American tadpole shrimp, Longtail tadpole shrimp
Triops longicaudatus (commonly called American tadpole shrimp or longtail tadpole shrimp) is a freshwater crustacean of the order Notostraca, resembling a miniature horseshoe crab. It is characterized by an elongated, segmented body, a flattened shield-like brownish carapace covering two thirds of the thorax, and two long filaments on the abdomen. The genus name Triops comes from Greek ὤψ or ṓps, meaning "eye" prefixed with Latin tri-, "three", in reference to its three eyes. Longicaudatus is an Latin neologism combining longus ("long") and caudatus ("tailed"), referring to its long tail structures. Triops longicaudatus is found in fresh water ponds and pools, often in places where few higher forms of life can exist.
Triops longicaudatus is widespread in North America. In Canada, it is found only in the provinces of Alberta, Saskatchewan and Manitoba. It is widespread throughout the contiguous United States, Mexico, and Hawaii, but not Alaska. Tadpole shrimps may be found in parts of South America (Argentina and Galápagos Islands), in the West Indies, and the Pacific Islands, including Japan and New Caledonia.
T. longicaudatus is the most widespread notostracan species, and may be found in western North America, South America, Japan, South Korea, and several Pacific Islands. It is most active at a temperature of approximately 20 °C (68 °F), and is usually found scratching the mud at the bottom of pools, searching for benthic food. Triops collect food particles by straining the water with hairs on their limbs. Loose food particles are collected in a groove running down the underside of the body lengthwise, and held together by a sticky secretion until they are swallowed by their very small (2 mm wide) mouth. The tiny mouth is deep in its underbelly, and while the animal is capable of breaking up plant roots or dead fish, it is incapable of chasing down and eating prey larger than it is.
The feeding method of tadpole shrimps is similar to that proposed for the ancestral crustacean. The anterior appendages (second pair to tenth pair) stir sediments and swirl muddy water into the wide, midventral food groove. The gnathobases (inward-facing lobes at the base of the leg) guide food anteriorly to the mouth. The large flat exopods (outward-facing lobes at the end of the leg) stir and lift the sediments. Fine silt particles and water escape easily, but large, coarse food particles are torn into smaller pieces by the blade-like, inward-facing lobes called endopods at the end of the leg.
Tadpole shrimps are omnivorous and may eat algae, insects, and other organic debris; they are known to chase very small fry, tadpoles, and oligochaete worms. In general, they eat anything organic that is smaller than they are, which even may include their siblings (they are cannibalistic). In turn, Triops longicaudatus are eaten by frogs and birds.
Triops longicaudatus displays several reproductive strategies. Individuals may reproduce sexually, but this is rare, as most populations are highly male- or female-biased. Parthenogenesis (development from unfertilized eggs) is the most common reproductive strategy. Some populations, however, consist of hermaphrodites who fertilize each other. Different populations display different strategies or combinations of strategies, and therefore, in the future may be considered separate species or subspecies.
In females, the eleventh pair of legs is modified into egg sacs, where the eggs are carried for several hours. The eggs are released in batches, have a thick shell, and can stand freezing temperatures as well as drought, enabling the population to survive from one season to the next. The eggs have to dry out completely before being submerged in water again in order to hatch successfully; they may remain in a state of diapause for up to 20 years. These eggs may have helped Triops longicaudatus, as well as other notostracans, to survive the various natural disasters and mass extinctions to date.
To complete their lives, tadpole shrimps depend on the changing nature of the temporary waters they inhabit. During the dry season (summer and autumn), their offspring stay inside the eggs. As the pool fills with rainwater during the winter and spring, they hatch and feed on fairy shrimps and other invertebrates. The first larval stage (the metanauplius) is orange in colour. It has a single eye, six legs, and develops through instars (growth stages). Each instar ends with shedding the exoskeleton. The number of segments and appendages increases as Triops grow, and they slowly change to greyish brown. In approximately eight days, they reach maturity and lay eggs. Adult Triops die as the pools dry up. Triops generally live for about 20–90 days if the pool does not dry up.
