Prey Preference Research Articles

Trace Element Concentrations in Livers of Pacific Harbor Seals (Phoca vitulina richardii) from San Juan County, Washington, USA

Approximately 5,000 Pacific harbor seals (Phoca vitulina richardii) reside year-round in San Juan County (SJC), Washington (US) in the center of the binational Salish Sea. We retrospectively analyzed total cadmium (Cd), copper (Cu), iron (Fe), mercury (Hg), magnesium (Mg), manganese (Mn), lead (Pb), selenium (Se), and zinc (Zn) in livers of dead stranded harbor seals (n=57) collected in SJC between 2009 and 2012 to identify age-related and regional patterns of trace element exposure. Consistent with prior studies of contaminants in pinnipeds, Hg, Cd, and Se concentrations increased with age, and Se:Hg molar ratios approached 1:1 in adult seals. Concentrations of Cd and Hg were below putative marine mammal toxicity thresholds. Mercury concentrations were comparable among Salish Sea populations. Although SJC is less urbanized with fewer industrial inputs than South Puget Sound (SPS), SJC nonpups had greater concentrations of Cd, Cu, and Zn, and pups had greater concentrations of Zn compared to SPS seals. We hypothesize these regional differences could be due to prey preference and availability or to natural geochemical processes. Reported concentrations inform future sampling protocols and can assist in tracking long-term temporal and spatial trends of trace elements in marine organisms.

Prey stage preference and functional response of Orius albidipennis (Hetetroptera, Anthocoridae) to Aphis fabae (Homomoptera, Aphididae)

The black been aphid, Aphis fabae Scopoli (Homoptera: Aphididae), is an important pest of many garden, vegetable and ornamental plants in the world. To evaluate Orius albidipennis potential for biological control of the aphid, prey stage preference of the predator to different life stages of the aphid and functional responses of it to different densities of the most preferred life stage were studied under laboratory conditions. Free choice experiment was conducted for evaluation of the bug prey preference between different instar nymphs and wingless female adults of A. fabae. In functional response trials, densities of 5, 10, 15, 20, 25 and 30 s instar nymphs, as the most preferred life stage, per arena were exposed to male and female of the predatory bug for 24-h period. The logistic regression analysis and Roger’s model were used for determinate the type and the parameters of predator’s functional response, respectively. The results showed that O. albidipennis preferred first and second instar nymphs of the aphid in comparison with other life stages. Both sexes of O. albidipennis showed the type II functional response to different densities of the nymphs. The handling times (Th) of O. albidipennis female and male were 1.344 and 1.584 h, respectively. The attack rate value (a) of the female and male bugs were 0.0839 and 1.4814 h−1, respectively. No significant differences were observed between the calculated Th and a for the female and male bugs. From the findings of this study, O. albidipennis can be used in biological control program of A. fabae.

Hunting practices of the Jahai indigenous community in northern peninsular Malaysia

Humans have been part of the ecology of Southeast Asian rainforests for millennia. Understanding the hunting practices of forest-dwelling people is important for designing policies and practices aimed to protect both vulnerable wildlife populations and human communities. The Jahai people are forest-dwelling hunter-gatherers living in northern Peninsular Malaysia and believed to be direct descendants of the first anatomically modern humans that arrived to the Malay Peninsula at least 50,000 years ago. We conducted semi-structured interviews in three Jahai villages around the Royal Belum State Park, asking about their knowledge and hunting habits of 11 wild mammal species. Specifically, we asked whether they were able to identify and whether they hunted the 11 animals, their relative prey preference, perceived trends of the animals’ populations, and how they hunted and handled them. Our respondents were familiar with all the species in the survey. None of the 87 respondents claimed to hunt tigers and elephants. The most preferred and commonly hunted species were medium-sized arboreal animals (gibbons and giant squirrels, hunted by >80% of respondents), whereas larger and more dangerous animals (gaur, sun bear, and tapir) were only hunted by a minority (<10%). The Jahai use traditional hunting methods, mainly blowpipes, spears, traditional snares, and fire traps (for smoking animals out of burrows). Only two respondents reported using firearms. Elephant numbers were perceived to be stable; all the other species were perceived to be declining moderately. Almost all the meat caught by the Jahai is for self-consumption, very little is traded with outsiders. The impacts of Jahai hunting on wildlife populations remain unclear, but our study provides a fundamental understanding of Jahai hunting practices for future management and conservation purposes.

Disentangling the effects of habitat biogeochemistry, food web structure, and diet composition on mercury bioaccumulation in a wetland bird

Methylmercury (MeHg) is a globally pervasive contaminant with known toxicity to humans and wildlife. Several sources of variation can lead to spatial differences in MeHg bioaccumulation within a species including: biogeochemical processes that influence MeHg production and availability within an organism’s home range; trophic positions of consumers and MeHg biomagnification efficiency in food webs; and individual prey preferences that influence diet composition. To better understand spatial variation in MeHg bioaccumulation within a species, we evaluated the effects of habitat biogeochemistry, food web structure, and diet composition in the wetland-obligate California black rail (Laterallus jamaicensis coturniculus) at three wetlands along the Petaluma River in northern San Francisco Bay, California, USA. The concentration of MeHg in sediments differed significantly among wetlands. We identified three sediment and porewater measurements that contributed significantly to a discriminant function explaining differences in habitat biogeochemistry among wetlands: the porewater concentration of ferrous iron, the percent organic matter, and the sediment MeHg concentration. Food web structure and biomagnification efficiency were similar among wetlands, with trophic magnification factors for MeHg ranging from 1.84 to 2.59. In addition, regurgitation samples indicated that black rails were dietary generalists with similar diets among wetlands (percent similarity indices > 70%). Given the similarities in diet composition, food web structure, and MeHg biomagnification efficiency among wetlands, we concluded that variation in habitat biogeochemistry and associated sediment MeHg production was the primary driver of differences in MeHg concentrations among black rails from different wetlands.

Parasites influence cannibalistic and predatory interactions within and between native and invasive amphipods.

In Northern Ireland, the amphipods Gammarus duebeni celticus (native) and G. pulex (invasive) coexist in some places, whilst in others the native species has been replaced by the invader. We explored the role of parasites in mediating interactions between these amphipods, which demonstrate mutual intraguild predation (IGP: predation between animals that also compete for prey). IGP and cannibalism can be important factors in structuring populations and communities. We investigated the effects of parasitism on rates of IGP between G. d. celticus and G. pulex and on cannibalism within each species by comparing functional responses (FRs: relationships between the use of a prey resource and its availability). Infection with the microsporidian Pleistophora mulleri caused an increase in IGP and cannibalism by G. d. celticus, which showed increased attack rates and reduced prey handling times. In contrast, infection with the acanthocephalan parasite Echinorhynchus truttae did not alter IGP or cannibalism by G. pulex. A prey preference experiment revealed that both amphipods were more likely to feed on heterospecific rather than conspecific prey, and this was also corroborated by the fact that overall IGP FRs were higher than cannibalism FRs. This may be selectively advantageous, as feeding on heterospecific prey removes possible competitors without the risk of consuming juvenile kin or acquiring parasites from infected conspecifics. Infection of the native G. d. celticus with P. mulleri enhanced IGP on the invasive G. pulex, which is likely to facilitate the coexistence of the 2 species.

Marine mammal distribution in the Gulf of Alaska from long-term passive acoustic monitoring

The Gulf of Alaska is an important habitat for a diverse array of marine mammals. Conservation of these populations requires understanding how they utilize different habitats. Passive acoustic monitoring can be used to study marine mammal distributions because the unique call types produced by many species are well documented. Between 2011 and 2015, a total of 3793 days of recordings were collected from autonomous acoustic recorders at five locations broadly distributed around the continental shelf, slope, and offshore seamounts in the Gulf of Alaska. The spatial and temporal patterns of seven cetacean species were examined and differences in habitat use and behavior were compared. Inshore habitats featured higher detections of humpback (Megaptera novaeangliae) and gray (Eschrichtius robustus) whales, while blue (Balaenoptera musculus) and sperm (Physeter macrocephalus) whales were more commonly detected offshore. The presence of most species was seasonal, with peak calling in spring and summer for gray and sperm whales, and fall and winter for fin (B. physalus) and humpback whales. Call type distributions within a species differed spatially for fin and killer (Orcinus orca) whales, and seasonally for blue whales. Spatial differences may indicate prey preferences, while seasonal changes are likely related to behavioral shifts from feeding to mating.

Ecological Character Displacement in Non-Congeneric Frogs.

Character displacement is phenotypic divergence driven by competition (ecological character displacement) or reproductive interference (reproductive character displacement). Although previous studies have examined these phenomena separately, recent evidence suggests that reproductive interference can drive both reproductive and ecological character displacement, in that certain traits are related to both competition and reproduction. Thus, to evaluate the effect of competition, the effect of reproductive interference must be excluded. Here, we analysed ecological character displacement between non-congeneric frogs, which show little reproductive interference. Odorrana amamiensis inhabits the Amami and Tokunoshima Islands, Japan, whereas its non-congeneric competitor Babina subaspera inhabits the Amami Island. We tested three of the Schluter (2000) criteria for ecological character displacement: phenotypic changes in O. amamiensis between the two islands, phenotypic change related to prey preference, prey availability between the two islands. We demonstrated that the three criteria in Schluter (2000) were likely to be satisfied, indicating the occurrence of ecological character displacement in non-congeners without reproductive interference. Thus, we conclude that competition is potentially the main driver of this phenotypic divergence, and that non-congeners may be a suitable model for evaluating ecological character displacement in a variety of organisms, as the influence of reproductive interference can be excluded.

Predator-prey body size relationships of cod in a low-diversity marine system

How predators select their prey largely defines ecosystem trophic structure, function and dynamics. In aquatic systems, organism body size is an important trait explaining predator− prey interactions. Here, we used a unique Atlantic cod Gadus morhua stomach content dataset with diet information from over 100 000 individuals collected from the Baltic Sea in 1963−2014, to explore prey size distribution and predator−prey mass ratios in the diet of Eastern Baltic cod. Maximum and average prey sizes increased with predator size, as for cod in other systems. However, the prey size spectra found in Eastern Baltic cod stomachs reflect the low species diversity in the Baltic Sea. In general, Eastern Baltic cod feed on smaller prey in relation to their body size than other cod populations. Due to the truncated prey size distribution in the Baltic Sea, cod cannibalism functions as a compensatory mechanism that allows Baltic cod to reach their prey size potential. On the other hand, small- and intermediate-sized cod prey mainly on a few invertebrate prey species, potentially making them vulnerable to changes in these prey populations. Our results encourage further studies disentangling the relative effects of prey preference and prey availability on cod trophodynamics in species-poor systems such as the Baltic Sea.

Diversified feeding strategies of Pleuragramma antarctica (Nototheniidae) in the Southern Ocean

The Antarctic silverfish Pleuragramma antarctica is a plankton feeder playing an important role at midtrophic level in High Antarctic ecosystems. Over years, a reasonable bulk of information has been collected on the trophic ecology of the species, nevertheless there are several aspects that still deserve full elucidation. Here, we explore the trophic plasticity of the Antarctic silverfish, its ability to change and optimize the diet according to the prey availability. A comparative analysis of stomach contents, fish condition, and feeding selectivity was performed on specimens from three different locations in the Southern Ocean: Ross Sea, Adelie Land, and Antarctic Peninsula. While confirming copepods and euphausiids as the most important items in the diet of the Antarctic silverfish, present study revealed significant local differences in diet composition and fish condition. The specimens from the Antarctic Peninsula showed the highest body condition and euphausiids were the preferential prey. Conversely, specimens from the Ross Sea and Adelie Land, where euphausiids are not as abundant as in those of the Antarctic Peninsula waters, showed a flexible and opportunistic trophic habit strategy, profiting of being able to rely on the great variety of planktonic prey. Being P. antarctica a key component of the pelagic ecosystem, improved knowledge of the trophic ecology of this fish would contribute to a throughout understanding of its response to environmental change.

Lack of prey switching and strong preference for mosquito prey by a temporary pond specialist predator

1. The strengths of trophic interactions within ecosystems can be mediated by complex mechanisms that require elucidation if researchers are to understand and predict population‐ and community‐level stabilities. Where multiple prey types co‐occur, prey switching (i.e. frequency‐dependent predation) by predators may facilitate low‐density prey refuge effects which promote coexistence. On the other hand, lack of switching and strong preferences by predators can strongly suppress prey populations, which is especially important considering vector species such as mosquitoes.2. The present study quantifies prey switching and preference patterns of the temporary pond specialist copepod Lovenula raynerae towards larvae of the medically important Culex pipiens mosquito complex in the presence of different proportions of alternative Daphnia pulex prey. Further, it examines whether prey switching and preferences are contingent on the sex of the predator.3. Lovenula raynerae exhibited a lack of prey switching and strong preference for larval mosquito prey overall, irrespective of predator sex. Also, when larval mosquitoes were available in higher proportions over daphniids, the strength of this positive selectivity increased. There was very little low‐density refuge for mosquitoes where they were rare.4. Lack of prey switching and strong preferences towards mosquitoes by predatory paradiaptomid copepods may enhance population‐level regulation of disease vector mosquitoes that exploit temporary pond‐style habitats. Accordingly, the conservation and promotion of these predators might enable better management of medically important species across landscapes.

Short communication:The relationships between gut length and prey preference of three pipefish (Syngnathus acus, Syngnathus typhle, Nerophis ophidion Linnaeus, 1758) species distributed in Aegean Sea, Turkey

In this research, gut lengths and numerical occurrence of gut contents (NO%) of Syngnathus acus, Syngnathus typhle and Nerophis ophidion were examined. The specimens were captured with beach seine net in different habitats (vegetated and sandy-vegetated etc.) at the coasts of Aegean Sea between 2006-and 2008. The zooplanktonic preys were mostly consumed by Syngnathus acus and Nerophis ophidion, whereas, Syngnathus typhle showed the most prey diversity. Thus, zooplanktonic organisms and other vegetative materials were consumed by three pipefish, however, vegetative tissues were mostly preferred by Syngnathus typhle. Feeding habits of three pipefish species are associated with gut lengths that show differences in increases or decreases depending on fish size and shape. In this study, S. typhle that has longer gut length than the other pipefish species explains the situation of being rich in organic gut content such as benthic zooplankton. Accordingly, probable presence or absent of the relationships between gut length and prey groups for three pipefish were discussed. The obtained results have supported low index values explaining the exact feeding habits of the pipefish.

Evaluation of food competition and resource partitioning of recruiting fish with permanent residents in a seagrass habitat

ABSTRACT Recruiting King George whiting Sillaginodes punctatus were studied to assess the potential for food competition with permanent resident fish species in a nursery habitat. Marine migrant post larval S. punctatus (<60 mm TL) consumed primarily harpacticoid copepods and had high (>0.6) diet overlap with permanent resident fish species Favonigobius leteralis and Stigmatopora nigra. Food electivity index indicated that S. punctatus juveniles preferred harpacticoid copepods and amphipods, while juvenile Heteroclinus adelaide and Gymnapistes marmoratus migrating to the nursery habitat targeted larger prey such as amphipods. Preference for larger prey by H. adelaide and G. marmoratus species coupled with differences in prey composition in the stomach was due to mouth size and feeding habits, resulting in different food preferences to S. punctatus. The whiting showed an ontogenetic shift in diet with early settlers (>60 mm TL) consuming less copepods and more amphipods, while previous year recruits (>120 mm TL) consumed polychaete worms. This study indicates that competition for food resources between the new recruiting S. punctatus juveniles and permanent resident juveniles is reduced through differences in temporal and spatial feeding behaviours, mouth morphology, and ontogenetic shift in prey consumption.

Ant Morphology Mediates Diet Preference in a Neotropical Toad (Rhinella alata)

Despite the widespread occurrence of myrmecophagy in anurans, it is unclear whether ant-specialists feed on ants opportunistically or whether they prefer species with specific morphological, ecological, or nutritional traits. We flushed 105 stomachs of a lowland neotropical toad, Rhinella alata, and identified each consumed ant to species level. We calculated linear selectivity to determine predator preference for ants by comparing the abundances of consumed species to their relative abundances in the leaf litter community on Barro Colorado Island, Panama. We conducted linear regression models to test whether linear selectivity or general predator preference related to seven morphological characteristics and two measurements of nutritional content. Rhinella alata preferentially ate 24 ant species. Other species were either avoided (n = 34) or were eaten opportunistically (n = 26). Preferred ant species were large and textured with hair and/or rugosity. We found that prey preference did not relate to prey nutritional content, that small ants were avoided even if they were superabundant in the environment, and that chemically defended and aggressive ants were preferred if they were large enough. We propose that R. alata prefers large ants because they represent a more efficient prey item in terms of prey handling time and because they are easier to see than are smaller ants. Furthermore, we hypothesize that predation attempts are more successful when prey are textured because microstructures on the tongue and prey surface may increase prey adhesion. The ant specialist R. alata is not specializing on any particular ant species but rather maximizing prey quantity over quality by only eating the largest ants, despite their scarcity in the environment.

Density and diet of invasive alien anuran species in a disturbed landscape: A case in the University of the Philippines Mindanao, Davao City, Philippines

Abstract. Jabon KJD, Gamalo LED, Responte MA, Abad RG, Gementiza GDC, Achondo MJMM. 2019. Density and diet of invasive alien anuran species in a disturbed landscape: A case in the University of the Philippines Mindanao, Davao City, Philippines. Biodiversitas 20: 2554-2560. The population density and diet composition of three invasive alien anuran species (IAS) (Kaloula pulchra, Hoplobatrachus rugulosus, and Rhinella marina), recorded from a disturbed landscape in the University of the Philippines Mindanao campus, were determined in this study. With the total area of 18 km2 covered, R. marina (3.89 ind/km2) showed to have the highest density followed by K. pulchra (0.83 ind/km2) then H. rugulosus (0.056 ind/km2). Furthermore, 20 samples of R. marina and 10 samples of K. pulchra were used for gut analysis in which all have stomach contents. Thirteen prey orders were identified in both species with different degrees of prey digestion. The Frequency of Occurrence (FOO) showed orders Spirobolida, Coleoptera, and Hymenoptera were constantly occurring in R. marina while only Hymenoptera for K. pulchra. On the other hand, Degree of Food Preference showed similarities with Hymenoptera as the most preferred prey in both species, especially for K. pulchra (2.8) as compared to R. marina (1.8). Accidental occurrence and occasionally preferred food items such as organic and inorganic matter were also present in both species. A notable record of a skink (Reptilia, Scincidae) as prey item was found in R. marina, indicative of its generalist and carnivorous diet and its potential impact on the native vertebrate fauna in the area. This study proposes that the occurrence of IAS with regards to their high density and variable prey preferences can be a factor disturbing biological diversity in an altered landscape.

Prey preferences of notonectids towards larval mosquitoes across prey ontogeny and search area.

Predatory biological control agents can be effective natural means of managing pests, vectors and invasive species. However, the strength of predator-prey interactions can be regulated through context-dependencies that often remain unquantified. In particular, refuge effects can influence the efficacy of biological agents towards target species, and such effects are often driven by prey size and search area differences. In this study, we quantify the prey preferences of two predaceous notonectids, Anisops breddini and Anisops sardeus, towards four different aquatic larval instar stages of the medically important mosquito Culex quinquefasciatus across variations in surface area and water depth. Consumption rates differed significantly among the four larval sizes but not between the notonectids. Search area variations also elicited differences in consumption rates. Both predators tended to prefer second-instar mosquito prey among surface area and water depth variations, while generally avoiding the largest (fourth instar) and smallest (first instar) prey instar stages. For both predators, differential selectivity traits were emergent across surface area variations and water depth, with refuge effects for small prey generally greatest at intermediate-large depths with high surface areas. We thus demonstrate that predatory impacts of notonectids towards mosquito larvae differ significantly according to prey size, and likely peak at intermediate size classes. Different mosquito size classes often coexist and compete, selectivity has important implications for adult mosquito proliferations. Further, in ephemeral aquatic habitats where surface areas and water depths are highly variable spatiotemporally, the efficacy of notonectids in controlling mosquito prey may differ substantially. © 2019 Society of Chemical Industry.

Prey-specific experience affects prey preference and time to kill in the soil predatory mite Gaeolaelaps aculeifer Canestrini

Generalist predators potentially have access to a wide array of prey, but it is little studied how experience with specific prey affects preference for this prey. In particular, it is unknown how experience with pest prey affects predator foraging decisions in cases where the pest is nutritious but protected by a repelling, potentially deadly defence. We investigated preference of the soil predatory mite Gaeolaelaps aculeifer Canestrini for the risky pest springtail Protaphorura fimata Gisin relative to the safe non-pest springtail Folsomia candida Willem. Egg production under foraging on live or dead individuals of either prey showed that the two species had equal nutritional quality for G. aculeifer, but indicated that live F. candida were more difficult to catch than live P. fimata. Importantly, some G. aculeifer were killed by P. fimata defence secretions, demonstrating that live P. fimata are risky prey. Preference for P. fimata was generally high when mites were given a choice between a live individual of either prey, but this preference was reduced following exposure to live individuals of P. fimata. Furthermore, fewer G. aculeifer killed a prey and time until kill was longer after experience with live P. fimata. These findings indicate that live P. fimata induced a partial aversion on G. aculeifer during exposure. Our study shows that generalist predators can reduce their preference for risky prey following exposure. This indicates that generalist predators used in biological control against risky prey are most efficient against this prey if not exposed to it prior to release.

Turbidity effects on feeding by larvae of the endemic Ambystoma mexicanum and the introduced Oreochromis niloticus in Lake Xochimilco, Mexico

We tested the effects of turbidity on the prey consumption by larvae of an endemic predator (Ambystoma mexicanum) and an introduced predator (Oreochromis niloticus). Prey visibility is influenced by turbidity due to organic (algae, detritus) or inorganic (sediment) matter. We hypothesized that the axolotl would be more adversely influenced by high turbidity as compared to Tilapia, resulting in a decrease in the population of the former under natural conditions. In order to test this, we conducted prey preference experiments and tested the reactive distance of both predators at different turbidity levels. Prey species used were the cladocerans Alona glabra, Moina macrocopa, Macrothrix triserialis and Simocephalus vetulus and the ostracod Heterocypris incongruens. In both, clear and turbid waters, the fish consumed about 17.5% and 2.5%, respectively, more prey. The axolotl had a greater preference for H. incongruens and M. triserialis, and the fish for S. vetulus and M. triserialis. These results indicate that turbidity decreases the visual ability during the larval stages of A. mexicanum which in turn influences its capacity to compete against the invasive fish. Implications of these results on conservation efforts for the highly endangered Ambystoma mexicanum are discussed.

Functional responses of three guilds of spiders: Comparing single‐ and multiprey approaches

AbstractSpiders are successful natural enemies of pests occurring throughout the different strata of an agroecosystem. The study of their functional responses can provide information related to the potential effectiveness of different species and guilds on reducing a pest population. However, multiple prey availability may change the functional response of a predator. In this study, the functional responses of three species of spiders in single‐prey and multiple‐prey experiments were modelled. The spider species Haplodrassus rufipes, Araniella cucurbitina and Synema globosum were chosen as being representatives of ground runners, orb‐weavers and ambushers, respectively. Three prey species were selected: a target prey, the Mediterranean fruit fly Ceratitis capitata and two alternate prey species, the flour moth Ephestia kuehniella and the house cricket Acheta domesticus. When the selected target prey C. capitata was supplied in the single‐prey experiments, the most and the least efficient spider species were H. rufipes and S. globosum, respectively. However, opposite results were obtained when alternative preys were supplied. Also, A. cucurbitina significantly changed its functional response with the presence of alternative prey species. Considering the prey preference during the multiple‐prey experiments, A. domesticus, used as representative of a heavy, long‐sized and highly motile prey, was avoided whereas C. capitata, the target pest used as representative of light, small‐sized and moderately motile prey was preferred by the three species of spiders. Ephestia kuehniella, used as representative of light, medium‐sized and low motile prey was occasionally consumed. Each guild could include efficient predators against pests according to its hunting strategies and the ecological exploited niches. Orb‐weavers could be efficient predators against flying pests; ambushers such as S. globosum could contribute to the reduction of the populations of flower‐visiting pests, whereas active ground hunters may also play an important role preying on pests that develop a part or all of its life cycle in the ground. However, further research on feeding behaviour such as prey switching is needed for a better understanding of the effectiveness of spiders as natural enemies.

Prey preference, environmental tolerances and ichthyotoxicity by the red-tide dinoflagellate Noctiluca scintillans cultured from Tasmanian waters

AbstractThe large phagotrophic dinoflagellate Noctiluca has become a prominent red tide organism in southeast Australian waters since the 2000s, raising concerns for beach tourism, grazing impacts as well as ichthyotoxicity for finfish aquaculture. Satisfactory culture growth rates (0.23–0.56 per day) were obtained by feeding with small Thalassiosira diatom and Tetraselmis flagellate diets, while optimal growth rates sustained for up to 8 months (0.69 per day) were achieved by feeding in a plankton wheel with the large chain-forming dinoflagellate Gymnodinium catenatum. Noctiluca was highly tolerant towards salinities from 20 to 35 and growth was stimulated by temperatures increasing from 10 to 23°C, which in combination with the key factor of prey abundance explains the incidence in southeast Australia of predominantly summer and spring but occasionally also winter blooms. Fatty acid biomarkers suggest that Tasmanian field populations indiscriminately feed on available diatom and dinoflagellate mixtures. Noctiluca exhibited very limited ichthyotoxicity, and only at the highest cell concentrations of 2 000 000/L (50% reduction in RTgill W1 cell viability). Only the densest red tide surface slicks contained acutely toxic levels of unionized ammonia of 242 to 510 μg/L while inshore slicks generated oxygen concentrations as low as 0–1.5 ppm. Lipid phycotoxins (eicosapentaenoic acid, docosahexaenoic acid) did not appear to contribute to Noctiluca ichthyotoxicity. The fatty acid 20:0 eicosanoic acid may serve as a potential Noctiluca biomarker in marine food webs and sediments.

Male preferences for nuptial gifts and gift weight loss amongst the nursery web spider, Pisaura mirabilis

Sexual signals produced by males are costly. Thus, sexual selection may favour males who are able to minimise these costs, but are still reproductively successful. I investigated male preferences for large and small prey used to produce nuptial gifts, which serve as sexual signals in the nursery web spider, Pisaura mirabilis. Males showed strong preferences for large prey compared to small prey for gift production, which is likely to be more attractive for females and provide males with longer copulation. These preferences were strongest in the presence of female pheromonal cues. Unexpectedly, carrying nuptial gifts did not influence male mobility measured in a 5-min-long behavioural test. Both gifts carried by males and the intact gifts showed similar decreases in mass, suggesting that males did not actively suck the content of the gift. Wrapping the gift with silk did not effectively reduce the gift’s dehydration, meaning that the function of the silk is not to protect the gift’s contents. Males in poor body condition were more likely to feed on the prey before wrapping it. This is consistent with condition dependent signalling, as males in low condition are predicted to feed on the prey as they are not able to pay the cost of gift production. Males preferred large-sized prey for gift production and males in poor body condition reduced the cost of gift production by feeding on the prey, supporting the idea that nuptial feeding is condition dependent.