J.T.A. Dick

Widespread vertical transmission and associated host sex ratio distortion within the eukaryotic phylum Microspora

Vertical transmission (VT) and associated manipulation of host reproduction are widely reported among prokaryotic endosymbionts. Here, we present evidence for widespread use of VT and associated sex–ratio distortion in a eukaryotic phylum. The Microspora are an unusual and diverse group of eukaryotic parasites that infect all animal phyla. Following our initial description of a microsporidian that feminizes its crustacean host, we survey the diversity and distribution of VT within the Microspora. We find that vertically transmitted microsporidia are ubiquitous in the amphipod hosts sampled and that they are also diverse, with 11 species of microsporidia detected within 16 host species. We found that infections were more common in females than males, suggesting that host sex–ratio distortion occurs in five out of eight parasite species tested. Phylogenetic reconstruction demonstrates that VT occurs in all major lineages of the phylum Microspora and that sex–ratio distorters are found on multiple branches of the phylogenetic tree. We propose that VT is either an ancestral trait or evolves with peculiar frequency in this phylum. If the association observed here between VT and host sex–ratio distortion holds true across other host taxa, these eukaryotic parasites may join the bacterial endosymbionts in their importance as sex–ratio distorters.

Probing motivational state during agonistic encounters in animals

Animals commonly compete for resources by direct aggression: for example, spiders fight for web sites, male red deer fight for females, and scorpionflies fight for prey. The application of game theory has considerably advanced our understanding of the evolution of such contests. A general conclusion is that, if possible, animals should assess both the relative fighting abilities and the value of resources before making tactical decisions during contests. These tactical decisions are assumed to be mediated by differing motivational state,, but this fundamental assumption has yet to be tested. Here we test the accumulated theory by probing the motivational state of hermit crabs during fights over the ownership of gastropod shells. The test uses a stimulus, novel to the crabs, that produces a startle response, the duration of which is an independent measure of the motivation to fight. We demonstrate that motivational state differs at an early stage of the contest according to the potential gain in resource value. There was no effect of relative size of the opponent on motivational state. In these contests, relative size neither predicted the likely cost of the contest nor the probability of victory.

Analysis of repeated signals during shell fights in the hermit crab Pagurus bernhardus.

Shell exchanges between hermit crabs may occur after a period of shell rapping, when the initiating or attacking crab brings its shell rapidly and repeatedly into contact with the shell of the non–initiator or defender, in a series of bouts. There are two opposing models of hermit crab shell exchange and the function of shell rapping. The negotiation model views shell exchange as a mutualistic activity, in which the initiator supplies information about the quality of its shell via the fundamental frequency of the rapping sound. The aggression model views shell rapping as either detrimental to the defending crab, or as providing it with information about the initiators ability or motivation to continue, or both. The negotiation model makes no predictions about the temporal pattern of rapping, but under the aggression model it would be expected that crabs that rapped more vigorously would be more likely to effect an exchange. Repeating the signal could be expected under either model. Crabs that achieve an exchange rap more vigorously, rapping is more persistent when a clear gain in shell quality may be achieved, and the vigour is greater when the relative resource–holding potential (or ‘fighting ability’) is high. These findings support the aggression model rather than the negotiation model. Contrary to the predictions of game theory, crabs that do not effect an exchange appear to signal that they are about to give up. The data suggest that rapping is performed repeatedly because the accumulation of all of the performances acts as a signal of stamina.

Consequences of shell-species preferences for female reproductive success in the hermit crab Pagurus bernhardus

Littoral hermit crabs, Pagurus bernhardus, show a strong preference for Littorina obtusata shells rather than those of Gibbula species. The fitness consequences, in terms of fecundity, for this shell preference is examined for female crabs. Females in the preferred species produced eggs earlier in the season, produced more eggs in the first brood, and produced a second brood more often than did females in the less preferred species. The smaller brood for Gibbula spp. was not a consequence of egg loss from the pleopods due to an unfavourable shape of shell, but rather reflected lower egg production. It is not clear, however, if this differential reproduction is due to direct costs of carrying an unfavourable shell, i.e. the shell impedes reproduction, or whether crabs compete aggressively for favoured shells so that only crabs of low quality inhabit lowquality shells.

Potential roles for differential body size and microhabitat complexity in mediating biotic interactions within invasive freshwater amphipod assemblages.

Cannibalism and intraguild predation (IGP) are common amongst freshwater amphipod crustacean as- wsemblages, particularly between individuals of different body size, with IGP of smaller by larger species. The decline of Gammarus tigrinus populations in mainland Europe has been accompanied by the arrival of the Ponto- Caspian invader Dikerogammarus villosus and previous studies have implicated IGP of G. tigrinus by the larger D. villosus, as the principal driving force in this replacement. We examined how factors such as microhabitat and body size may mediate both cannibalism within G. tigrinus populations and IGP by D. villosus and thus contribute to fi eld patterns of coexistence and exclusion. A fi eld survey of an invaded Dutch lake indicated that G. tigrinus and D. villosus differed in distribution, with D. villosus being the numerically dominant amphipod (80-96 %) on the rocky boulder substrate of the shoreline and G. tigrinus being the dominant amphipod (100 %) in the crushed shell/sand matrix immediately adjacent to this. Laboratory microcosm experiments indicated that G. tigrinus cannibalism, particularly of smaller by larger size classes, may be common. In addition, although D. villosus predation of all G. tigrinus size classes was extreme, the smallest size classes suffered the highest predation. Indeed, when exposed to D. villosus, predation of larger G. tigrinus was lowest when smaller G. tigrinus were also present. Increasing mi- crohabitat complexity from a simple bare substrate littered with Dreissena polymorpha zebra mussels to a crushed shell/sand matrix signifi cantly reduced both cannibalism and IGP. Our study emphasizes the need to consider both life history stages and habitat template, when considering the impacts of biotic interactions and it also emphasizes that complex, interacting factors may be mediating the range expansion of D. villosus.

Coexistence and exclusion among Gammarus species: behavioural avoidance of interspecific precopulation by male G. pulex (Amphipoda).

The significance of interspecific mate choice behaviour in explaining distributional patterns of Gammarus species is investigated. Male G. pulex from a pure population and from mixed G. pulex/G. duebeni populations were given a choice between G. pulex and G. duebeni females. Males discriminate between conspecific and congeneric females during grabbing and holding of the females, and this behaviour increases in frequency towards G. duebeni females the closer those females are to moulting. Male G. pulex with no prior experience of congenerics appear to require a more thorough period of assessment in order to determine species

Ecological impacts of the microsporidian parasite Pleistophora mulleri on its freshwater amphipod host Gammarus duebeni celticus

The microsporidian parasite, Pleistophora mulleri, infects the abdominal muscle of the freshwater amphipod Gammarus duebeni celticus. We recently showed that P. mulleri infection was associated with G. d. celticus hosts being more vulnerable to predation by the invasive amphipod Gammarus pulex. Parasitized G. d. celticus also had a reduced ability to prey upon other co-occurring amphipods. We suggested the parasite may have pervasive influences on host ecology and behaviour. Here, we examine the association between P. mulleri parasitism and parameters influencing individual host fitness, behaviour and interspecific interactions. We also investigate the relationship between parasite prevalence and host population structure in the field. In our G. d. celticus study population, P. mulleri prevalence was strongly seasonal, ranging from 8.5% in summer to 44.9% in winter. The relative abundance of hosts with the heaviest parasite burden increased during summer, which coincided with high host mortality, suggesting that parasitism may regulate host abundance to some degree. Females were more likely to be parasitized than males and parasitized males were paired with smaller females than unparasitized males. Parasitism was associated with reduction in the hosts activity level and reduced both its predation on the isopod Asellus aquaticus and aggression towards precopula pairs of the invasive G. pulex. We discuss the pervasive influence of this parasite on the ecology of its host.

Survival of juvenile European eels (Anguilla anguilla) transferred among salinities, and developmental shifts in their salinity preference

After their oceanic migration, juvenile European eels Anguilla anguilla enter estuaries as glass eels, develop into pigmented elvers and migrate into fresh water. Fisheries often transfer such eels abruptly between salinities, principally glass eels and elvers from estuarine to fresh water. It is usually assumed that survival rates are high, but this required systematic investigation. Survival was found to be 100% over 21 days of glass eels and semi-pigmented elvers transferred abruptly from estuary conditions into fresh water, 50% sea water and full sea water. Fully pigmented elvers, however, showed significantly reduced survival when transferred into sea water. Salinity preference experiments with juvenile eels have historically been inconclusive. Here, in a choice chamber design, a clear developmental shift in salinity preference was found, with glass eels preferring 100% sea water, semi-pigmented elvers showing no clear preference and fully pigmented elvers preferring fresh water. We conclude that eel fisheries enhancement by abrupt transfer of juveniles among salinities is largely vindicated. In addition, developmental shifts in salinity preference have been clarified and this aids in the interpretation of eel migration patterns.

A preliminary assessment of the distribution of the sea lamprey (Petromyzon marinus L.), river lamprey (Lampetra fluviatilis (L.)) and brook lamprey (Lampetra planeri (Bloch)) in Northern Ireland.

Lampreys are endangered in Europe, and European states are legally required to take measures to ensure their protection. However, there is currendy little information on the distribution of the three species present in Northern Ireland. Anecdotal records of adult lampreys were collated from anglers and other sources, and a systematic electrofishing survey was undertaken to establish the distribution of lamprey ammocoetes. Lampreys were found in seven of the nine Northern Irish river catchments. Brook lampreys (Lampetra planen (Bloch)) were widely distributed, but the two anadromous species, sea lamprey (Petromyzon marinus L.) and river lamprey (Lampetra fluviatilis (L.)), were more limited in their distribution, possibly due to barriers restricting migration. CE. Goodwin (corresponding author; email: claire.goodwin? gmail.com), Ulster Museum, National Museums Northern Ireland, 153 Bangor Road, Holywood, Co. Down, BT18 0EU; J.T.A. Dick, R.W. Elwood, School of Biological Sciences, Medical Biology Centre, Queens University Belfast, 97 Lisburn Road, Belfast, Co. Antrim, BT9 7BL.