Thomas A. Schram

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.

Studies on the meroplankton in the inner Oslofjord I. Composition of the plankton at Nakkholmen during a whole year

Abstract The meroplankton at Nakkholmen in the inner Oslofjord has been studied during one annual cycle (December 1963–December 1964). Each of the 70 samples taken consisted of the larvae from 1000 litres of sea water delivered by a suction pump and filtered through a 125 μ plankton net. In total, these samples contained about 182,000 larvae belonging to 54 species (polychaetes: 27 species, bivalves: 16 species, gastropods: 6 species, echinoderms: 5 species). For almost the whole year the meroplankton was dominated by larvae of polychaetes which constituted 90.6 % of the total number of larvae. In particular Polydora ciliata and P. antennata were found in enormous numbers. Thus, about 78 % of the total number of larvae were identified as P. ciliata, followed in frequency by larvae of Mytilus edulis (8.3 %) and of Polydora antennata (6.7 %). The number of echinoderm and gastropod larvae were negligible. The observed composition of the larval plankton agrees well with that from the southern part of the Cope...

Intra- or inter-specific difference in genotypes of Caligus elongatus Nordmann 1832?

Two mitochondrial and one nuclear genetic marker were used to study the phylogenetic position of the two reported CO1-genotypes of Caligus elongatus in a group of closely related caligid parasites. Molecular analysis of the two mitochondrial genes (CO1 and 16S), indicate genetic distances of the two C. elongatus genotypes in the lower range of distances previously reported between other crustacean species, but higher than comparable reported within-species differences. Analyses of nuclear 18S sequences indicate no detectable differentiation between these genotypes, but may be due to expected differences in the resolution of these genetic markers. Investigation of two of three selected morphological characters reveals phenotypes supporting the division based on the molecular division. The species status on the two C. elongatus genotypes cannot be drawn conclusively, although the molecular and morphological data presented here suggests the presence of sibling species.

A systematic review of the Oslofjord species of Polydora Bosc and Pseudopolydora Czerniavsky, with some new biological and ecological data (Polychaeta: Spionidae)

Polydorids are an important component of both the bottom fauna and the plankton of the origanically enriched Oslofjord. Some species have an opportunistic life pattern and morphological variation within species is known, but their systematic relationships have not been solved. To clarify the situation those adults which are known from the inner Oslofjord, i.e. Polydora ciliata (Johnston), P. ligni Webster, P. socialis (Schmarda), P. caulleryi Mesnil, and Pseudopolydora paucibranchiata Pseudopolydora paucibranchiata (Okuda) are described and illustrated. Reproductive and ecological data which are of importance for an understanding of life histories, are included. A procedure for identification of polydorids is recommended, stressing the importance of working with living animals. There is a key to adults of the species known from the area, including those which have only been found as larvae. Variations in different morphological characteristics of Polydora ciliata are described. Data supporting the opinion...

On the Enigmatical larva nauplius y type I Hansen

Abstract Eighty-five nauplius y I larvae were collected from different localities in the inner part of Oslofjorden. Almost all larvae were caught within the first six months of the year. Furthermore, about 81 % of the larvae, for which the approximate vertical distribution is known, were collected below the discontinuity layer. Nauplius I larvae are being both transported into the inner Oslo-fjord by currents that havc passed the Drobak sill (19.5 m), and are also being hatched in areas where the adults must have been already established. The length of the carapace of 66 nauplius I (processes excluded) varied from 247 μ to 450 μ, the mean length being 284 μ. Older larvae have a more slender form than the younger ones. Selected specimens, representing different size and developmental stages, are illustrated, described, and compared with the y larvae described by Hansen. It is concluded that the four specimens from the Atlantic Ocean, representing Hansens nauplius I, II, and, III, must be referred to naupl...

Ectoparasites on northeast Atlantic mackerel (Scomber scombrus L.) from western and north sea stocks

Mackerel caught in the North Sea (130), Skagerrak (148) and southwest of Ireland (176) were examined for ectoparasites. The most common parasites were Kuhnia scombri and K. sprostonae (Monogenea), ...

MALE MATE CHOICE IN A NATURAL POPULATION OF THE PARASITIC COPEPOD LERNAEOCERA BRANCHIALIS (COPEPODA: PENNELLIDAE)

Variations in age and sex structure of a natural population of the copepod Lernaeocera branchialis, parasitic on flounder, Platichthys flesus were examined in a 15-month study. Recent laboratory studies and literature on reproductive strategies indicate that male mate choice in this species should depend on the sex ratios of the parasite on its hosts, and on the age and mating status of females. Sex ratios suggested a strong intra-male competition for females. The ratio of chalimus 4 and virgin adult females (preferred stages) to adult males exceeded 1 female: 4 males on 50% of the hosts. In four out of the six sampling periods, more than 50% of hosts harboured more adult males than the total number of females. The males preference for the different female stages was estimated from the number of precopula and copula associations. The data were fitted to a logistic regression model. At most sex ratios, males preferred chalimus 4 and virgin adult females, and discriminated against younger stages. Mated females were about as attractive as the youngest larval stages at female-biased sex ratios, but they were chosen more frequently at strongly male-biased sex ratios. Most adult L. branchialis females had copulated more than once, and some had accommodated at least 5 ejaculates in the their sperm storage organs. The patterns of mate guarding and potential for sperm competition strongly suggest that L. branchialis males take the intrasexual competition into account when choosing mates. On this basis, they minimize guarding time and maximize the possibility of paternity.

Ectoparasites on the Atlantic halibut, Hippoglossus hippoglossus (L.), from northern Norway — potential pests in halibut aquaculture

Abstract Adult (174) and immature (72) wild halibut were examined for skin parasites. Immature fish did not carry parasites. The prevalence of the caligid Lepeophtheirus hippoglossi infection at the two localities, Soroysund and Malangen, was 78 and 66 % respectively. There was an average of 1 male cope pod per halibut, 3–4 female parasites on male and 5–6 on female hosts. The size of the copepods from the two localities was not significantly different. Of the cope pods from Soroysund and Malangen 60 and 56 %, respectively, were egg-producing females, 23 and 25 % juvenile females, and 17 and 19 % males. The morphology of the sternal furca and genital complex is described and illustrated. There are clear sex differences in size and shape. The monogenean Udonella caligorum was found as commensal mainly on L. hippoglossi from Soroysund. No less than 50-60 % of the halibut from Soroysund and Malangen were infected by the monogenean Entobdella hippoglossi. Marc females than males were parasitized. Three times ...

Further records of nauplius y type IV Hansen from Scandinavian waters

Abstract Nauplius y IV larvae of different size and developmental stages from Oslofjorden are illustrated, described, and compared with y IV larvae described by Hansen (1899) and others. The material includes both shorter and longer y IV larvae than have been recorded previously and the longer y IV larvae possessed a longer carapace than that of the oldest nauplius I on record. Although the known number of nauplius I larvae from Oslofjorden is much higher than that of nauplius IV the latter type seems to be more abundant outside the fjord. Nauplius IV larvae also have a much wider geographical distribution, in fact amphi-Atlantic. A summary of this distribution is revealed, as well as an outline of misconceptions in ealier literature. Nauplius I and IV represent two different species which have been inaccurately treated in the literature. The parental forms of the y larvae are probably close to the Cirripedia, but the final answer cannot be given at present.

14. Cypris y, a later developmental stage of nauplius y Hansen

Abstract Four specimens of cypris y caught pelagically in April and May 1967 in the Bahamas are described and figured. They are compared with the single cypris from the Sound (Oresund, Denmark) described by Bresciani (1965) and with the last developmental stages found inside metanauplius y larvae. The larvae from the Bahamas differ somewhat from the Sound specimen, but the gross morphology is the same, and all five specimens known are certainly cypris forms representing the last stage of the nauplius y described by Hansen (1899). The connection between the types of nauplius y larva and the known cypris is at present an unsolved problem, as well as the systematical placing of the y larva. The latter question is discussed and although the larvae from the Bahamas show similarities to the preimaginal stages of Cirripedia and Ascothoracida, different features seem to be specific for the larvae in question. It is concluded that the relationship of the Bahama larva to either of the systematical groups mentioned,...