The life-cycle of the flatfish nematode Cucullanus heterochrous.

Mature specimens of Cucullanus heterochrous Rudolphi, 1802 (Nematoda: Cucullanidae) were obtained from the intestine of the flounder, Platichthys flesus, from Danish waters. Eggs embryonate in seawater but do not hatch. Fully developed larvae pressed out of eggs are 430 microm long with amphids and dereids and enclosed within the cuticle of a previous larval stage. Infective larvae are believed to be in their third stage. Experimental studies showed that the polychaetes, Nereis spp., Scoloplos armiger, Brada villosa and Capitella sp., may act as intermediate hosts. In N. diversicolor the larvae increase their length to 1 mm within four weeks (15 degrees C) without moulting. Experimental infections showed that larvated eggs are not infective to fish, whereas >550 microm long larvae from polychaetes survived in 4-24 cm long flounders and plaice, Pleuronectes platessa. Third-stage larvae 550 microm to 1.1 mm long were found in the submucosa of the intestine one week post infection. At a length of about 800 microm to 1.4 mm they moult to fourth-stage larvae. Fourth-stage larvae, immature and mature worms occur in the intestine and rectum. Fourth-stage larvae and adults survived experimental transfer from one flounder to another. Similar developmental stages survived for two weeks in the intestine of experimentally infected cod, Gadus morhua.