Impact of an introduced Crustacean on the trophic webs of Mediterranean wetlands

Invasive predatory crayfish do not trigger inducible defences in tadpoles

Invasive species cause deep impacts on ecosystems worldwide, contributing to the decline and extinction of indigenous species. Effective defences against native biological threats in indigenous species, whether structural or inducible, often seem inoperative against invasive species. Here, we show that tadpoles of the Iberian green frog detect chemical cues from indigenous predators (dragonfly nymphs) and respond by reducing their activity and developing an efficient defensive morphology against them (increased tail depth and pigmentation). Those defensive responses, however, were not activated against a highly damaging invasive predator (red swamp crayfish). Induced defences increased tadpole survival when faced against either indigenous dragonflies or invasive crayfish, so its inactivation in the presence of the invasive predator seems to be due to failure in cue recognition. Furthermore, we tested for local adaptation to the invasive predator by comparing individuals from ponds either exposed to or free from crayfish. In both cases, tadpoles failed to express inducible defences against crayfish, indicating that ca 30 years of contact with the invasive species (roughly 10-15 frog generations) have been insufficient for the evolution of recognition of invasive predator cues.

The paradox of the long-term positive effects of a North American crayfish on a European community of predators

Invasions of non-native species are one of the major causes of losses of native species. In some cases, however, non-natives may also have positive effects on native species. We investigated the potential facilitative effects of the North American red swamp crayfish (Procambarus clarkii) on the community of predators in southwestern Spain. To do so, we examined the diets of predators in the area and their population trends since introduction of the crayfish. Most predator species consumed red swamp crayfish, which sometimes occurred in over 50% of their diet samples. Moreover, the abundance of species preying on crayfish increased significantly in the area as opposed to the abundance of herbivores and to predator populations in other areas of Europe, where those predators are even considered threatened. Thus, we report the first case in which one non-native species is both beneficial because it provides prey for threatened species and detrimental because it can drive species at lower trophic levels to extinction. Increases in predator numbers that are associated with non-native species of prey, especially when some of these predators are also invasive non-natives, may increase levels of predation on other species and produce cascading effects that threaten native biota at longer temporal and larger spatial scales. Future management plans should include the complexity of interactions between invasive non-natives and the entire native community, the feasibility of successful removal of non-native species, and the potential social and economic interests in the area.

Factors influencing epibenthic assemblages in the Minho Estuary (NW Iberian Peninsula)

The epibenthic community of the Minho Estuary was studied during the summer of 2006. Diversity was generally low and a total of 14 fish and five crustacean taxa were identified. Multivariate analysis revealed two site clusters (A and B). Water conductivity and percentage of fine sand were the abiotic variables that most contributed to the spatial distinction between clusters. The species contributing the most to the average similarity within Cluster A were Crangon crangon and Pomatoschistus microps, while in Cluster B was Atyaephyra desmarestii. Possible factors responsible for the low diversity of the epibenthic community in Minho Estuary were the low macrozoobenthic abundance and diversity, and the high abiotic oscillations between tides (mainly salinity) acting on the ecosystem.

Procambarus clarkii as a bioindicator of heavy metal pollution sources in the lower Ebro River and Delta

In the Ebro River basin, point and diffuse pollution of heavy metals stems mainly from industry and agriculture. Bioaccumulation patterns were examined under different pollution sources (point and diffuse) using levels of heavy metals (As, Cd, Cr, Cu, Hg, Pb and Zn) in abdominal muscle tissue of Procambarus clarkii. P. clarkii captured under point source effects presented the highest concentrations of Hg, Pb and As; and were related with distance to the source of industrial waste sediments. Mean Hg levels in crayfish exposed to point sources of metals significantly exceeded legal allowed values established by the European Union legislation. In the Ebro Delta, high levels of As, Cr, Cu and Zn were associated with traditional agriculture activity (diffuse pollution) as well. These results demonstrate the potential of P. clarkii to bioaccumulate heavy metals from both point and diffuse sources and hence potentially transfer these metals to higher trophic levels.

Investigational piscivory of some juvenile Australian freshwater fishes by the introduced Mozambique tilapia Oreochromis mossambicus

Experimental tanks were used to observe predatory effects in three different size classes of Mozambique tilapia Oreochromis mossambicus (one of the world's most widespread exotic species and generally regarded to be a herbivore or both herbivore and detritivore) when tested against 10 juvenile Australian freshwater fish species, and significant levels of predation against all were recorded. There was a general trend for larger O. mossambicus to kill more prey and this was also reflected in a separate series of experiments using juvenile barramundi Lates calcarifer over a range of size classes. Predatory effects by O. mossambicus broadly reflected the accepted models of predator-prey interactions, being that mortality (and survival) was closely related to relative body size and mouth gape limitation. Experimental evidence for piscivory in O. mossambicus was supported by field sampling that detected prey fish remains in 16% of all fish surveyed (n = 176). The recognition of active piscivory by O. mossambicus in laboratory and field situations is the first such evidence, and suggests a need to re-evaluate the nature of their effects in introduced environments.

Ecological Effects of Invasive Arthropod Generalist Predators

Arthropod generalist predators (AGP) are widespread and abundant in both aquatic and terrestrial ecosystems. They feed upon herbivores, detritivores, and predators, and also on plant material and detritus. In turn, AGP serve as prey for larger predators. Several prominent AGP have become invasive when moved by humans beyond their native range. With complex trophic roles, AGP have diverse effects on other species in their introduced ranges. The invaders displace similar native species, primarily through competition, intraguild predation, transmission of disease, and escape from predation and/or parasites. Invasive AGP often reach higher densities and/or biomass than the native predators they replace, sometimes strengthening herbivore regulation when invasive AGP feed on key herbivores, but sometimes weakening herbivore suppression when they eat key predators. The complexity and unpredictability of ecological effects of invasive AGP underscores the high risk of adverse consequences of intentional introductions of these species (e.g., for biological control or aquaculture).

Acclimatable cardiac and ventilatory responses to copper in the freshwater crayfish Procambarus clarkii

Mortality and physiological tests following exposure to waterborne copper were performed in the red swamp crayfish Procambarus clarkii from a central Italian population. Mortality tests gave an estimated 96 h LC50 value (with 95% confidence limits) of 162 (132-211) mg L(-1) waterborne copper II. Variations in cardiac and ventilatory rates were simultaneously monitored using a non-invasive plethysmographic technique. In experiments with different sub-lethal copper concentrations (control, 0.5, 1 and 10 mg L(-1)) performed at different times (3, 6, 96 h), copper exposure elicited a reduction in both heart and scaphognatite rates. Following exposure to 10 mg L(-1) copper for 96 h, the heart and scaphognatite rates decreased to about 35% of the initial values. The reduction was fully reversible, since crayfish exposed to 0.5, 1 and 10 mg Cu L(-1) for 96 h resumed control rates after a 3-h residence in clean water. In crayfish pre-exposed (96 h) to sub-lethal copper concentrations (0.1 and 1 mg L(-1)) and then held in control water (3 h), the reduction of heart and scaphognatite rates after exposure to 10 mg Cu L(-1) were significantly lower than in specimens pre-exposed to control water. Therefore, copper induces a concentration and time dependent reduction of both cardiac and ventilatory activity in P. clarkii; these responses can be reduced or fully abolished by pre-exposure to sub-lethal levels of the metal.

The use of the red swamp crayfish (Procambarus clarkii, Girard) as indicator of the bioavailability of heavy metals in environmental monitoring in the River Guadiamar (SW, Spain)

A translocation experiment of red swamp crayfish (Procambarus clarkii) to different sites located in the River Guadiamar was performed in order to assess the ability of this species as bioindicator of heavy metal and metalloid contamination. Crayfish were placed in cages and exposed to polluted environment during either 6 or 12days in the three sites with different concentration of contaminants. Their tissues (exoskeleton+gills, hepatopancreas and abdominal muscle) were dissected and analysed by ICP-MS to assess for concentration of Cd, Cu, Zn, Pb and As. Both exposure times result in significant bioaccumulation of some metals in crayfish tissues as compared to their concentration in the environment. According to overall metal concentration, crayfish tissues rank as follows: hepatopancreas/viscera>exoskeleton/gills>abdominal muscle. Essential metals for crayfish metabolism (Cu and Zn) are always found in high concentrations independently of their quantities in the environment because of the ability of crayfish to manipulate their levels for their own metabolic profit. Metals not involved in crayfish metabolism (Cd, Pb, As) tend to increase with increasing concentration in the surrounding environment and with longer exposure times. Thus crayfish could be used as bioindicator of these pollutants because their dose- and time-dependent accumulation may be reflective of the levels of non-essential metals present in contaminated wetlands. Future guidelines in plans for monitoring contamination on polluted Mediterranean rivers and wetlands should take into account the implementation of the incubation of crayfish during 6days and their subsequent analyses of metal contents, as a routine.