Scavenger-driven fish carcass decomposition and phosphorus recycling: Laboratory experiments with freshwater fish and crayfish

1. Fish carcass decomposition can generate fluxes of nutrients to the water column at levels comparable to other major nutrient sources. However, relatively little is known about the biological processes modulating fish decay or the rates at which carcass-bound nutrients are made available to the biota.
2. This study focused on quantifying scavenger-mediated phosphorus (P) recycling, because the availability of this essential element can regulate the trophic state of aquatic ecosystems.
3. To explore the role of some important macroscopic aquatic scavengers in P recycling, laboratory experiments were conducted where carcasses of two fish species (common bleak Alburnus alburnus; pumpkinseed Lepomis gibbosus) were offered to two size classes of black bullhead (Ameiurus melas) and two crustacean species (spiny-cheek crayfish Faxonius limosus; narrow-clawed crayfish Astacus leptodactylus).
4. Our results show that the black bullhead and the two crayfish species are highly efficient macroscopic decomposers as the P contents of scavenged carcasses were reduced at significantly higher rates compared to those of microbially decomposed (control) carcasses.
5. Pumpkinseed carcasses proved to be more resistant to rapid decomposition, as they typically lost lower proportions of their P content than bleak carcasses during the course of the experiments.
6. Scavengers sequestered a relatively large fraction (up to 33% in black bullhead and 36% in crayfish) of total carcass P in their bodies. This suggests that the consumer species used in this study can transfer/return considerable quantities of carcass-derived nutrients directly to higher trophic levels incorporated into their own tissues, and may serve as an additional, short-term sink of these nutrients.

     

Fish mortality and physicochemistry in a managed floodplain wetland

Patterns of fish mortality and associated physicochemical factors werestudied during late spring in a managed wetland canal along the lowerMissouri River, Missouri. Mean dawn dissolved oxygen was lower and meanun-ionized ammonia and turbidity were higher during the fish kill thanbefore or after the kill, or than was observed in a nearby wetland canalwhere no fish kill occurred. Dissolved oxygen at dawn and un-ionizedammonia concentrations were at critically low and high levels respectively,so that both likely contributed to the fish mortality. Timing and magnitudeof observed carcasses suggested that Ameiurus melas Rafinesques wasthe most tolerant species for the sizes observed compared to Ictiobuscyprinellus Valenciennes, Lepomis macrochirus Rafinesque, Cyprinus carpio Linneaus, and Lepomis cyanellus Rafinesque.Decreasing mean lengths of fish carcasses during the fish kill for C.carpio, L. cyanellus, and A. melas, indicate that smaller fishes mayhave been more tolerant of harsh environmental conditions than largerindividuals of the same species. Differential mortalities among species andsizes during drawdowns in actively managed wetland pools may haveintentional and unintentional ramifications on wetland and riverine fishcommunity structure, fish-avian interactions, and implementing anecosystem management perspective to restoring more naturalized riverfloodplain wetland functions. Late summer and early autumn draining ofmanaged wetlands might be used to benefit a wider diversity of wildlife andfishes.     

Olfactory discrimination of complex mixtures of amino acids by the black bullhead Ameiurus melas

On the basis of previous findings of behavioural discrimination of amino acids and on the knowledge of electrophysiology of the catfish (genera Ictalurus and Ameiurus) olfactory organs, behavioural experiments that investigated olfactory discrimination of amino acid mixtures were carried out on the black bullhead Ameiurus melas. Repeated presentations of food‐rewarded mixtures released increased swimming activity measured by counting the number of turns >90° within 90 s of stimulus addition. Non‐rewarded amino acids and their mixtures released little swimming activity, indicating that A. melas discriminated between the conditioned and the non‐conditioned stimuli. Two questions of mixture discrimination were addressed: (1) Are A. melas able to detect components within simple and complex amino acid mixtures? (2) What are the smallest differences between two complex mixtures that A. melas can detect? Three and 13 component mixtures tested were composed primarily of equipotent amino acids [determined by equal electroolfactogram (EOG) amplitude] that contained L‐Cys at ×100 the equipotent concentration. Ameiurus melas initially perceived the ternary amino acid mixture as its more stimulatory component alone [i.e. cysteine (Cys)], whereas the conditioned 13 component mixture containing the more stimulatory L‐Cys was perceived immediately as different from L‐Cys alone. The results indicate that components of ternary mixtures are detectable by A. melas but not those of more complex mixtures. To test for the smallest detectable differences in composition between similar multimixtures, all mixture components were equipotent. Initially, A. melas were unable to discriminate the mixtures of six amino acids from the conditioned mixtures of seven amino acids, whereas they discriminated immediately the mixtures of four and five amino acids from the conditioned mixture. Experience with dissimilar mixtures enabled the A. melas to start discriminating the seven‐component conditioned mixture from its six‐component counterparts. After fewer than five training trials, A. melas discriminated the mixtures of nine and 10 amino acids from a conditioned mixture of 12 equipotent amino acids; however, irrespective of the number of training trials, A. melas were unable to discriminate the 12 component mixture from its 11 component counterparts.     

Predatory and nesting behaviour of Cocalus gibbosus,a spartaeine jumping spider (Araneae: Salticidae) from Queensland

Abstract

Cocalus gibbosus was studied in the field in Queensland and in the laboratory. This is the first behavioural study of a species from the spartaeine genus Cocalus. C. gibbosus often omitted elements which are usually present in the predatory sequences of typical salticids and tended to lunge at prey from close range rather than leap from afar. Experiments showed that C. gibbosus prefers moths to other prey. In nature, C. gibbosus moulted and oviposited on silk sheets spun against tree trunks, and in the laboratory on sides of cages or blocks of wood, but this species never built an enclosing nest like typical salticids nor a large prey-catching web like some other spartaeines. C. gibbosus stalked across alien webs to catch spiders and insects, but it did not make vibratory signals. It did not stick to cribellate or ecribellate glue on alien webs. The behaviour of C. gibbosus is compared to that of other spartaeine salticids.     

Vertebrate scavenger guild composition and utilization of carrion in an East Asian temperate forest

Scavenging is a common feeding behavior by many species that plays an important role in ecosystem stability and function while also providing ecosystem services. Despite its importance, facultative scavenging on large animal carcasses has generally been overlooked in Asian temperate forest ecosystems. The aim of this study was to determine the composition and feeding behavior of the facultative scavenger guild as it relates to sika deer (Cervus nippon) carcasses in Japanese forests. There are no obligate scavengers or large predators that kill adult ungulates, but humans fill the role of large predators by culling deer for population management. We documented nine vertebrate species scavenging on deer carcasses and found that mammals were more frequent scavengers than birds and also fed for longer durations. This result suggests that there is a facultative scavenger guild composed mainly of mammals in our forest ecosystem and that carcass utilization by birds was restricted to only forest species. Raccoon dogs (Nyctereutes procyonoides) and Asian black bears (Ursus thibetanus) were the most frequent scavenger species and also fed for longer durations than other scavengers. There were significant seasonal differences in scavenging by Asian black bear, Japanese marten (Martes melampus), and mountain hawk‐eagle (Nisaetus nipalensis), suggesting the availability of other food resources may alter facultative scavenging by each species. Our results support that scavenging is widespread in this system and likely has important functions including building links in the food web.     

Introduction of Alien Host–parasite Complexes in a Natural Environment and the Symbiota Concept

When a species is introduced in a new territory its parasites may also follow, thus forming a sort of biotic unit. We propose the term symbiota to indicate this host–parasite complex. This concept is at the basis of the present research in which 11 species of alien and native freshwater fish were collected by electrofishing in the Po river area (Northern Italy). The alien host fish Ictalurus melas and Silurus glanis were parasitized by only one monogenean species each. Lepomis gibbosus, of North American origin, hosted four monogeneans and Carassius carassius three. All these parasites should be considered as alien species, except those collected from C. carassius that should be considered in part natives and in part aliens.     

Life‐history variability of non‐native centrarchids in regulated river systems of the lower River Guadiana drainage (south‐west Iberian Peninsula)

Life‐history variability of two non‐native centrarchids, pumpkinseed Lepomis gibbosus and largemouth bass Micropterus salmoides, was evaluated in three stream stretches of the lower River Guadiana drainage (south‐west Iberian Peninsula) with different degrees of regulated flows. Abundance, condition and population structure differed among populations for both species, but invasion success was lower in the least regulated river. Lepomis gibbosus were abundant and had multiple age classes in the three river sites, whereas M. salmoides were less abundant and mainly represented by young‐of‐the‐year fish. Juvenile growth in L. gibbosus was similar in all three populations, though longevity was slightly greater in the population from the River Guadiana mainstream. Lepomis gibbosus exhibited a long reproductive season, but the duration of season, size at maturity and reproductive effort varied among populations. The life‐history differences found demonstrate the importance of species adaptation to local conditions which might favour their invasion success. Lepomis gibbosus were more adaptable and resilient to local conditions, whereas M. salmoides seemed dependent on reservoirs and large rivers for maintenance of riverine populations.     

Scavenger community response to the removal of a dominant scavenger

The alteration of scavenging communities can reduce basic ecosystem services and increase risks to human and wildlife health. Recent work demonstrated that scavenging communities in agricultural landscapes are extremely efficient: superabundant mesopredators sequestered system energy by dominating scavenging activity. To explore how the disturbance of these communities affects the stability of carrion removal as an ecosystem function, we experimentally manipulated a scavenging community within an agricultural landscape by reducing the abundance of the dominant scavenger, raccoons Procyon lotor. We then monitored the fates of 676 mouse Mus musculus carcasses placed in 13 control and 13 removal woodlots from June 2007–May 2008. The diversity of vertebrate scavengers did not change between control and removal woodlots and scavenging by invertebrates was unaffected by our experiment. Although Virginia opossums Didelphis virginiana and other scavengers exhibited a functional response when raccoons were reduced in abundance, the increases did not change the proportional allocation of carcasses among scavengers. Finally, the reduced abundance of a major scavenger affected system efficiency. More carcasses remained un‐scavenged at the end of trials in removal woodlots than in control woodlots. This experiment demonstrates the vulnerability of a critical ecosystem service, carrion removal, to perturbations of the scavenging community and serves to highlight the method by which scavenger communities may respond to perturbations.     

How do roads affect the habitat use of an assemblage of scavenging raptors?

Scavengers may benefit from the availability of dead animals along roads that result from collisions with vehicles. However, roads are also considered risky places for many species. Animal habitat selection patterns usually balance energy intake with mortality risk. In this work we analyzed the foraging space use of an assemblage of diurnal scavenging raptors in relation to distance from roads in northwest Patagonia. We selected patches at different distances from roads, and placed a sheep carcass in each patch during the night (n = 18 carcasses in total). In general, carcasses near roads were detected by diurnal scavenging raptors much faster than those far from roads. Smaller raptors such as southern caracaras (Caracara plancus), chimango caracaras (Milvago chimango), and black vultures (Coragyps atratus), were commonly associated with roads both in terms of overall detections and scavenging activities. Southern and chimango caracaras proved to be very good at detecting carcasses, were faster to land in order to feed from them, and were found in greater numbers near roads than far from them. Even though Andean condors (Vultur gryphus) and black-chested buzzard-eagles (Geranoaetus melanoleucus) flew all over the area, they chose to feed far from roads. Our work emphasizes that some scavengers have taken advantage of the novel food resources provided by roads whereas others are reluctant to feed near them. Within a scenario of an increasing number of roads, some species can extend their distributions favoring competition and biotic homogenization processes within original communities. We highlight the importance of taking into account large flying scavengers in land-use planning. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Comparison of Day and Night Fyke Netting, Electrofishing and Snorkelling for Monitoring a Population of the Threatened Golden Galaxias (Galaxias auratus)

The littoral zone of small off-stream water storage containing a translocated population of Galaxias auratus was sampled fortnightly at day and night with fyke nets, electrofishing and snorkelling over 3 months. Variation in population data provided by each method, including relative abundance indices, size structure, and habitat preferences, were examined. Aspects of behaviour and activity patterns were also investigated. Night sampling using all methods consistently yielded larger catches than day sampling. The size structure of catches varied, with electrofishing at night and fyke netting during the day having higher proportions of juveniles, whilst snorkelling at night and electrofishing during the day had higher proportions of adults. Fyke netting at night yielded by far the largest catches (~3-fold more than other methods) and also captured balanced proportions of juveniles and adults. Galaxias auratus had a strong diel activity pattern and were most active at night. The majority of the population migrated into the littoral zone during the night and back into deeper water during the day. A small number of juveniles remained in the littoral zone and some adults sheltered in the dense cover of species-rich littoral vegetation during the day. Shores with shallow depth profiles appeared to be preferred due to higher catches in these areas using all methods. Based on the results of this study, fyke netting at night in littoral habitats is recommended for monitoring populations of G. auratus. Fyke netting is likely to be an effective method for monitoring other lacustrine galaxiid species; however, further work is required to investigate the effects of habitat variables and fish community structure on activity patterns of galaxiids, and hence their catchability with various methods, in more extensive lentic environments.     

Thermal influences on the habitat preference and the diurnal activity in three European Rana species

The diurnal behaviour of frogs was recorded quantitatively in a habitatlike experimental environment by a new method of automatic registration via thermocouples. Three species with different habitat preferences during their summer activity period were chosen: the terrestrial common frog Rana temporaria LINNÈ and the semiaquatic water frog Rana lessonae CAMERANO and Rana ridibunda PALLAS. The activity, the location and the skin temperatures of these frogs were recorded continuously in four different temperature ranges (8.1° C–31.0° C) within the temperature span of the summer activity period. The thermal requirements and behavioural adaptations to the habitat of each species were analysed. The experimental results coincided with field observations and showed some details not yet known. The northern species R. temporaria exhibited a distinctive behavioural thermoregulation including cooling behaviour at high temperatures and warming behaviour at low temperatures during the day. In all temperatures tested these frogs remained on land changing their preferred location from the open area to the hiding places. The main activity period shifted from night to day with decreasing temperatures. The more southern species R. lessonae preferred higher temperatures than the other species showing basking behaviour during the day independent of the ambient temperature. With decreasing temperatures the preferred location changed from the shore to the water and the activity during the night almost disappeared. The closely related R. ridibunda tolerated high temperatures too, but only rarely basked. This species, however, reacted contrarily to decreasing temperatures: it changed its preference from the water to the land. This behavioural differenciation between the water frog species may be of importance for the actual habitat preference of these central european water frogs.     

Morphological variability of black bullhead Ameiurus melas in four non‐native European populations

External morphology in black bullhead Ameiurus melas, a fish species considered to have high invasive potential, was studied in its four non‐native European populations (British, French, Italian and Slovak). The aim of this study was to examine this species' variability in external morphology, including ontogenetic context, and to evaluate its invasive potential. Specimens from all non‐native populations reached smaller body size compared to individuals from native populations. Juvenile A. melas were found to have a relatively uniform body shape regardless of the population's origin, whereas adults developed different phenotypes depending upon location. Specimens from the U.K., Slovak and French populations appeared to be rather similar to each other, whereas the Italian population showed the most distant phenotype. This probably results from the different thermal regime in the Italian habitat. Ameiurus melas from non‐native European populations examined in this study showed some potential to alter the body shape both within and between populations. The phenotypic plasticity of A. melas, however, was not found to be as significant as in other invasive fish species. The results suggest that morphological variability itself is not necessarily essential for invasive success. The invasiveness of A. melas is therefore probably favoured by variations in its life‐history traits and reproduction variables, together with some behavioural traits (e.g. voracious feeding and parental care) rather than by phenotypic plasticity expressed in external morphology.     

Ecological role of vertebrate scavengers in urban ecosystems in the UK

Recent research has demonstrated how scavenging, the act of consuming dead animals, plays a key role in ecosystem structure, functioning, and stability. A growing number of studies suggest that vertebrate scavengers also provide key ecosystem services, the benefits humans gain from the natural world, particularly in the removal of carcasses from the environment. An increasing proportion of the human population is now residing in cities and towns, many of which, despite being highly altered environments, contain significant wildlife populations, and so animal carcasses. Indeed, non‐predation fatalities may be higher within urban than natural environments. Despite this, the fate of carcasses in urban environments and the role vertebrate scavengers play in their removal have not been determined. In this study, we quantify the role of vertebrate scavengers in urban environments in three towns in the UK. Using experimentally deployed rat carcasses and rapid fire motion‐triggered cameras, we determined which species were scavenging and how removal of carcass biomass was partitioned between them. Of the 63 experimental carcasses deployed, vertebrate scavenger activity was detected at 67%. There was a significantly greater depletion in carcass biomass in the presence (mean loss of 194 g) than absence (mean loss of 14 g) of scavengers. Scavenger activity was restricted to three species, Carrion crows Corvus corone, Eurasian magpies Pica pica, and European red foxes Vulpes vulpes. From behavioral analysis, we estimated that a maximum of 73% of the carcass biomass was removed by vertebrate scavengers. Despite having low species richness, the urban scavenger community in our urban study system removed a similar proportion of carcasses to those reported in more pristine environments. Vertebrate scavengers are providing a key urban ecosystem service in terms of carcass removal. This service is, however, often overlooked, and the species that provide it are among some of the most disliked and persecuted.     

Deer Carcass Decomposition and Potential Scavenger Exposure to Chronic Wasting Disease

ABSTRACT Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy afflicting the Cervidae family in North America, causing neurodegeneration and ultimately death. Although there are no reports of natural cross-species transmission of CWD to noncervids, infected deer carcasses pose a potential risk of CWD exposure for other animals. We placed 40 disease-free white-tailed deer (Odocoileus virginianus) carcasses and 10 gut piles in the CWD-affected area of Wisconsin (USA) from September to April in 2003 through 2005. We used photos from remotely operated cameras to characterize scavenger visitation and relative activity. To evaluate factors driving the rate of carcass removal (decomposition), we used Kaplan-Meier survival analysis and a generalized linear mixed model. We recorded 14 species of scavenging mammals (6 visiting species) and 14 species of scavenging birds (8 visiting species). Prominent scavengers included American crows (Corvus brachyrhynchos), raccoons (Procyon lotor), and Virginia opossums (Didelphis virginiana). We found no evidence that deer consumed conspecific remains, although they visited gut piles more often than carcasses relative to temporal availability in the environment. Domestic dogs, cats, and cows either scavenged or visited carcass sites, which could lead to human exposure to CWD. Deer carcasses persisted for 18 days to 101 days depending on the season and year, whereas gut piles lasted for 3 days. Habitat did not influence carcass decomposition, but mammalian and avian scavenger activity and higher temperatures were positively associated with faster removal. Infected deer carcasses or gut piles can serve as potential sources of CWD prions to a variety of scavengers. In areas where surveillance for CWD exposure is practical, management agencies should consider strategies for testing primary scavengers of deer carcass material.     

MORBILLIVIRUS INFECTION IN TWO SPECIES OF PILOT WHALE (GLOBICEPHALA SP.) FROM THE WESTERN ATLANTIC

We report evidence of enzootic morbillivirus infection among long-finned, Globicephala melas, and short-finned, G. macrorhynchus, pilot whales in the western Atlantic. A retrospective serologic survey, using five morbilliviruses, was carried out on 99 G. melas from 14 stranding events between 1982 and 1993 and from 25 G. macrorhynchus stranded in 5 events between 1986 and 1994. A blood sample was also obtained from an adult G. melas by-caught in the western North Atlantic. Tissues were collected from 24 G. melas and 15 G. macrorhynchus for histology and immunoperoxidase staining. Neutralizing antibody titers were found in 92 (92%) of 100 G. melas and 16 (64%) of 25 G. macrorynchus, and titers were highest against cetacean morbilliviruses. Seroprevalence was similar between age classes and sexes. The earliest evidence of infection was in a G. melas that stranded in 1982. Stable antibody titers were observed in pilot whales under rehabilitation for up to eight months. Clinical disease consistent with morbillivirus pneumonia was detected in a G. melas calf. Immunoperoxidase staining confirmed that viral antigen was present in the lesions. We propose that enzootic infection in pilot whales is facilitated by population size, social structure, and migration patterns. Furthermore, through mixing with other odontocetes, pilot whales could act as vectors through the Atlantic. Clinical morbillivirus infection may precipitate mass strandings of highly social odontocetes.     

Significant fish predation on zebra mussels Dreissena polymorpha in Lake Champlain,U.S.A.

The objectives of this study were to: (1) determine which fishes were consistently eating zebra mussels Dreissena polymorpha in Lake Champlain and document their feeding behaviour and (2) quantify the diet composition of the fish predators that were found to consume zebra mussels. From 2002 to 2005, freshwater drum Aplodinotus grunniens, pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens and rock bass Ambloplites rupestris all consumed zebra mussels at varying frequencies and amounts. Aplodinotusgrunniens and L. gibbosus chewed clumps of zebra mussels, expelling shells, whereas P. flavescens and A. rupestris swallowed small individuals whole. Lepomis gibbosus consumed zebra mussels at the highest frequency (65–89% of prey consumed) and zebra mussels comprised a large part of this fish’s diet (up to 40% by dry mass). Zebra mussels were also an important component of the diet of A. grunniens (up to 59% of the diet by dry mass, 40–63% frequency of consumption). The percentage of the diet comprising zebra mussels in P. flavescens and A. rupestris varied significantly from year to year but never exceeded 10%. Because A. grunniens and L. gibbosus crushed zebra mussels, the nutritional return from consuming zebra mussels would be similar to other prey; for P. flavescens and A. rupestris zebra mussels were only partially digested and the nutritional return would probably be low. As predation on zebra mussels is widespread and significant, it is possible that fish predators could contribute to regulating the population of zebra mussels in Lake Champlain.     

Identification of vertebrate scavengers of small mammal carcasses in a forested landscape

We identified vertebrate scavengers of small mammal carcasses at the 780-km² Savannah River Site during the winter of 2000–2001. Rodent carcasses, differing in size and visual conspicuousness, were placed in upland pine forests and bottomland hardwood forests during six 2-week periods. Sixty-two of the 96 carcasses (65%) were removed by vertebrates. With the aid of remote photography, we identified 11 species of scavengers removing carcasses. RaccoonsProcyon lotor, gray foxesUrocyon cinereoargenteus, and feral pigsSus scrofa scavenged most frequently. The mean elapsed time for carcass removal was 5.6 days. The number of carcasses removed by vertebrates did not differ significantly with respect to carcass size, visual conspicuousness, or habitat type; however, air temperature was strongly correlated (positively) with carcass removal. Our study demonstrates that many mammal species are capable of utilizing small carrion items as a food resource, and suggests that scavenging may account for a higher proportion of the diet of some facultative scavengers than is now widely assumed.     

Scavenging affects persistence of avian carcasses resulting from window collisions in an urban landscape

ABSTRACT Collisions with windows remain an important human‐related threat to bird survival in urban landscapes. Accurately estimating the magnitude of avian mortality at windows is difficult and may be influenced by many sources of error, such as scavenging of carcasses. Failure to account for removal of carcasses by scavengers can bias estimates of window mortality. We tested the hypothesis that carcass survival depends on local habitat factors known to influence scavenger behavior. Scavenger activity on bird carcasses was documented at 20 buildings in an urban landscape in northwestern Illinois for 1 week during each season of a year. Known‐fate models were used to relate carcass survival to local habitat composition and to evaluate temporal variation in survival. We also documented species of scavengers and the timing of scavenging using motion‐triggered cameras. Daily carcass survival was greater in winter than during spring, summer, and fall. Survival was related negatively to canopy cover (trees and shrubs within a 50‐m buffer) and window area, and positively to pavement cover. Using an exponential model of survival time, estimated mean time of survival of carcasses (t± SE) was 82.9 ± 11.7 d for winter and 11.8 ± 7.2 d for other seasons. Raccoons (Procyon lotor) scavenged more carcasses than other species. Our results suggest that (1) carcass survival times may be short at locations with preferred habitats of known scavengers and predictable sources of food, and (2) knowledge of scavenger distribution and activity can inform predictive models of persistence. In studies of bird‐window collisions, the influence of scavenger bias can be minimized by maintaining short time intervals between carcass searches. Search intervals can be inferred by estimating the number of days that a carcass should persist at a site, which can be calculated using predicted daily survival probabilities of carcasses at study buildings.     

Effects of size on predation risk,behavioural response to fish,and cost of reduced feeding in larval Ischnura verticalis (Coenagrionidae: Odonata)

Summary We used laboratory studies to examine the role of predation risk and cost of anti-predator behaviour in determining the behavioural response of several larval instars of Ischnura verticalis to a fish predator (Lepomis gibbosus). Smaller larvae were less susceptible to fish predation than larger larvae. Smaller larvae depressed movement to a greater degree in the presence of fish than did larger larvae; large larvae were generally less active than small larvae regardless of fish presence. Reduced feeding resulted in smaller larvae suffering more in terms of reduced growth than did large larvae. In general, our results tend to support the hypothesis that individuals that suffer high costs of anti-predator behaviour but little risk of predation may only exhibit anti-predator behaviours in the presence of predators, whereas individuals with a higher risk of predation and a lower cost of anti-predator behaviour may evolve anti-predator mechanisms that are in effect even in the absence of predators.     

Genetic isolation within the malaria mosquito Anopheles melas

Anopheles melas is a brackish water–breeding member of the Anopheles gambiae complex that is distributed along the coast of West Africa and is a major malaria vector within its range. Because little is known about the population structure of this species, we analysed 15 microsatellite markers and 1161 bp of mtDNA in 11 A. melas populations collected throughout its range. Compared with its sibling species A. gambiae, A. melas populations have a high level of genetic differentiation between them, representing its patchy distribution due to its fragmented larval habitat that is associated with mangroves and salt marsh grass. Populations clustered into three distinct groups representing Western Africa, Southern Africa and Bioko Island populations that appear to be mostly isolated. Fixed differences in the mtDNA are present between all three clusters, and a Bayesian clustering analysis of the microsatellite data found no evidence for migration from mainland to Bioko Island populations, and little migration was evident between the Southern to the Western cluster. Surprisingly, mtDNA divergence between the three A. melas clusters is on par with levels of divergence between other species of the A. gambiae complex, and no support for monophyly was observed in a maximum‐likelihood phylogenetic analysis. Finally, an approximate Bayesian analysis of microsatellite data indicates that Bioko Island A. melas populations were connected to the mainland populations in the past, but became isolated, presumably when sea levels rose after the last glaciation period (≥10 000–11 000 bp ). This study has exposed species‐level genetic divergence within A. melas and also has implications for control of this malaria vector.