Małgorzata Poznańska

Effect of substratum drying on the survival and migrations of Ponto-Caspian and native gammarids (Crustacea: Amphipoda)

Organisms inhabiting shallow near-shore waters are at risk of desiccation during water level fluctuations. Using laboratory experiments, we investigated the survival and behavioural defences of four freshwater amphipod species during substratum drying: three Ponto-Caspian invaders (Pontogammarus robustoides, Dikerogammarus haemobaphes and Dikerogammarus villosus) and the native Gammarus fossarum. We hypothesized that they would be able to survive air exposure events as well as to adjust their behaviour by following the decreasing water level and/or burying in the sediments. To test these hypotheses, we examined survival of each species on gradually drying sandy substratum as well as their horizontal and vertical migration behaviours. P. robustoides was most resistant to substratum drying and was the only species burying into the substratum. On the other hand, G. fossarum exhibited distinct horizontal migrations following the retreating waterline. These two species seem to be particularly well adapted to the drying environment. Defence mechanisms of D. haemobaphes and D. villosus were less efficient, though the former species also followed the retreating waterline to some extent. Our study demonstrates that exotic and native gammarids have several adaptations that enable them to invade and persist in habitats experiencing common water level fluctuations.

The role of solid waste materials as habitats for macroinvertebrates in a lowland dam reservoir

Because of the increasing anthropogenic pressure, the amount of solid waste discarded in water is growing considerably. It can affect aquatic organisms, modifying their distribution and abundance. This study was designed to investigate the effect of several types of solid waste (bottles, textiles, bundles of string, pieces of polythene and rubber) upon the taxonomic composition and biodiversity of macroinvertebrates inhabiting the near-shore zone (up to 1-m depth) of the Włocławek Reservoir (the River Vistula, central Poland). The epifauna developing on these materials was compared with organisms living on natural substrata: bare sandy bottom (from which the waste was collected), macrophytes and sandy bottom overgrown by plants. The communities inhabiting the artificial substrata (except strings) were much more diverse (in terms of taxon richness, Shannon–Wiener index and evenness) than the surrounding bare sandy bottom. The biodiversity on the waste was similar to that found on the macrophytes and bottom among them, located nearby. The most suitable materials appeared to be bottles, providing shelters in their interiors. The artificial substrata differed considerably from all natural habitats with respect to taxonomic composition, though they were more similar to the macrophytes than to the bottom sediments. Furthermore, the variability among the assemblages developing on the particular waste objects, even of the same type, was much larger than that observed on the macrophytes, indicating the patchy nature of the former habitat. Thus, discarded waste can constitute alternative habitats for invertebrates, especially when natural substrata are unsuitable (e.g. sandy bottom), but they cannot fully replace natural substrata, such as plants, in their habitat-forming role in ecosystems.

Macrozoobenthos communities from two types of land-water transition zones in a European lowland dam reservoir.

We investigated the abundance, diversity and taxonomic composition of macrozoobenthos from the land-water interfaces of a lowland dam reservoir developing on sandy and organic-rich substrata. We also checked the possibilities of the occurrence of the edge effect consisting in increased abundance and diversity of the commu- nity developing in the transition zone between two habitats, as well as in the presence of taxa specifi c for only that zone. The study area was the middle part of the Wocawek Reservoir (the lower Vistula River, central Poland). In 2002-2003, we sampled air-exposed (above the water line, at the water line) and submerged (0.5 m and 1 m depth) sites from a sandy area situated in the fl ooded part of the reservoir and a small cove overgrown with macrophytes, with sediments rich in organic matter. In the organic-rich sediments, biodiversity and abundance were higher at the exposed sites than at the submerged ones. Furthermore, we found numerous specifi c taxa, which occurred only in the land-water interface of the organic-rich habitat (semi-terrestrial Diptera, terrestrial and amphibious Oligo- chaeta, Endochironomus sp. (impar group), Valvata cristata, Anisus contortus and Coleoptera larvae). Thus, all symptoms of the edge effect were observed there. On the other hand, the sandy shore was a simple transition zone. Some symptoms of the edge effect appeared (high biodiversity and the presence of a few amphibious taxa) but the abundance was extremely low compared with that found at the submerged sites. Individuals inhabiting the sandy transition zone formed accidental assemblages, widely differing from one sample to another. The homogeneous unstable land-water interface on the sandy shore is a poor habitat for macroinvertebrates, which are washed out by wave action, have no protection against air exposure and freezing, and their substratum may be totally destroyed in winter by ice drifting downstream. On the other hand, organic-rich sediments are more stable and provide much better protection and food conditions for the fauna living near the water line.

The impact of zebra mussel (Dreissena polymorpha) periostracum and biofilm cues on habitat selection by a Ponto-Caspian amphipod Dikerogammarus haemobaphes

Dikerogammarus haemobaphes is one of several Ponto-Caspian gammarids invading Europe in recent decades. Previously, it exhibited active preferences for habitats associated with another Ponto-Caspian alien, zebra mussel. Now we tested gammarid preferences for living mussels and their empty shells with biofilm and/or periostracum present or absent, to find the exact cues driving gammarid responses. We observed a strong preference of gammarids for biofilmed shells, even if the biofilm was relatively young (2-day old). However, the biofilm quality, related to the substratum on which it had developed (shells with or without the periostracum, or coated with nail varnish) did not affect their behaviour. In the absence of biofilm, gammarids positively responded to the shell periostracum. Furthermore, they clearly preferred living zebra mussels over old empty shells, independent of the presence or absence of biofilm, confirming the importance of a periostracum-associated cue in their substratum recognition. On the other hand, shells obtained shortly after mussels’ death were preferred over living bivalves. Thus, the attractant is associated with fresh mussel shells, rather than with living mussels themselves. The ability of alien gammarids to locate sites inhabited by zebra mussels may contribute to their invasion success in novel areas inhabited by this habitat-forming bivalve.

Behavioural changes of zebra mussel Dreissena polymorpha (Bivalvia) induced by Ponto-Caspian gammarids

Colonies of an invasive Ponto-Caspian zebra mussel constitute suitable habitats for many invertebrate organisms, including Ponto-Caspian gammarids that are now spreading throughout Europe. There is, however, little known of the impact of these gammarids on their mussel hosts which they live amongst. We studied the behaviour of zebra mussels in the presence of three gammarid species that differed in their origin and degree of association with Dreissena habitats. In the presence of the Ponto-Caspian invader Dikerogammarus villosus that was commonly observed in mussel habitats, mussels responded by increasing their attachment strength and reducing upward movement. The responses to the Ponto-Caspian Pontogammarus robustoides, a species that showed no preferences for mussel habitats, were weaker and limited to the increase of attachment strength. There were no behavioural changes in mussels exposed to the native Gammarus fossarum. As mussels only responded to gammarids moving freely among their shells and not to those kept behind a mesh barrier, the most likely factor causing these changes was mechanical irritation of their soft parts by amphipod appendages. This hypothesis was further confirmed in an additional experiment with a South-American catfish Corydoras paleatus used as another potential source of mechanical irritation. The responses of zebra mussels to gammarids were similar to their anti-predator defences shown earlier in the presence of molluscivorous roach. Thus, it appears likely that Ponto Caspian gammarids within mussel colonies have the capacity to compromise the normal functioning of bivalves through inducing their defensive responses.

Zebra mussel beds: an effective feeding ground for Ponto-Caspian gobies or suitable shelter for their prey?

Aggregations of the Ponto-Caspian invasive zebra mussel (Dreissena polymorpha) constitute a suitable habitat for macroinvertebrates, considerably increasing their abundance and providing effective antipredator protection. Thus, the overall effect of a mussel bed on particular predator species may vary from positive to negative, depending on both prey density increase and predator ability to prey in a structurally complex habitat. Alien Ponto-Caspian goby fish are likely to be facilitated when introduced into new areas by zebra mussels, provided that they are capable of utilizing mussel beds as habitat and feeding grounds. We ran laboratory experiments to find which prey (chironomid larvae) densities (from ca. 500 to 2,000 individuals m−2) in a mussel bed make it a more beneficial feeding ground for the racer goby Babka gymnotrachelus (RG) and western tubenose goby Proterorhinus semilunaris (WTG) compared to sandy and stone substrata (containing the basic prey density of 500 ind. m−2). Moreover, we checked how food availability affects habitat selection by fish. Mussel beds became more suitable for fish than alternative mineral substrata when food abundance was at least two times higher (1,000 vs. 500 ind. m−2), regardless of fish size and species. WTG was associated with mussel beds regardless of its size and prey density, whereas RG switched to this habitat when it became a better feeding ground than alternative substrata. Larger RG exhibited a stronger affinity for mussels than small individuals. WTG fed more efficiently from a mussel bed at high food abundances than RG. A literature review has shown that increasing chironomid density, which in our study was sufficient to make a mussel habitat an attractive feeding ground for the gobies, is commonly observed in mussel beds in the field. Therefore, we conclude that zebra mussels may positively affect the alien goby species and are likely to facilitate their establishment in novel areas, contributing to an invasional meltdown in the Ponto-Caspian invasive community.

The Karyotype and External Morphology of Stictochironomus sticticus (Fabricius, 1781) (Diptera: Chironomidae) from Poland

Abstract. This paper presents the karyotype and external morphology of larva, pupa and adult male of Stictochironomus sticticus (Fabricius, 1781) from Poland. The species of the genus Stictochironomus are generally identified on the basis of pupal and adult morphology. However, the species described in this paper has species-specific markers in the salivary gland chromosomes which allow identification at the larval stage. Karyological markers are evaluated for the Polish population in comparison with other European populations and Stictochironomus species.