Volume 14

Energetic costs of group-living? A reversed “group effect” in shoaling minnows (Phoxinus phoxinus)

Group-living provides major benefits to the individual group members; e.g., teleost fishes often form shoals to reduce piscine and avian predation risk. A number of studies reported that shoal members can have a calming effect on individual fish, whereby individuals show typical stress responses (like increased oxygen consumption) in a lone compared to a grouped situation (the so-called “group effect”). We tested whether European minnows (Phoxinus phoxinus) also show such a group effect by comparing oxygen consumption (via respirometry) of individuals in a lone or grouped situation. Surprisingly, focal fish showed higher metabolic rates when a group was presented during respirometric measurements. Two alternative explanations for this effect seem possible: (a) presence of the group per se stressed the focal fish, or (b) being restricted to the respiratory chamber focal fish were stressed as they could not join the group. We predicted that, if hypothesis (b) was true, fish should show reduced metabolic rates when allowed to interact freely with a group, but should show increased metabolic rates if hypothesis (a) was true. As measuring individual oxygen consumption is impossible in free-ranging fish, we measured gill ventilation frequencies as a proxy of metabolic rates in a second experiment where focal fish were tested either alone, within a group, or, like in experiment one, with visual perception of a group. Again, metabolic rates were increased when focal fish could see a group nearby, but the increase was even higher when they could interact freely with the group. Thus, presence of the group per se might indeed elicit a stress response, and we tentatively argue this is due to costs arising from increasing competition among shoal members.

Ecology of host-parasite relationships in the brackish SW Baltic Sea, with special regard of small-size fish – a review

This review examines the relationships of parasites to hosts and host communities in a brackish water environment. After six local parasite communities have been investigated at the SW coast of the Baltic, the existence of a regional parasite fauna became evident. Density of parasites and hosts were examined by analysis of supra-populations. The investigation of supra-communities revealed the existence of core and satellite species, which vary between single hosts. Specifity of several parasite species became obvious by comparison of several host species of a guild. The numbers of specialist and generalist parasites within guild communities may be used as indicator for the quality of the environment. Prevalence, which reveals the range of microhabitats occupied by parasites in diverse hosts, is a characteristic measure of component populations. Component communities describe the diversity of parasites in host populations, which also allows conclusions on the quality of the environment. The calculation of the intensity of infra-populations may indicate zero, normal or massive parasite infections; the last case may be caused either by high infection of the intermediate hosts, accumulation or special host-parasite-relationships. The infra-community is a measure for parasite capacity and richness in a single host. The calculation of an infra-community index makes affi nities and competition between parasites evident. The combined results are the basis of a model, in which colonizing parasites were filtered by genetic, phylogenetic and ecological distances. The respective strengths of two environmental stressors in the Baltic Sea, brackish water and eutrophication, can influence colonization of parasites and parasite composition in different localities.

Parasite community and heavy metal contents of roach (Rutilus rutilus L.) from different freshwater habitats in North Rhine-Westphalia, Germany

The parasite fauna and heavy metal contents of roach, Rutilus rutilus L., was studied from four different freshwater habitats in North Rhine-Westphalia. A total of 115 roach were sampled from Lake Dörpfeld, Lake Masuren, River Ruhr and Diergardt’s side canal from April to August 2007, revealing 19 generalistic parasite species belonging to the Trichodina (2), Sporozoa (1), Myxozoa (3), Digenea (2), Monogenea (2), Cestoda (2), Nematoda (3), Acanthocephala (2) and Crustacea (2). The most diverse component community was found in the River Ruhr (twelve species) and the lowest in Lake Masuren (seven species). The infracommunity was highest in the River Ruhr (3.7) and lowest in the Diergardt’s side canal (2.1). The two helminths Diplostomum spathaceum (Rudolphi, 1819) (Digenea) and Dactylogyrus sp. (Monogenea) were the only ubiquitary species. Acanthocephalans were recorded only from the River Ruhr, while the cestodes and the nematode Philometra rischta (Skrjabin, 1917) occurred only in the lakes. Ten roach from each sampling site were analysed for their heavy metal contents in muscles and kidney. Highest values in muscles were recorded from the River Ruhr and lowest from Lake Dörpfeld. The heavy metal concentration in the kidney was, in most cases, higher compared to that of the muscle. Especially high nickel contents were found in the muscle of fishes from River Ruhr and high cadmium contents in the kidney from Lake Masuren. A positive correlation was found between the heavy metal content and the level of infestation with trichodinid ectoparasites. Similarly, the size of the water bodies and the connectivity to nearby lakes and rivers enforced a higher level of infestation with the recorded fish parasites.

Pigment cell retention in cavernicolous populations of Poecilia mexicana (Poeciliidae)

The Atlantic molly (Poecilia mexicana) is a small livebearing fish that has colonized two cave systems in the southern state of Tabasco, Mexico. Unlike many obligate cave-dwellers (i.e. troglobites) all cave P. mexicana retain some pigmentation, as well as a functional visual system. In the Cueva del Azufre the fish occupy habitats (i.e. cave chambers) that differ along a gradient of hydrogen sulfide (H₂S) concentration, as well as a patchwork of light exposure due to several large skylights. While the relationship between eye size and opsin expression with cave distance has been explored, the extent of differences in pigmentation has not yet been quantitatively evaluated. In this study we compared pigment cell (melanophore and xanthophore) count in wild-caught fish from one surface population (Arroyo Bonita) and two cave populations: chambers V (featuring a skylight) and chamber X (exists in perpetual darkness) of the Cueva del Azufre over a 120 day period. Surface populations had significantly higher total numbers of pigment cells than both cave populations, which did not differ significantly from each other. We speculate that skylights in the Cueva del Azufre, paired with a recent evolutionary origin of the cave population and genetic homogenization, have allowed for trait maintenance in cavernicolous P. mexicana.

Food finding ability in several molly species (Poecilia spp.), including the unisexual Amazon molly

Unisexual Amazon mollies (Poecilia formosa) and their bisexual host species are remarkably similar in most aspects of their ecology. Here we studied one aspect of feeding ecology, food finding ability, in several clonal lineages of Amazon mollies (including triploids), two of their hosts and a laboratory produced F1. We found only minor differences between the studied species and lineages; most notably the triploids were able to locate food faster than the sexual Poecilia mexicana. Our findings are in agreement with several other studies all showing that niche overlap in this species complex is very high and likely has to do with the unusual reproductive mode of Amazon mollies, gynogenesis.

Distinctive individual marks in Red Sea wrasses (Labridae)

Digitale Unterwasser-Fotos von Lippfischen aus dem Roten Meer belegen gut sichtbare individuelle Zeichnungsmuster vor allem im Kopfbereich von Cheilinus lunulatus (ein gelbes Zeichen am oberen Kiemendeckel), Cheilinus abujubbe (radiäre Augenstreifen) und Thalassoma rueppellii (grüne Kopfstreifen). Laminierte Porträts dieser Indivduen können bei späteren Tauchgängen mitgeführt werden und erlauben eine eindeutige Identifizierung, die z.B Verhaltensstudien dieser revierbildenden und in Harems lebenden Fische erleichtern könnte.

Partnerschaften versus Parasitismus bei Fischen

[Partnership vs. parasitism in fishes]

The different categories of partnership between organisms are divided according to advantage or disadvantage of the partners into mutualism (advantage for both partners), commensalism (advantage for one, no advantage or disadvantage for the other partner) and parasitism (advantage for the parasite, disadvantage for the host). Parasitism may have been evolved in partnerships of fishes from commensalism with stinging invertebrate like jelly-fish and sea urchins and from mutualism between cleaner-fish. But, if additional energy consumption is regarded, which needs one of the partner in order to maintain the partnership also some of the symbiotic relationship must be classified as parasitism. This situation is profited by the sand dwelling gobies, the remoras, the anemone fishes and the followers of burrowing goat fish. It is proposed to call the classical, obvious parasitism eu-parasitism, and those partnerships that are not obvious and based on energy exploitation, crypto-parasitism.