Notostraca (Tadpole Shrimps)

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Notostraca

(Tadpole shrimps)

Phylum Arthropoda

Subphylum Crustacea

Class Branchiopoda

Order Nostostraca

Number of families 1

Thumbnail description
Pale-colored or translucent crustacean with a flattened shield-like carapace that covers twothirds of the body and a thin, elongated abdomen ending in paired tail filaments

Evolution and systematics

Tadpole shrimps are considered living fossils because their basic body plan has not changed in the last 300 million years. In fact, some fossil species known from the Paleozoic are basically indistinguishable from modern types. The oldest fossil of a tadpole shrimp dates from the carboniferous Paleozoic, and Triops cancriformis is the oldest living animal species on the planet, with fossils of this same species dating from the Triassic. Given the innumerous changes that Earth has undergone during the last 300 million years, tadpole shrimps are considered biological marvels of survival and adaptation.

Notostracans belong to the subclass Calamanostraca, which, together with the subclasses Diplostraca and Sarsostraca, form the three living groups of the class Branchiopoda. The fossil order Kazacharthra dates from the Jurassic and also belongs to the subclass Calamanostraca; they are distinguished from the Notostraca mainly by the shape of their carapace, which has spikes on the margin, rendering them a fierce appearance. Kazacharthran fossils were found exclusively in the now Republic of Kazakhstan, of the former USSR, and some authors consider them a very specialized group of Notostracans. Tadpole shrimps share with the other groups of the Branchiopoda the characteristic "gilled feet," which are leaf-like and divided into lobes, each containing a gill plate.

Modern Notostracans are grouped into a single family: Triopsidae. Only two genera are recognized (Triops and Lepidurus), comprising 15 living species with 11 subspecies.

Physical characteristics

Tadpole shrimps are among the largest Branchiopoda, with most species ranging in size from 0.4 to 1.6 in (10–40 mm). Nevertheless, some species may be larger; Triops cancriformis has been reported to reach a length of 4.0 in (11 cm). The head and all the limb-bearing segments of the trunk of tadpole shrimps are covered by a large and shield-like carapace, which is dorsoventrally flattened. They possess a pair of sessile compound eyes on the anterior portion of this carapace just behind a group of two to four ocelli; just behind the eyes there is a nuchal or dorsal organ that may act in chemoreception. The antennae are reduced or absent. The abdomen is thin, elongated, and flexible with very reduced appendages. The telson bears two long protuberances known as caudal rami. The genus Triops may be easily distinguished from Lepidurus because the latter possesses an extended supraanal plate on the telson between the caudal rami, while in Triops this plate is reduced or absent. The body of tadpole shrimps is generally pale or translucent, although it may present a pink or reddish coloration due to the presence of hemoglobin in the haemolymph. The carapace varies in color depending on the species, ranging from silvery gray, yellowish, olive, dark brown, and sometimes mottled, rendering them well camouflaged.

Distribution

Tadpole shrimps can be found almost exclusively in freshwater ephemeral pools worldwide, except for Antarctica. One species, Lepidurus arcticus, inhabits permanent lakes in Norway and Greenland.

Habitat

With the single exception of Lepidurus arcticus, which inhabits a few lakes in Norway and Greenland and may coexist with one fish species, all other notostracans can be found exclusively in ephemeral pools where fish predation can be avoided. These pools may last only a few weeks during the rainy season, and tadpole shrimps are specially adapted to this kind of niche, developing extremely quickly and producing drought-resistant eggs that may remain dormant for decades, and which may require a period of drought before they are capable of hatching. In general, Triops species are mostly found in warmer and short-lasting pools, while Lepidurus are more common in cooler and longer-lasting pools. Tadpole shrimps are usually found swimming in the benthos of these pools.

Behavior

The most interesting aspect of tadpole shrimp behavior is their numerous reproductive strategies. Notostracans may reproduce sexually, with species either having two separate sexes or being hermaphrodites. They may also reproduce parthenogenetically, and most species are both parthenogenetic and sexual. There are also cases in which hermaphrodites must cross-fertilize with other hermaphrodites, and they may reproduce by parthenogenesis as well. In fact, most species of tadpole shrimps use more than one of these strategies, and different populations of the same species may use different reproductive strategies in a population-dependent manner. This plasticity of reproduction in tadpole shrimps is one of the reasons they have survived through the millenia.

Feeding ecology and diet

One of the key adaptations of tadpole shrimps is their capability to eat almost anything available in their restricted habitat. This adaptation is important in order to maintain the rapid development needed to colonize temporary ponds, requiring about 40% of their body mass in food per day. Notostracans are facultative detritus feeders, scavengers, or even predators; they eat anything from bacteria, algae, protozoa, lower metazoans, insect larvae, tender plant roots and shoots, and even prey on smaller tadpole shrimps, fairy shrimps, and amphibian tadpoles. Tadpole shrimps use their numerous appendages to channel food down a ventral groove between the mandibles.

Reproductive biology

Tadpole shrimps may reproduce sexually with populations composed of separate sexes (males and females), males and hermaphrodites, or solely hermaphrodites. The latter either self-fertilize or cross-fertilize one another. In most species,

females or hermaphrodites also reproduce parthenogenetically, which is the most common means of reproduction in the Notostraca. During mating, the male holds the female above it while swimming in an upside-down position. Tadpole shrimps breed throughout their adult life.

The female or hermaphrodite carries the fertilized eggs in a brood pouch for several hours before dropping them in the water. These eggs are drought- and freeze-resistant and may remain dormant for up to several decades. Furthermore, through a mechanism that is not yet understood, a small percentage of these eggs may hatch shortly after being laid, while another portion may need only one period of drought before hatching, and some others may need two or more drought periods to hatch. In this way, tadpole shrimps increase the chances of their offspring surviving if any given pool should not last long enough to complete their development. In addition, the eggs are sensitive to light, osmotic pressure, and temperature, which allows them to hatch only in new (temporary) freshwater pools where predators can be avoided.

Tadpole shrimps hatch as nauplius larvae, and their development is extremely rapid, going through larval molts in as little as 24 hours if conditions are optimal. After each molt, the individual gains a pair of appendages. In most species, sexually mature tadpole shrimps may be found a couple of weeks after hatching.

Conservation status

The IUCN lists Lepidurus packardi as Endangered due to habitat destruction in California, where this species is endemic.

Significance to humans

Triops longicaudatus and T. cancriformis are considered pests of rice fields in countries where rice is directly planted. These tadpole shrimps may occur in enormous numbers, and they expose and eat the roots of rice seedlings while paddling through the mud in search of food. In Japan, however, the rice is planted as plantlets and T. longicaudatus is considered a biological control agent of weeds. Dormant cysts of some species of Triops are sold throughout the world in kits for rearing as aquatic pets.

Species accounts

List of Species

Vernal pool tadpole shrimp
Longtail tadpole shrimp

Vernal pool tadpole shrimp

Lepidurus packardi

family

Triopsidae

taxonomy

Lepidurus packardi Simon, 1886, California, United States. No subspecies recognized.

other common names

None known.

physical characteristics

Large tadpole shrimp, reaching up to 1 in (2.5 cm) in length, with a long pad-like supraanal plate on the telson between the caudal rami. Its carapace is olive or gray, sometimes mottled, allowing it to blend in with aquatic plants. Females can be distinguished from the males by the presence of ovisacs, also known as foot capsules, attached to the eleventh pair of gilled feet.

distribution

Endemic to the northern Central Valley of California (United States), where it is locally abundant and widespread in spite of the losses sustained to its vernal pool habitat.

habitat

Found in a variety of natural and artificial pond habitats, such as vernal pools, swales, ephemeral drainages, stock ponds, reservoirs, ditches, backhoe pits, and ruts caused by vehicular activity.

behavior

During the evening hours, when the oxygen concentration of the water is low, L. packardi is usually found foraging at the water's surface on blades of grasses. During the day, it spends most of its time stirring the muddy pool bottom searching for prey.

feeding ecology and diet

Larval stages of the vernal pool tadpole shrimp are probably obligate filter feeders. However, adult L. packardi also prey actively on insect larvae, lower metazoans, other small crustaceans, and even smaller vernal pool tadpole shrimps. The food is collected by their gilled appendages while scavenging over vegetation or paddling through the mud.

reproductive biology

Adult L. packardi are dioecius (males and females). The eggs carried by the female are orange in color. Cysts usually hatch after at least one drought and two to four days after rehydration. The nauplius larva develops into an adult after six or seven weeks, depending on food availability and temperature. This species reproduces only during the rainy season; the shrimps die out during the dry season or periods of drought.

conservation status

Lepidurus packardi is listed as Endangered by the IUCN. The main reason for decline seems to be the elimination and degradation of its vernal pool habitat due to agricultural and urban development; other reasons include grazing, off-road vehicle

use, and hydrologic modification. On September 19, 1994, the U.S. Fish and Wildlife Service listed L. packardi as endangered because of its limited distribution, the small number of remaining populations, and the amount and nature of threats to this species. The U.S. Federal Government recognized this species as threatened once scientific research demonstrated that the vernal pool tadpole shrimp could go extinct without the protection afforded by the Endangered Species Act.

significance to humans

None known.

Longtail tadpole shrimp

Triops longicaudatus

family

Triopsidae

taxonomy

Triops longicaudatus LeConte, 1846, United States. Some authors recognize two subspecies: Triops longicaudatus longicaudatus and Triops longicaudatus intermedius, but these have not been officially accepted.

other common names

English: Rice tadpole shrimp, American tadpole shrimp; Spanish: Tortugueta ("little turtle").

physical characteristics

Large tadpole shrimp reaching a length of up to 1.5 in (4.0 cm). In this species, the second maxilla is absent and there is no anal plate.

distribution

North America (including Hawaii but not Alaska), Central America, South America, Japan, West Indies, Galápagos Islands, and New Caledonia.

habitat

This is the most widespread species of notostracan, being found in a variety of temporary waters, including rice fields.

behavior

Usually found scratching the soil surface in search of benthic food. When oxygen levels in the water are low, they will swim upside-down close to the surface of the water.

feeding ecology and diet

Omnivorous; it may eat detritus, scavenge dead organisms in its environment, or actively prey upon other animals, such as protozoa, insect larvae, other small crustaceans, and even cannibalize its siblings. In order to obtain food, adults actively paddle through the soil surface.

reproductive biology

T. longicaudatus is known to exhibit several major reproductive strategies: individuals can be sexual (either male or female with populations that are either half male and half female or are female-biased), parthenogenetic, or hermaphroditic. What is interesting about T. longicaudatus is that different populations exhibit a different reproductive strategy or combination of strategies. This fact suggests that these different reproducing populations might be considered different subspecies or species in the future.

conservation status

Not listed by the IUCN.

significance to humans

Triops longicaudatus is also known as the rice tadpole shrimp and is considered a pest species of rice fields where the rice is germinated in the field (mainly in the United States and Spain). T. longicaudatus damages the roots and leaves of seedling rice plants and also muddies the water, impeding the access of sunlight to the developing plants. In Japan, where the rice is transplanted, the long tail tadpole shrimp cannot harm the rice because it is too big and resistant to its attack. Instead, Japanese rice farmers use T. longicaudatus as a biological control agent against weeds. Dried cysts of T. longicaudatus are sold in kits to be bred as aquatic pets.

Resources

Books

Bliss, Dorothy. E. Biology of the Crustacea. New York: Academic Press, 1982–1985.

Pennak, Robert W. Fresh-Water Invertebrates of the United States. New York: John Wiley and Sons, 1978.

Schram, Frederick R. Crustacea. New York: Oxford University Press, 1986.

Periodicals

Ahl, J. S. B. "Factors Affecting Contributions of the Tadpole Shrimp, Lepiduris packardi, to Its Oversummering Egg Reserves." Hydrobiologia 212 (1991): 137–143.

Goettle, Bradley. "Living Fossil in the San Francisco Bay Area?" Tideline 16, no. 4 (1996): 1–3.

Linder, F. "Contributions to the Morphology and Taxonomy of the Branchiopoda Notostraca, with Special Reference to the North American Species." Proceedings of the U.S. National Museum 102 (1952): 1–69.

Longhurst, A. R. "A Review of the Notostraca." Bulletin of the British Museum of Zoology 3 (1955): 1–57.