that’s where most killifish are found. So once again, when the names are explained, they become much less exciting.

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A mangrove killifish. They reach up to 2 inches in length. Image by Jean-Paul Cicéron (KCF80036), Public domain, via Wikimedia Commons

Now on to the important part, the cool facts about the mangrove killifish. Ever heard the expression like a fish out of water? Well the killifish doesn’t think being out of water is that big a deal. The environment in which this fish lives is harsh and unstable, so the fish have a backup plan. If a pool or pond that the fish lives in becomes too extreme, or other killifish are too aggressive, this little guy flips and slithers across the ground until it finds a new home. During the dry season, when the pools it calls home dry out, the fish moves into moist crab burrows, or into hollow logs. If the area is moist, the fish can spend over sixty days out of water. Researchers discovered this trait while trying to figure out how the fish survives the frequent dry spells that characterize its habitat. Apparently one researcher kicked a log, and out came a slew of killifish, hiding in the moist log until rain came and replenished the ponds.

During its time out of water the fish modifies its gills, in order to prevent the loss of water and other nutrients. It uses capillaries on its skin for gas exchange, in order to keep breathing, as well as for waste removal. Because of this amazing adaptation, the mangrove killifish can survive in areas other fish would dare not go. If conditions get too salty, or there isn’t enough oxygen in the killifish’s prime puddle, or it starts to dry out, this tiny fish has a solution! Just hop on out and move somewhere else.

Biological processes aren’t the only thing that changes when the killifish moves out of water. Its behaviour does as well. Normally mangrove killifish are very territorial and aggressive (I would be too if there were only a few nice places to live). But this behaviour is relaxed when the fish are in shelters like logs. Many fish can be crammed into one small log or crab burrow, and while normally the fish would be attacking each other, they seem content to wait out the harsh conditions in peace. Which seems like a pretty good idea to me.

Quite the plain little guy, isn’t he? Image by Cardet co6cs, CC BY-SA 3.0, via Wikimedia Commons

The mangrove killifish also has a pretty interesting mode of reproduction. It is a functioning hermaphrodite, able to fertilize and produce offspring internally. The mangrove killifish is the only vertebrate known to self-fertilize in the wild. There are two types of males, primary and secondary males. Primary males develop from fertilized eggs, while secondary males only develop under certain conditions. Although they can be hermaphrodites, there is evidence that sexual outcrossing does occur in the population, presumably producing genetically unique offspring.

Whatever the mode of reproduction (which is confusing), the mangrove killifish has somehow evolved a unique method of surviving in a harsh, constantly changing environment. Not many species could do this, but somehow the killifish manages. Kudos to you, little guy.

Cover image by Vassil, CC0, via Wikimedia Commons