Associated to the crustacean leptostracan Nebalia
Paraseison annulatus is unable to swim and move only by creeping, but it does not move easily. One way rotifers colonize young crustaceans is to move to them when they are still in the brood chamber. However, it is not known how Paraseison transfer during the crustacean moult. Isolated specimens Seison are apparently able to attach themselves to a free swimming Nebalia in an experimental vessel.
This species is exclusively marine/brackish. Paraseison annulatus lives only as epibiont on leptostracan crustaceans of genus Nebalia and it is reported to occur, on the same host, together with Seison nebaliae. However the former lives attaches to pereiopods edges of the carapace, and occasionally to the antennae and abdomen while the latter on the pleopods.
This species has been described on specimens found near Trieste (North Adriatic Sea). Later on it has been collected in the Mediterranean Sea at Naples (Italy), at Venice Lagoon (Italy) and at Majorca (Balearic Islands, Spain). Out of Mediterranean sea this species has been reported for Roscoff (France), Germany and Southern England.
Exclusively marine
The body can be divided into four parts: head, neck, trunk and foot. Neck and foot are segmented and can be retracted telescopically, while head and trunk are not segmented. The overall body shape of P. annulatus is slightly different in the two sexes, with females being generally larger and slightly humped. The head is oval and somewhat flattened laterally. Many secretory cells with granular cytoplasm are present in the central and posterior regions, close to the brain. The brain occupies the posterior half of the head. It has an outer cellular layer and a uniform inner mass of nerve fibers. Around this unpaired structure, and connected to it, five masses of nervous tissue are visible. Dorsal to the brain one small mass is clearly connected to an unpaired, short antenna. The external morphology of the head reveals two apical pseudosegments that can be retracted telescopically The first one forms a mobile lobe, the rostrum, which can cover the ventral mouth. The mouth opens within a ciliated buccal field lined bilaterally by five or six mobile ciliar tufts arranged in a row. Ventrally, the mouth is bordered by a V-shaped, bilobed structure with short cilia on its inner edge. The mastax, with its strong muscles, is visible apically. No eyes are evident. A sort of ventral longitudinal groove, shaped like a spoon, is seen in males, but this feature is almost lacking in females. The trunk is generally oval, tapering posteriorly, with longitudinal folds. Paraseison annulatus exhibits four discernable constrictions on the rear trunk, which gives it the name. The digestive system is composed of pharynx with mastax, long esophagus, oval stomach, and short intestine with anus . Many glands open at different levels along the digestive system. Female genital apparatus consists of two sac-like ovaries in which growing eggs at different stages of maturation can be seen. The two ovaries join caudally in a short oviduct which ends in the cloaca, dorsal at the posterior end of the trunk. The male genital system of P. annulatus is similar to that of S. nebaliae in its general organization, but no detailed description is available. The foot is segmented into six parts. It contains many ducts and adhesive glands of different lengths. Some glands open at different positions along the foot and look like cutaneous glands. Most open at the terminal adhesive disc, which is surrounded by a cuticular ring. The last segment of the foot, ending with the disc, is always bent dorsally. Ventrally it possesses a tubercle, which is the opening of a mucous gland. Trophi are fulcrate and consist of paired manubria, rami and unci, and an elongated fulcrum.
Some authors observed that the stomach contents of P. annulatus are the same color as the eggs of Nebalia and argued a possible parasitism of the rotifer on the crustacean eggs. In theory, this is possible, as N. bipes incubates its eggs on the pleopods till they hatch into miniature adults; the same position is occupied by P. annulatus, which could easily feed on its hosts' eggs. However, because it is thought that the crustacean is unable to reproduce throughout the year, an alternative food for the rotifer should be assumed. Probably Paraseison species are ectoparasites sucking out haemolymph of their hosts.
Dioic species that reproduce only sexually
In mature specimens, it is about 130-150 μm long and about 50-60 µm wide. Females are larger than their males, 0.4-0.7 mm long.
