F.P.L. Collas

Oxygen can limit heat tolerance in freshwater gastropods: differences between gill and lung breathers

AbstractInsufficient oxygen delivery to tissues is hypothesised to limit thermal tolerance, but evidence in ectotherms is mixed. We assessed heat tolerance under hypoxia, normoxia and hyperoxia to test whether the extent in which oxygen can lower or increase heat tolerance differed with mode of respiration, comparing gill-breathing caenogastropods and lung-breathing pulmonates with or without an accessory gill. Hypoxia lowered heat tolerance in three of the four pulmonates (Physa fontinalis, Physa acuta and Planorbis carinatus) by 1.2–2.1°C. Hyperoxia, however, did not increase the heat tolerance in any of the pulmonate species. Thus, heat tolerance limits of these pulmonates does not appear to be oxygen limited under normoxia, possibly because of their high capacity to regulate oxygen consumption associated with aerial gas exchange. Instead, other processes may become limiting at thermal extremes such as loss of protein function, loss of membrane stability or neuronal dysfunction. The caenogastropod species tested (Potamopyrgus antipodarum, Bithynia tentaculata) closed their operculum during the warming experiments. This behavioural response prevented us from obtaining clear results. Nevertheless, our results suggested hyperoxia may increase heat tolerance in B. tentaculata. This could be related to its lower capacity to regulate oxygen, owing to its fully aquatic gas exchange mechanism.

Longitudinal training dams mitigate effects of shipping on environmental conditions and fish density in the littoral zones of the river Rhine

The stability of habitat conditions in littoral zones of navigated rivers is strongly affected by shipping induced waves and water displacements. In particular, the increase of variability in flow conditions diminishes the suitability of these habitats for juvenile fishes. Recently, a novel ecosystem based river management strategy has resulted in the replacement of traditional river training structures (i.e., groynes) by longitudinal training dams (LTDs), and the creation of shore channels in the river Waal, the main, free-flowing and intensively navigated distributary of the river Rhine in the Netherlands. It was hypothesized that these innovative LTDs mitigated the effects of shipping on fishes by maintaining the natural variability of habitat conditions in the littoral zones during ship passages whereby shore channels served as refugia for juvenile fishes. Measurements of abiotic conditions showed a significantly lower water level fluctuation and significantly higher flow stability in shore channels compared to groyne fields. Flow velocity did not differ, nor did the variation in flow velocity fluctuation during ship passage between these habitats. Densities of fish were found to be significantly higher in the littoral zones of shore channels compared to nearby groyne fields. Moreover, electrofishing along the inner side of the newly constructed LTD showed a significant linear relationship between fish density and distance from highly dynamic in- and outflow sections and to lowered inflow sections in the LTD. Results of our field sampling clearly indicate successful ecological rehabilitation of littoral zones that coincides with a facilitation of navigation in the main river channel and increased flood safety.

GROWTH, SURVIVAL, AND MORTALITY OF JUVENILE AND ADULT ALIEN CONRAD'S FALSE MUSSEL MYTILOPSIS LEUCOPHAEATA (CONRAD, 1831) (MOLLUSCA, BIVALVIA, DREISSENIDAE) IN A BRACKISH CANAL

ABSTRACT The false mussel, Mytilopsis leucophaeata originating from the Atlantic coast and Gulf of Mexico in North America invaded brackish water systems in Europe. Seasonal sessile population structure is determined by spat fall, growth, and mortality. Juvenile and adult M. leucophaeata were kept in cages in a brackish water canal to study their mortality and to estimate annual and seasonal shell growth rate in relation to shell size and water temperature. Mortality increased with increasing temperature with the strongest effect on the 4–6 mm and 8–10 mm size classes. The average mortality of all size classes of M. leucophaeata in cage experiments was lowest in winter, increased from April to July, and peaked in July and August. Between a water temperature of 18 and 23°C, a strong growth was observed for mussels of size classes 2–14 mm, whereas mussels of size class 14–16 mm showed only a slight growth (<20 µm day-1) and size classes 16–18 mm and 18–22 mm showed no growth at all. When water temperature was between 9 and 18°C, there was no or slight growth, and at lower than 9°C, hardly any growth. Especially for the small mussels, the influence of the water temperature on shell growth is strong. Polynomial regression showed a mean summer growth rate of 94 µm day-1 for the 2–4 mm class, 37 µm day-1 for the 10–12 mm class, and no growth for the 18–22mmclass. Analysis of all available (literature) data yielded mean summer and annual growth rates of M. leucophaeata collected in brackish western European harbors and canals ranging from 30 to 133 µm day-1 and 8 to 49 µm day-1, respectively. Increase of water temperature by climate change or thermal discharges will increase growth rates and mortality of this mussel.