Jarosław Kobak

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.

Factors influencing taxonomic composition and abundance of macrozoobenthos in extralittoral zone of shallow eutrophic lakes

Zoobenthos is an essential part of shallow lake ecosystems, exerting a considerable impact upon their functioning. We studied 13 eutrophic, shallow, polymictic lakes from Northern Poland to find out which environmental factors influence taxonomic composition, abundance and biodiversity of their zoobenthos. The Canonical Correspondence Analysis allowed to distinguish three lake types: (1) macrophyte-dominated lakes, with high plant cover and well illuminated bottom, inhabited by abundant, diverse benthic taxa; (2) deeper phytoplankton-dominated lakes, with shaded bottom, high sediment oxygen demand (SOD) and rather sparse zobenthos community, dominated by Chironomus and Chaoborus larvae; (3) shallower phytoplankton-dominated lakes, with intermediate amount of light at the bottom and lower SOD values and comparatively diverse zoobenthos, but with lower number of taxa than in the first group. Apart from plant presence, distinguishing between macrophyte-dominated lakes and the other types, the most important variable in the CCA was amount of light reaching the bottom. Probably the impact of light on the bottom fauna was indirect: light stimulated development of macrophytes or phytobenthos (depending on its intensity) and thus improved food and oxygen conditions. Zoobenthos was also affected by oxygen conditions (mainly SOD), presumably by short-time oxygen depletions occurring in the deep phytoplankton-dominated lakes and preventing survival of some benthic taxa.

Attachment strength, aggregation and movement of the zebra mussel ( Dreissena polymorpha , Bivalvia) in the presence of potential predators

We carried out laboratory experiments to study the behaviour of zebra mussels in the presence of predators with different feeding habits: the roach Rutilus rutilus (an efficient molluscivore), the racer goby Neogobius gymnotrachelus, the crayfish Orconectes limosus (facultative molluscivores), and the perch Perca fluviatilis (prefers other kinds of food). We studied three size (length) classes of mussels: small ( 17mm). We measured attachment strength and numbers of individuals forming aggregations after 6-d exposure to predators, as well as horizontal and vertical distances moved by mussels after 24h of exposure. Attachment strength and tendency to form aggregations of small and medium mussels increased in the presence of roach. Medium mussels exposed to perch were also more strongly attached than those in the control treatment (no predators). Large mussels were unaffected by any predators. Predators did not affect horizontal distances moved by all sizes of mussels. Small individuals exposed to roach reduced their upward movement. In most cases, the lack of responses to perch (a non-molluscivore) ruled out any potential influence of fish movements or faeces upon mussels, suggesting an effect of predator kairomones. The exception was the attachment of medium individuals, which also responded to the presence of perch. In this case, responses to other stimuli, e.g. fish movements, could be involved.

Factors influencing the attachment strength of Dreissena polymorpha (Bivalvia).

Abstract The effects of several factors (shell length, exposure time, substratum orientation in space, illumination, temperature, conspecifics) upon the attachment strength (measured with a digital dynamometer) of the freshwater, gregarious bivalve Dreissena polymorpha were studied under laboratory conditions. A rapid increase in attachment strength was observed on resocart (a thermosetting polymer based on phenol-formaldehyde resin, with paper as filler) substrata during the first 4-d exposure, after which it stabilised at ca 1 N. The attachment strength increased also with mussel size. Mussel adhesion on variously oriented surfaces (vertical, upper horizontal and lower horizontal) was similar. Illumination inhibited attachment strength, as expected for a photophobic species, but only after a 2-d exposure. After 6 d, no effects of light were detected. Thus, illumination seemed to influence the attachment rate, rather than the final strength. The optimum temperature for mussel attachment was 20 – 25°C. At lower and higher temperatures (5 – 15°C and 30°C), their adhesion strength decreased. The presence of conspecifics stimulated mussel attachment strength.

Preferences of invasive Ponto-Caspian and native European gammarids for zebra mussel (Dreissena polymorpha, Bivalvia) shell habitat

We investigated habitat preferences of two invasive Ponto-Caspian gammarids (Dikerogammarus haemobaphes and Pontogammarus robustoides) and a native European species (Gammarus fossarum) in laboratory experiments. The habitats consisted of the following objects: (1) living zebra mussels; (2) empty mussel shells (clean or coated with nail varnish) with both valves glued together using aquarium silicone sealant to imitate a living mussel; (3) stones (clean or varnished); (4) empty plates. Ten objects of the same type were glued to a plastic plate (10 × 10 cm) with methyl acrylic glue. The plates were placed in experimental tanks in various combinations. A single gammarid was put into the tank and its position was determined after 24 h. The studied species responded differently to the presence of zebra mussels. D. haemobaphes preferred living mussels rather than their empty shells and these two habitats over stones and empty plates. It responded positively to shell shape, selecting varnished shells rather than varnished stones, and to shell surface properties, selecting clean shells rather than varnished shells. It did not respond to waterborne mussel exudates. P. robustoides did not exhibit any preferences for the above-mentioned substrata. G. fossarum was attracted by empty mussel shells (but not by living mussels). It responded only to their shape, not to surface properties. The strong affinity for zebra mussels, exhibited by D. haemobaphes, might help it survive and develop stable populations in newly invaded areas.

Effectiveness of zebra mussels to act as shelters from fish predators differs between native and invasive amphipod prey

Biological invasions cause organisms to face new predators, but also supply new anti-predator shelters provided by alien ecosystem engineers. We checked the level of anti-predator protection provided to three gammarid species by an invasive Ponto-Caspian zebra mussel Dreissena polymorpha, known for its habitat modification abilities. We used gammarids differing in their origin and level of association with mussels: Ponto-Caspian aliens Dikerogammarus villosus (commonly occurring in mussel beds) and Pontogammarus robustoides (not associated with mussels), as well as native European Gammarus fossarum (not co-occurring with dreissenids). The gammarids were exposed to predation of two fish species: the racer goby Babka gymnotrachelus (Ponto-Caspian) and Amur sleeper Perccottus glenii (Eastern Asian). This set of organisms allowed us to check whether the origin and level of association with mussels of both prey and predators affect the ability of gammarids to utilize zebra mussel beds as shelters. We tested gammarid survival in the presence of fish and one of five substrata: sand, macrophytes, stones, living mussels and empty mussel valves. D. villosus survived better than its congeners on all substrata, and its survival was highest in living dreissenids. The survival of the other gammarids was similar on all substrata. Both fish species exhibited similar predation efficiency. Thus, D. villosus, whose affinity to dreissenids has already been established, utilizes them as protection from fish predators, including allopatric predators, more efficiently than other amphipods. Therefore, the presence of dreissenids in areas invaded by D. villosus is likely to help the invader establish itself in a new place.

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.

Differential shell strength of Cepaea nemoralis colour morphs—implications for their anti-predator defence

One of the most spectacular evolutionary forces is predation, evidenced to stimulate polymorphism in many prey species. Shell colour polymorphism of the land snail Cepaea nemoralis is a well-known model in evolutionary research. Nevertheless, the knowledge on the ecological causes driving its evolution remains incomplete and proximal factors shaping predatory pressure on C. nemoralis morphs are unknown. We evaluated shell crushing resistance and thickness, constituting crucial snail anti-predator defences in two shell areas (the apex and labium) of eight C. nemoralis morphotypes differing in shell colour and banding pattern. A GLM showed a significant effect of shell colour, banding pattern and shell thickness on shell strength. Pink shells were stronger than yellow ones, and banded forms had stronger shells than unbanded snails. The labium (usually attacked by mice) was generally thicker and more resistant than the apex (usually crushed by birds). Thicker shells were more resistant to crushing, and the rate of shell strength increase per unit of shell thickness was greater in pink and banded individuals compared to yellow and unbanded ones. Yellow and unbanded morphs have been found to be preferred by mice in the previous studies, which suggests that shell strength may be an important trait used in prey selection by these shell-crushing predators. The differences in potential anti-predator defences among snail morphs, found in the present study, justify future research on direct effect of C. nemoralis morphs shell strength on predator selectivity.

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.