Evolution and systematics
Tadpole shrimps are thought-about residing fossils as a result of their fundamental physique plan has not modified within the final 300 million years. In truth, some fossil species recognized from the Paleozoic are principally indistinguishable from fashionable varieties. The oldest fossil of a tadpole shrimp dates from the carboniferous Paleozoic, and Triops cancriformis is the oldest residing animal species on the planet, with fossils of this identical species courting from the Triassic. Given the innumerous modifications that Earth has undergone over the past 300 million years, tadpole shrimps are thought-about organic marvels of survival and adaptation. Notostracans belong to the subclass Calamanostraca, which, along with the subclasses Diplostraca and Sarsostraca, kind the three residing teams of the category Branchiopoda. The fossil order Kazacharthra dates from the Jurassic and likewise belongs to the subclass Calamanostraca; they’re distinguished from the Notostraca primarily by the form of their carapace, which has spikes on the margin, rendering them a fierce look. Kazacharthran fossils had been discovered solely within the now Republic of Kazakhstan, of the previous USSR, and a few authors contemplate them a really specialised group of Notostracans. Tadpole shrimps share with the opposite teams of the Branchiopoda the attribute “gilled feet,” that are leaf-like and divided into lobes, every containing a gill plate. Modern Notostracans are grouped right into a single household: Triopsidae. Only two genera are acknowledged (Triops and Lepidurus), comprising 15 residing species with 11 subspecies.
Tadpole shrimps are among the many largest Branchiopoda, with most species ranging in measurement from 0.4 to 1.6 in (10–40 mm). Nevertheless, some species could also be bigger; Triops cancriformis has been reported to attain a size of 4.Zero in (11 cm). The head and all of the limb-bearing segments of the trunk of tadpole shrimps are lined by a big and shield-like carapace, which is dorsoventrally flattened. They possess a pair of sessile compound eyes on the anterior portion of this carapace simply behind a bunch of two to 4 ocelli; simply behind the eyes there’s a nuchal or dorsal organ which will act in chemoreception. The antennae are lowered or absent. The stomach is skinny, elongated, and versatile with very lowered appendages. The telson bears two lengthy protuberances often called caudal rami. The genus Triops could also be simply distinguished from Lepidurus as a result of the latter possesses an prolonged supraanal plate on the telson between the caudal rami, whereas in Triops this plate is lowered or absent. The physique of tadpole shrimps is usually pale or translucent, though it might current a pink or reddish coloration due to the presence of hemoglobin within the haemolymph. The carapace varies in shade relying on the species, starting from silvery grey, yellowish, olive, darkish brown, and typically mottled, rendering them effectively camouflaged.
Tadpole shrimps will be discovered nearly solely in freshwater ephemeral swimming pools worldwide, aside from Antarctica. One species, Lepidurus arcticus, inhabits everlasting lakes in Norway and Greenland.
With the single exception of Lepidurus arcticus, which inhabits a couple of lakes in Norway and Greenland and should coexist with one fish species, all different notostracans will be discovered solely in ephemeral swimming pools the place fish predation will be averted. These swimming pools might final just a few weeks throughout the wet season, and tadpole shrimps are specifically tailored to this type of area of interest, growing extraordinarily rapidly and producing drought-resistant eggs which will stay dormant for many years, and which can require a interval of drought earlier than they’re able to hatching. In common, Triops species are largely present in hotter and short-lasting swimming pools, whereas Lepidurus are extra widespread in cooler and longer-lasting swimming pools. Tadpole shrimps are normally discovered swimming within the benthos of these swimming pools.
The most attention-grabbing facet of tadpole shrimp habits is their quite a few reproductive methods. Notostracans might reproduce ####ually, with species both having two separate ####es or being hermaphrodites. They may additionally reproduce parthenogenetically, and most species are each parthenogenetic and ####ual. There are additionally instances by which hermaphrodites should cross-fertilize with different hermaphrodites, they usually might reproduce by parthenogenesis as effectively. In truth, most species of tadpole shrimps use multiple of these methods, and totally different populations of the identical species might use totally different reproductive methods in a population-dependent method. This plasticity of replica in tadpole shrimps is likely one of the causes they’ve survived via the millenia.
Feeding ecology and food regimen
One of the important thing diversifications of tadpole shrimps is their functionality to eat nearly something accessible of their restricted habitat. This adaptation is essential so as to keep the fast growth wanted to colonize short-term ponds, requiring about 40% of their physique mass in food per day. Notostracans are facultative detritus feeders, scavengers, and even predators; they eat something from micro organism, algae, protozoa, decrease metazoans, insect larvae, tender plant roots and shoots, and even prey on smaller tadpole shrimps, fairy shrimps, and amphibian tadpoles. Tadpole shrimps use their quite a few appendages to channel food down a ventral groove between the mandibles.
Tadpole shrimps might reproduce ####ually with populations composed of separate ####es (women and men), males and hermaphrodites, or solely hermaphrodites. The latter both self-fertilize or cross-fertilize each other. In most species, females or hermaphrodites additionally reproduce parthenogenetically, which is the most typical technique of replica within the Notostraca. During mating, the male holds the feminine above it whereas swimming in an upside-down place. Tadpole shrimps breed all through their grownup life. The feminine or hermaphrodite carries the fertilized eggs in a brood pouch for a number of hours earlier than dropping them within the water. These eggs are droughtand freeze-resistant and should stay dormant for up to a number of a long time. Furthermore, via a mechanism that’s not but understood, a small proportion of these eggs might hatch shortly after being laid, whereas one other portion may have just one interval of drought earlier than hatching, and a few others may have two or extra drought durations to hatch. In this manner, tadpole shrimps enhance the possibilities of their offspring surviving if any given pool mustn’t final lengthy sufficient to full their growth. In addition, the eggs are delicate to gentle, osmotic stress, and temperature, which permits them to hatch solely in new (short-term) freshwater swimming pools the place predators will be averted. Tadpole shrimps hatch as nauplius larvae, and their growth is extraordinarily fast, going via larval molts in as little as 24 hours if circumstances are optimum. After every molt, the person positive factors a pair of appendages. In most species, ####ually mature tadpole shrimps could also be discovered a few weeks after hatching.
The IUCN lists Lepidurus packardi as Endangered due to habitat destruction in California, the place this species is endemic.
Significance to people
Triops longicaudatus and T. cancriformis are thought-about pests of rice fields in nations the place rice is straight planted. These tadpole shrimps might happen in huge numbers, they usually expose and eat the roots of rice seedlings whereas paddling via the mud in quest of food. In Japan, nonetheless, the rice is planted as plantlets and T. longicaudatus is taken into account a organic management agent of weeds. Dormant cysts of some species of Triops are bought all through the world in kits for rearing as aquatic pets.