Peter Andreas Heuch

Salinity and temperature effects on sea lice over-wintering on sea trout ( Salmo trutta ) in coastal areas of the Skagerrak

Both the salmon louse Lepeophtheirus salmonis and its close relative Caligus elongatus are ectoparasitic on Salmonidae in salt water in the northern hemisphere. In this study we monitored population dynamics of these parasites on anadromous brown trout, i.e. sea trout, on the Norwegian Skagerrak coast in the winters of 1998-1999 and 1999-2000. The low salinity was expected to reduce sea lice populations as lice do not tolerate sojourns of more than a few weeks, at most, in freshwater. Results confirmed the presence of both parasite species on estuarine sea trout in winter, and showed that the lice populations go through a bottleneck in this period. Prevalences of infection of both parasite species were very different in the two sampling periods, but fell below 10% in March in both winters. Median infection intensity was 1-2 fish -1 . Salinity was statistically related to the presence of C. elongatus both winters, and to L. salmonis in 1999-2000. Temperature appeared to be less important for the abundance of lice.

Temporal and spatial variations in lice numbers on salmon farms in the Hardanger fjord 2004–06

The long and narrow Hardanger fjord in western Norway has a high density of salmon farms and has had severe salmon lice, Lepeophtheirus salmonis, problems. In the years 2004-06, salmon lice numbers were recorded in selected salmon farms in the fjord as part of a larger research project. Most farm sites participated in a strategic control programme and were deloused between November and January in each year. The aim of the programme was to achieve a mean abundance of <0.3 adult female lice at this time and to minimize the infection pressure on wild smolts in the spring. Dedicated teams carried out detailed counting of lice on farmed fish in April-September each year. Temperature conditions were fairly similar throughout the fjord and amongst years, but wide variations in salinities were observed. The two innermost zones, B and C, had the lowest lice mean abundances, whereas the outermost zones, D and E, consistently had more lice. General linear model analyses showed that differences in adult female lice abundance between the zones were associated with differing levels of salinity and emamectin benzoate treatments strategically administered. Mean fish weight was significantly positively correlated with mean abundance of adult female lice.

The egg string attachment mechanism of selected pennellid copepods

The anatomy of the hook apparatus that attaches egg strings to the fish parasites Haemobaphes cyclopterina, Lernaeocera branchialis, Lernaeocera lusci, Lernaeenicus sprattae, Sarcotretes scopeli and Pennella balaenoptera (Copepoda: Pennellida) is described and illustrated. The hook rises from a cupulate base, extending posteriorly and anteriorly in the body cavity. The suspension of the apparatus in the trunk of the different species differs, but the function is similar. The hook tip enters the genital antrum, nearly penetrates the proximal end of the egg string, and continues into a notch on the antrum wall. The apex of the egg string acquires a concave depression like the finger end of a glove. In this way the string is mechanically attached inside the female genital segment. The mobile ectoparasites Lepeophtheirus salmonis and Hatschekia hippoglossi have hooks which function similarly, but perforate the strings.

A special issue on parasitic crustaceans.

The parasitic mode of life appears to be extraordinarily successful (e.g., Schmidt and Roberts, 1985); however, not all groups of parasites are equally well represented in the literature. This appears to be true even within journals devoted to parasitology. We humans tend to study most intensively what is readily available to us, or is affecting us directly or indirectly through economic costs. This may be why papers concerning parasites which occur in our own terrestrial habitat are most prevalent in parasitology journals. Most of the surface of the earth is covered with water, making the sea the largest biome on earth: 1347 million km of open water (Huys and Boxshall, 1991). Fresh and brackish water habitats come in addition. One particularly rich group of aquatic organisms is the crustaceans. A recent estimate suggests that there are more than 51,000 species in the subphylum Crustacea, and the known species have been arranged into six classes including 849 families (Martin and Davis, 2001). Several entire subclasses are parasitic, such as the Branchiura and the Tantulocarida. In this special issue we attempt to show some of the amazing morphological and ecological variation of the parasitic crustaceans. The differences in size and form are incredible, both within freeliving and parasitic crustaceans. Many parasitic crustaceans are hardly possible to recognize as crustaceans. Their English vernacular names sometimes suggest they were taken to be worms, e.g., the cod gill worm Lernaeocera branchialis (Copepoda) and eye-maggot of the sprat Lernaeenicus sprattae (Copepoda) (Schram, 1979), or insects e.g., gill lice Salmincola spp. (Copepoda) and carp lice Argulus spp. (Branchiura). Some crustaceans are parasitic on other crustaceans. Representatives from three groups are treated in this issue; cirripeds (barnacles), tantulocaridans, and isopods (relatives of woodlice and pill bugs). The parasitic barnacles (Rhizocephala) are one of the most transformed crustaceans, and some of the larger parasites. In its adult form, a female Rhizocephalan may fill up much of the shell of its host, at the expense of host internal organs. In addition the parasite makes an ‘‘externa”. This is a sac protruding from the host’s ventral side, from which parasite larvae are released. In this issue, Glenner and co-workers show the how variations in the complex life cycle of these parasites contribute to the understanding of their existing, genetically derived, phylogeny. On the other side of the size spectrum is the tantulocaridan Microdajus tschesunovi n. sp., described by Karbasov and Savchenko (in this issue). The adult male of this species attains 460 lm total length, and is probably one of the smallest parasitic crustaceans. The marine isopods have a high representation of parasitic taxa. The relationship between the isopod and its ‘‘host” can be anything from a fully obligate (adult found only in association with host) to a temporary association involving larvae or adults which consume host tissue. In the latter case the parasite may be called a ‘‘micro-