Members of the order Notostraca (colloquially referred to as notostracans, tadpole shrimp, shield shrimp or by the genus name Triops) are small crustaceans in the class Branchiopoda. Triops have two 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.
Triops have been marketed as a novelty pet, given that they can hatch from dried eggs in a similar fashion as Sea-Monkeys.
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 pairs of antennae are 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 and hermaphrodites, 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 from 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 dropped 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.