Parasites in larvae of the herring (Clupea harengus L.) fed with wild plankton

In winter 1965/66 larvae of Downsherring were reared in aquaria at the Marine Station of the Biologische Anstalt Helgoland. They were fed with wild plankton caught on Helgoland Roads. About 10% of the actively feeding larvae were lost due to two endo-parasites and two ecto-parasites. The endoparasites are: a nematode (Contracoecum sp.) which is taken in by the larvae with the food and enters the host's body cavity from the gut, and a cestode (Scolex pleuronectis) which is found in the posterior part of the gut. The ecto-parasites are: a copepodite stage of a lernaeocerid, which attaches itself preferably close to the anus area of the herring larvae, and the copepode Caligus rapax. The nematode and copepodite were observed when the larvae had reached a total length of 9 to 13 mm. The cestode was found for the first time in larvae of 15 to 18 mm, and Caligus rapax in larvae of 20 to 25 mm total length.