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January, 2016

Crustaceans of the Weirdest Kind

by Valerie

My husband, Larry, recently visited Enchanted Rock in central Texas and he mentioned finding fairy shrimp in one of the small pools on the granite outcropping. Seeing those delicate little beasts is a real treat, since their preferred habitat is ephemeral at best. They are frequently found in vernal pools, which don't necessarily occur only in the spring around here. After any good rain, there are numerous small basins of water on rocks, in parking lots and, later, after the creeks have had their brief run, in the usually dry stream beds.

Fairy Shrimp
Fairy shrimp swim slowly and have no defenses against their numerous predators, which include waterfowl, fish and predatory water beetles, so they are almost never found in permanent bodies of water with established fauna. Instead, they have evolved to take advantage of temporary pools in which they hatch, grow and lay eggs before any competitors can colonize the water. The eggs are tough enough to withstand heat, cold and desiccation so they wait, sometimes for years, until conditions once again favor another generation.

When I was a kid in Illinois, I loved to find the little crustaceans in the early spring. I'd sometimes put them in an aquarium and watch them float dreamily back and forth, their numerous legs propelling them in a graceful rhythmic pattern. If I put any other critters in with them, though, the fairy shrimp were soon history. Water beetle larvae, baby crayfish, dragonfly nymphs, and seemingly harmless water mites and scud inevitably attacked and ate them.

Clam Shrimp

There were large, colorful advertisements in comic books back in the '60s for Sea-Monkeys. As a typical child of that decade, I read plenty of comic books and saw the enticing ads for almost human-looking creatures that could be mail-ordered for about $1, lived in an aquarium and were reported to be trainable. My mother quenched my gullible excitement by explaining that the animals were really just brine shrimp, the salt-water equivalent of fairy shrimp, and that they were as different from the ads' drawings as the super heroes and gruesome monsters were from real people. Once I read about brine shrimp in a biology book, I finally accepted her decision NOT to waste money on brine shrimp eggs and fancy packaging. Besides, we could get fairy shrimp for free.

The crustaceans that I kept in my aquarium ranged in size from the nearly microscopic Cyclops to large crayfish that invariably managed to crawl out and expire in a secluded, far too dry, corner of our house. Cyclops are minuscule copepods that scoot around with jerky motions. The females trail pairs of egg sacs on either side of their tail. They often resided on the glass sides of the aquarium where they were easily observed. Another tiny denizen of our fish tanks was the water flea, or Daphnia. These didn't cling to the glass but swam around freely. The fish usually ate all they could catch and only a few that stuck close to the gravel at the bottom survived for any length of time.

Pillbugs mating
Fresh water hosts a few other unusual crustaceans. Tadpole shrimp and clam shrimp are both named for superficial similarities to other taxons. I've only found tadpole shrimp once, and it was in a rain-filled puddle in a gravel parking lot. The creatures were over an inch long and looked more like trilobites or other prehistoric arthropods than tadpoles. Several of them were swimming about in the silt-laden water and I wondered how they survived the car and truck tires that must occasionally splash through their environment. Clam shrimp are, of course, not mollusks, but are instead named for the clam-like pair of shells that encompass their body. The legs stick out a little, and the shells are transparent, so eggs are often visible within a female's body. Individuals swim rather slowly and clumsily, as their cumbersome shells are not particularly stream-lined.

Two other kinds of crustaceans were frequent residents in my aquariums. Scud are small amphipods that swim quickly and usually tend to hide amid the leaves of pond plants. Whenever they are taken out of the water, then dropped back in, they tend to get caught on the surface tension and cannot get back underwater without help. If a pond becomes uninhabitable or dries up, the scud will leave and seek new digs. They can hop a bit, but they are rather ungainly on land and must find water before they dry up and die. Like all the other crustaceans I've observed, they turn red when they are cooked. In the north, streams that hosted scud often also contained isopods. These relatives of pillbugs and sowbugs crawl around on the bottom just like their terrestrial cousins. They seem to need more dissolved oxygen than most other crustaceans, and I was never very successful at keeping them alive in an aquarium for long.

Though not as common as their comparative saltwater relatives, shrimp and prawns do inhabit some pristine freshwater streams. The San Marcos River just south of Austin contains small, almost colorless shrimp that look just like the tiny glass shrimp found in coastal waters. They made nice aquarium pets, helping keep the bottom of the tank clean. Freshwater prawns, on the other hand, are rare but big enough to be table fare. I once found the molted exoskeleton of a freshwater prawn in Juniper Creek in Florida. It was over 6 inches long!

Rock Slater

When I lead entomology hikes or give talks on the subject, I frequently ask participants if sowbugs and pillbugs, sometimes called roly-polies or woodlice, are insects. The answers are generally a confident "don't know" because they perceptively suspect this is a trick question. People are then surprised to find out that these very common little animals are crustaceans, but this is one of those interesting factoids that tend to stick in the brain. Like aquatic crustaceans, sowbugs and pillbugs have gills, but they are encased in a waterproof chamber. If submerged, these relatives of shrimp, crab and lobsters will drown.

Unlike insects, crustaceans have colonized both fresh and saltwater habitats. It's obvious that the sowbugs found in forest litter and compost bins are quite similar to the slaters (also called sea roaches or rock lice) found on pilings, walls, and rocks near the ocean. There is also a saltwater equivalent of the scud found in freshwater ponds,

Skeleton Shrimp
in the form of beach fleas or beach hoppers. While many ocean crustaceans are very typical looking (crabs, shrimps, lobsters), some are amazingly odd. The mantis shrimp is sort of a Frankenstein-ish looking creature, part shrimp and part lobster. Better yet are the mole crabs: egg-shaped sand dwellers that are loads of fun trying to catch as they move about with each wave.

A group of crustaceans that everyone has seen but few realize their true identity are the barnacles. Rock, leaf, volcano, acorn, and gooseneck are just some of the names of this diverse taxon. While their outer hard shells are usually seen, especially when stepped upon with bare feet, they are only active when underwater, so they don't even look alive during low tides when they are exposed or when their substrate washes up on the beach. View them in a tide pool, or place some in a cup of water, and they almost at once begin to repeatedly reach and withdraw their legs, an action that strains for small particles of food.

One very odd little animal that I once found washed up in algae was a skeleton shrimp. This tiny, elongate creature doesn't actually resemble much of anything in the Animal Kingdom, but it does resemble a bit of algae, especially when it strikes its characteristic plant-like pose.

I do have a special fondness for crustaceans, and it's not just because I like the taste of crab and shrimp. Some of the species are so strange that they look like aliens or imaginary beasts. It's nice that such fascinatingly bizarre beings can be found in relatively mundane places, like puddles, seashore detritus, creeks, and our compost bin.

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