Notostraca
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[Photo] Triops longicaudatus. Photograph taken by Dominik Tomaszewski from http://mytriops.com/

Members of the order Notostraca (colloquially referred to as notostracans, called Triops, tadpole shrimp or shield shrimp) are small crustaceans in the class Branchiopoda. Triops have two internal compound eyes and one naupliar eye in-between, a flattened carapace covering its head and leg-bearing segments of the body. The order contains a single family, with only two extant genera. Their external morphology has apparently not changed since the Triassic appearance of Triops cancriformis around 220 million years ago. Triops cancriformis may therefore be the "oldest living animal species on earth." The members of the extinct order Kazacharthra are closely related, having been descended from notostracans.

Characteristics
Although notostracans are related to fairy shrimps, they differ greatly in structure. The flat oval shield covers the crustacean's head and the front part of the thorax. Near the front edge of the shield, on its surface, is a small lump that bears two dark, stalkless, compound eyes. In between them is an unpaired naupliar eye, and behind the three eyes is an unusual four-celled organ. The function of this organ is uncertain. It is thought to be an internal secretion organ. The hind edge of the shield is a semicircular dent that leaves the hind portion of the thorax exposed. The abdomen ends with a telson that bears two long segmented uropods, known as furcae.

By looking at a tadpole shrimp from the ventral side one can clearly see the structure of its segments and limbs. In front, the carapace curves down to the ventral side, where it attaches to a large labrum (upper lip) that is almost square in shape. The first and second pair of antennae is extremely reduced in size, while the mandibles are quite large, with many toothlike projections. The mouth is located in between the mandibles, behind the labrum. Behind the mandibles are one or two pairs of flat jaws (maxillae). The next 10 thoracic segments each bear a pair of legs. As in fairy shrimps, the legs have 6 lobes on the inner side that push the food towards the mouth; on the outer side is one large swimming lobe and one breathing lobe, modified into a gill (fairy shrimps have two breathing lobes on each leg). Detailed muscle studies conclude that the legs of tadpole shrimps and fairy shrimps have no phylogenetic relationship, apparently having evolved independently, albeit with similar functions.

The first and, less obviously, second pair of a tadpole shrimp's legs differs from other legs by the four inner lobes, which are modified into elongated, segmented, flagellum-like structures that project outside from the edge of the carapace. They act as sensory organs and even resemble externally the flagella of the antennae of other crustaceans. This modification of the inner lobes of the front thoracic legs is unquestionably associated with the reduction of the tadpole shrimp antennae.

In females, the eleventh pair of limbs is quite unusual in structure: the large outer lobe, which is used for swimming in other limbs, is modified into a round egg capsule, where the eggs are carried. In males, the eleventh pair of legs does not differ from the other legs.

There is another surprising phenomenon in the structure of tadpole shrimps. Every thoracic segment, from the thirteenth one on, bears not one, but 4 to 6 pairs of legs. Therefore, the number of pairs of the crustacean's legs often reaches as many as 70. No other crustacean has as many legs as tadpole shrimps.

The legs gradually become smaller from front to back, while the posterior-most segments bear no legs at all.

An obvious difference between tadpole shrimp and fairy shrimp legs is that the former possess several spikes at the base of each front leg, pointing inward. Tadpole shrimps use the spikes to grab large food particles and pass it form one leg to another forward, to the mouth. While fairy shrimps filter small food particles suspended in the water, notostracans apparently cannot do this. Their hind limbs mostly aid in respiration. It is quite noticeable during the crustacean's brief stops that its rear limbs continue moving, while the front ones keep still. As a tadpole shrimp swims, its legs bend and straighten in a wavelike motion. The Swedish zoologist Lundblad once droppered some water mixed with carmine near a tadpole shrimp's hind legs. He observed as the water moved slowly forward through the groove between the crustacean's hind legs. However, as soon as the water reached the tenth pair of legs, it streamed forward steadily, which showed how important the front limbs are in the process of bringing food up to the mouth.

When a tadpole shrimp swims, its eyesight plays an important role. Lighting up the bottom of an aquarium in a dark room makes the crustacean roll over, ventral side up: the dorsally positioned eyes can then sense the light. Even when the crustacean's eyes are lacquered and it sits on the bottom of the aquarium ventral side down, the experiment brings the same result. Apparently, the tadpole shrimp reacts to the light because the naupliar eye compartment continues down as a connective passage and ends at the ventral side, in front of the upper lip, as a pigmentless "window". Thus, tadpole shrimps can sense light from above and below, simultaneously.

Notostracans do not use their eyes to find food. They use a special chemical sense, centred in the flagellum-like inner outgrowths on the first pair of legs. A tadpole shrimp can easily find an earthworm in an aquarium and eat it. However, studies show that when quinine is added to the worm, the crustacean feels the serving with its flagella and refuses to eat the distasteful food.

Life cycle
In the northern hemisphere, tadpole shrimps mainly hatch from unfertilized eggs. The populations of the North American and Eurasian puddles and pools are made up almost exclusively of females. There is often less than 1 male per 100 females, and some pools contain no males at all. As one travels north to south, however, the males increase in numbers; in the tropics they may even outnumber the females.

Due to the rareness of the males, mating tadpole shrimps have only been observed twice. The male attached himself to the female's carapace and bent his body so that a good deal of it was under the female. Next, he grabbed the female's eleventh pair of legs with his eleventh pair of legs.

Both fertilized and unfertilized eggs are stored in the female's egg compartment on the 11th pair of legs. She keeps the eggs there for a short period of time and releases them to the bottom of the pool. What is interesting about them is that no differences in the structure, hatching rate or development of the two kinds of eggs can be observed.

These tiny eggs are equipped with a strong, thick shell. They can withstand freezing, drought and can hatch as long as 9 years after the release. The eggs can stand temperatures of up to 80°C, or pass through a frog's intestine without any harm at all. They are easily carried by the wind, even for very long distances. Due to these qualities of eggs, tadpole shrimps are common around the world. Their sudden appearance in some small pool is explained by their diapaused eggs ending up in the pool, and not adult crustaceans falling from the sky, as certain local inhabitants think.

If the egg arrives in a pool with suitable conditions, the larva hatches a year later. In the common species Triops cancriformis this larva is a typical nauplius, with three pairs of limbs and no traces of segmentation. In other tadpole shrimp species the larva hatches at a later stage - the metanauplius, characterized by a segmented hind part of the body. In order to reach sexual maturity, Triops cancriformis must moult about 40 times. These moults follow one another very quickly and at summer temperature, the development from a nauplius to an adult crustacean takes only about two weeks. The spring tadpole shrimp (Lepidurus apus) reaches a length of 12 mm after 17 moults, while the sexual dimorphism appears after the 14th moult.

Ecology
Tadpole shrimps usually live near the ground of astatic pools, where they move with their ventral side down. However, lack of oxygen can force them to swim upside-down with their gill-like legs close under the surface of the water. Notostraca are omnivorous; they dig around in the mud using the frontal part of their shield, looking not only for plankton but also for larger prey such as worms, chironomid larvae, small, dying or weak tadpoles, and even each other. Anostracans, often associated with notostracans, can also be a considerable part of their prey (especially when weak or dying).

Notostracans sometimes even cannibalise freshly moulted members of the same species. In northern and central Europe, with few exceptions, all tadpole shrimps are female, whereas the sexes in southern and western Europe as well as in northern Africa are nearly equal in number. These "females" possess hermaphroditic glands, but instead of selfing, parthenogenesis takes place.

Triops survives in temporary pools all over the world, and are correspondingly short-lived. These ponds usually dry up during certain times of the year when there is no rainfall. Although the adult Triops die during these droughts, their embryos remain in a state of diapause (suspended animation) and can survive for several years until the next rains fill up the pools again, allowing them to hatch.

Triops are often the top predators in vernal pools, as they will eat anything smaller than themselves. They also are an important food source for visiting birds. In some areas, certain species of Triops are considered pests, as they damage young rice plants by uprooting them while searching for food.

The maximum life spans in the lab agree with data from field observations on Triops survival. In the lab, T. longicaudatus has a maximum lifespan of about 50 days and T. cancriformis a maximum lifespan of about 90 days, with some individuals beginning to die off as soon as two weeks after hatching. Secondarily, some may suffer premature deaths from moulting complications or other reasons.

Distribution
Notostracans can be found on every continent except Antarctica.

In Austria, two notostracan species (one spring species and one summer species) are documented: Lepidurus apus and Triops cancriformis. In the Americas, several species have been identified, including Triops longicaudatus and Triops newberryi. In Australia, Triops australiensis is found. Other common types are Triops numidicus from Africa, although it has been said that some papers refer to this as an outdated name for Triops granarius, which is from South Africa, China, Japan and Italy. Triops cancriformis is also found in elsewhere in Europe, for example in Britain where it is endangered, occurring only in pools in two localities, one being in England at New Forest the other in Scotland at Wildfowl & Wetlands Trust Caerlaverlock, which is near Dumfries.

Etymology of the word "Triops"
"Triops" is a loanword from Greek, meaning "three eyes." Notostracans, indeed, have three eyes, in that, unlike in the related Fairy shrimps, the naupliar ocellus (the eye of the nauplius) is retained in the adult form, even as the two compound eyes develop. The two compound eyes look out for food and predators, but they can only see clearly for a short distance. The naupliar ocellus senses light, acting as an internal "compass".(In their natural habitat, the sun shines from above, therefore the animals believe that light is where "up" is.). Both the singular and plural is Triops.

Diversity
In spite of the wide distribution in still freshwater pools from subarctic ponds to hot tropical puddles, the order Notostraca comprises a paradoxically small number of species. Modern zoologists, taking the variety of all crustaceans into account, recognize only 16 species of tadpole shrimps worldwide - 6 species of the genus Triops and 10 species of the genus Lepidurus. Some of them, such as Triops cancriformis and Lepidurus apus, generally occur worldwide, while others, such as Lepidurus arcticus, are confined to Arctic pools. There are certain exclusively African species and exclusively Australian species.

Classification

Genus Lepidurus Leach, 1819
Lepidurus apus (Linnaeus, 1758)
Lepidurus arcticus (Pallas, 1776)
Lepidurus batesoni Lonhorst, 1955
Lepidurus bilobatus Packard, 1883
Lepidurus couesii Packard, 1875
Lepidurus cryptus Rogers, 2001
Lepidurus lemmoni Holmes, 1894
Lepidurus lynchi
Lepidurus mongolicus Vekhoff, 1992
Lepidurus packardii

Genus Triops Schrank, 1803
Triops australiensis (Spencer & Hall, 1895)
Triops cancriformis (Bosc, 1801)
Triops granarius Lucas, 1864
Triops longicaudatus (LeConte, 1846)
Triops numidicus
Triops newberryi (LeConte, 1846)

Geological history
Another notable characteristic of tadpole shrimps is their unusual geological longevity. The living species Triops cancriformis has been found fossilized in Triassic strata of Germany. A detailed analysis of the traces of the crustaceans and their appendages concluded that the Triassic tadpole shrimp is nearly identical to modern ones. It inhabited the same type of pools that modern notostracans inhabit. Modern African species had also been found fossilized, but those were found in Jurassic and more recent strata.

No other living animal species is known to have existed so long ago. Tadpole shrimps are quite rightfully referred to as "living fossils".

Notostracans in the aquarium
Notostracans, primarily Triops longicaudatus, are often sold to children in small packets as eggs as an interesting science project or as a beginner's kit for their first aquarium. One pours distilled or spring water onto the eggs, and they will hatch out very quickly--often within less than a day. They will also grow at a remarkable pace; growth is actually noticeable from one day to the next until they reach their maximum size of approximately 2 inches, including their caudal appendages. If food is not available in plenty--and sometimes even if it is--they will not hesitate to eat each other.

These creatures can make a useful addition to a freshwater aquarium, as they keep the substrate very clean and eat almost anything they find. They may, however pose a slight danger to very small fish or other crustaceans, and may in turn be eaten by large fish. Also, they may damage the roots of aquarium plants they encounter if not kept well fed. They are also very short-lived pets, but are easily replaceable.

By thoroughly drying the tank in which they are kept after a die-off and then allowing the completely dry tank to sit for at least a month and finally reflooding it with distilled or spring water, another generation of pets is easily generated, as the previous brood will have left eggs in the substrate. This simulates the periodic drying and flooding of the natural habitat of the tadpole shrimp.

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