Nicothoe astaci

Nicothoe larvae are released from the egg-sacs into surrounding water. These are cyclopid larvae. The morphology of a first-stage larva includes full mouthparts, two pairs and rudimentary third pair of legs, and a sucking mouth tube. The cephalothorax is as long as the rest of the body and some legs are free. There is a gap in knowledge of the life cycle of this parasite. It is unknown if this species requires any intermediate hosts; the earliest stage of Nicothoe seen on lobster gills was a last-stage copepodid. This last stage resembles the adult except for the absence of wings and egg-sacs. Growth continues, with a whole succession of juvenile stages, at which wings and gonads develop. Once the wings have reached a length of 2.0–2.2 mm (0.079–0.087 in), eggs are forced out of the gonads into egg-sacs and it is at which the organism becomes an adult.[8]

The series of changes in the reproductive system of the juvenile and adult Nicothoe are divided into six stages:[8]

• Immature – No development of ovaries are externally visible in juvenile; adult ovaries are empty or with few residual eggs.
• Egg-sacs contain spherical eggs.
• Developing – Ovaries become externally visible as granular with no definite oocytes visible. Larvae are in egg-sacs.
• Half-full – Ovaries are darker with oocytes externally visible. Young oocytes and dividing oogonia can be seen. Larvae in egg-sacs are showing signs of segmentation.
• Full – Ovaries are large and full with plots of oognia and oocytes (few are young). Larvae are showing rudimentary limbs.
• Ripe – Ovaries appear dense. Eggs are closely packed together and showing signs of maturation. Egg-sacs are full of cyclopid larvae with mouthparts and swimming legs. Fertilization by sperm occurs before the eggs enter the egg-sacs and the development into embryos happens once the egg-sacs are full of eggs. At this point, the ovaries have already begun forming new eggs.
• Spawning – Old egg-sacs are lost and larvae is released. New egg-sacs are being formed and the cycle is repeated.

The feeding mechanism of the female Nicothoe astaci has been studied extensively. The parasite feeds upon the haemolymph (blood) of the lobster by attaching to the gill filament using its circular ('oral disc') mouthparts and limbs.[4] Recent cryo-scanning electron microscopy has revealed structural adaptations that facilitate attachment of these parasites to the gill filaments of their lobster host.[6] It is thought that sharp mandibles pierce the wall of the gill filament before the suctorial oral disc attaches to the gill, which in conjunction with an observed peristaltic mechanism of the stomach,[9] ensures a ‘vacuum seal’ to the host, extracting haemolymph to be ingested. The aperture of the feeding channel, through which host haemolymph is drawn, is only approximately 5 micrometers in diameter.

The effect of Nicothoe astaci upon the host European lobster has only recently been studied.[4][7] Internally, lobsters respond to the parasite by encapsulating haemocytes in an attempt to partition off damaged gill areas, and Wootton et al. (2011)[4] hypothesized that this response may impair gill respiratory function. Various haemolymph factors can provide a general health indicator of crustaceans.[10] For example, haemolymph levels of hyperglycaemic hormone, proteins, haemocyanin levels, nitrogenous wastes (including ammonia, urea and uric acid) and glucose change in response to environmental stressors such as temperature, salinity and hypoxia, as well as to physiological stressors such as moult stage and exogenous stressors such as temperature, handling, disease and parasitization. The data suggest that lobsters with gills damaged by the feeding activities of N. astaci respond by producing higher levels of haemocyanin, which is both a key defence response and may compensate for their decreased respiratory functioning (i.e. to bolster the oxygen transport capability of haemolymph).

The most important moulting season of the lobster in Scottish waters occurs between May and August. Male lobster cast their exoskeleton every year while female cast their exoskeleton every other year. It is important for Nicothoe astaci to enter the gills while the exoskeleton is soft right after the moult. Any further hardening of the exoskeleton prevents female Nicothoe astaci from attaching to the gills. If the gills are soft, the parasite is able to pierce its mandibles on the gills to attach. This enables the female to feed on the host's blood which leads to the formation and growth of their wings.[8]