Reproductive behavior of three tube-building peracarid crustaceans: the amphipods Jassa falcata and Ampithoe valida and the tanaid Tanais cavolinii

Laboratory observations indicate that Jassa falcata, Ampithoe valida, and Tanais cavolinii have quite similar patterns of behavior: males attend females until ovulation and copulation occur, then the male and female separate. Females tend to remain inside their own tubes, and males tend to move between the tubes of receptive females. This pattern of behavior (called “cruising males”) may be common in crustaceans that exhibit some fidelity to a specific site, especially if the females produce several broods in succession, do not store sperm, and reproduce asynchronously relative to each other. Male J. falcata have a terminal molt which is marked by the presence of a specific sexually dimorphic characteristic, the “thumb” on the second gnathopod. Alternating ovulatory and anovulatory molts occur in T. cavolinii.     

Modelling habitat overlap among sympatric mesocarnivores in southern Illinois,USA

Few researchers have developed large-scale habitat models for sympatric carnivore species. We created habitat models for red foxes (Vulpes vulpes), coyotes (Canis latrans) and bobcats (Lynx rufus) in southern Illinois, USA, using the Penrose distance statistic, remotely sensed landscape data, and sighting location data within a GIS. Our objectives were to quantify and spatially model potential habitat differences among species. Habitat variables were quantified for 1-km² buffered areas around mesocarnivore sighting locations. Following variable reduction procedures, five habitat variables (percentage of grassland patches, interspersion–juxtaposition of forest patches, mean fractal dimension of wetland patches and the landscape, and road density) were used for analysis. Only one variable differed (P < 0.05) between red fox and coyote sighting areas (road density) and bobcat and coyote sighting areas (mean fractal dimension of the landscape). However, all five variables differed between red fox and bobcat sighting areas, indicating considerable differences in habitat affiliation between this pair-group. Compared to bobcats, red fox sightings were affiliated with more grassland cover and larger grassland patches, higher road densities, lower interspersion and juxtaposition of forest patches, and lower mean fractal dimension of wetland patches. These differences can be explained by different life history requirements relative to specific cover types. We then used the Penrose distance statistic to create habitat models for red foxes and bobcats, respectively, based on the five-variable dataset. An independent set of sighting locations were used to validate these models; model fit was good with 65% of mesocarnivore locations within the top 50% of Penrose distance values. In general, red foxes were affiliated with mixtures of agricultural and grassland cover, whereas bobcats were associated with a combination of grassland, wetland, and forest cover. The greatest habitat overlap between red foxes and bobcats was found at the interface between forested areas and more open cover types. Our study provides insight into habitat overlap among sympatric mesocarnivores, and the distance-based modelling approach we used has numerous applications for modelling wildlife–habitat relationships over large scales.     

Asymmetric mate choice,hybridization, and hybrid fitness in two sympatric grasshopper species

Hybridization between animal species has long been regarded as unusual, but is meanwhile accepted as a widespread phenomenon. Typically, sexual interactions among species are studied in secondary contact zones of closely related species (hybrid zones) or between invasive and native species, whereas hybridization between sympatric congeners has received little attention. Here, we present a study on the hybridization potential of two grasshopper species, Chorthippus parallelus and Chorthippus montanus, which occur sympatric in large parts of Eurasia. We performed a mate choice experiment with specimens from an area of local parapatry in northwestern Germany. Most copulations were conspecific, but males of both species and females of C. parallelus were indiscriminate in their mate choice, while females of C. montanus clearly preferred conspecific males. Although these results suggest asymmetric introgression, a no-choice hybridization experiment revealed that hybridization is possible in both directions. The hatching success of the hybrids was intermediate between the parental species. Female hybrids showed no clear mate preferences, indicating that back crossing in both directions is possible in principle, but the fertility of the hybrids and the fitness of the F2 generation remain unknown. Our study suggests that hybridization between sympatric species might occur more often than assumed. It might affect the ecology and local distribution of animals in a similar way as competition does.     

Feeding habits and food requirements of some amphipods in the Black Sea

Food composition and food requirements of four amphipod species of the Black Sea — Dexamine spinosa (Mont.), Amphithoe vaillanti Lucas, Gammarellus carinatus (Rathke), Gammarus locusta L. — were studied and an attempt made to assess the quantitative composition of the food, daily rhythms of feeding, and daily food rations. The composition of the food of the above-named amphipods is similar and consists primarily of seaweeds. Daily feeding intensity is characterized by a distinct increase during the dark period (D. spinosa, A. vaillanti); however, in G. locusta feeding intensity increases in the morning. The daily food rations of amphipods vary greatly (from 1.7 to 360% of the specimens' body weight), depending upon factors such as the kind of food, age, sex, the physiology of specimens, and the water temperature. The total quantity of food eaten by the populations of these amphipods in the coastal zone of the Black Sea amounts to 4000 g organic matter per square metre within 1 year.     

Determinants of habitat association in a sympatric clade of marine fishes

Triplefin fishes reach their greatest diversity in New Zealand with 26 endemic species, and habitat diversification has been implicated as a key factor in the divergence of this group. Despite this, it is unknown whether species-specific habitat patterns in these sympatric fishes are established by passive processes (e.g. differential mortality) or by habitat selection during settlement. We investigate this question by comparing the habitat associations of new recruits with those of conspecific adults in five species. In addition, the amount of variation in habitat use of conspecific recruits and adults was calculated to identify ontogenetic shifts in habitat association. The results indicated that while there were some differences between recruit and adult habitats, these differences were small in magnitude and habitat use of new recruits was similar to that of adult conspecifics. This finding was further supported by the small difference in variation of habitat use between conspecific recruits and adults. The study suggests that new recruits are actively involved in the selection of habitats at settlement and maintain the use of these throughout demersal life. Habitat use in these territorial species has a large influence on mate choice, thus habitat selection by new recruits would provide a powerful mechanism for pre-zygotic isolation between individuals with different habitat preferences. Together these findings support the notion that habitat diversification has been a major component in the radiation of this sympatric group.     

Feeding,tube-building and particle-size selection in the terebellid polychaete Eupolymnia nebulosa

The present study was aimed at obtaining new information on the feeding biology of the terebellid polychaete Eupolymnia nebulosa (Montagu). Feeding, tube-building, particle-size selection, and tentacular morphology are described. Experiments were carried out during the fall of 1984 on specimens collected in the vicinity of Banyuls-surmer, France. Feeding activity takes place almost entirely in darkness, and E. nebulosa shows a marked preference for smaller particles when feeding and selects larger ones for tube-building. The control of its activity pattern by light intensity may result in seasonal modification of its energy balance. The mechanisms controlling particle-size selection in this species are discussed.     

The role of different types of detached macrophytes in the food and habitat choice of a surf-zone inhabiting amphipod

Allorchestes compressa is the dominant macroinvertebrate species in wrack accumulations on surf zones of south-western Australia. These amphipods were provided with a choice of macrophyte material representing brown and red algae and seagrass in a series of preference experiments in the laboratory and field. Feeding experiments showed that A. compressa exhibited a strong preference for particular types of macrophytes (P < 0.01). Amphipods primarily consumed brown algae, with 69–98% of the biomass of Ecklonia radiata and 64% of the biomass of Sargassum sp. lost over the experiments. This study has shown that the amphipod A. compressa exhibits a clear preference for brown algae over red algae and seagrass as food. In terms of habitat preference, tank experiments using a series of pair-wise comparisons showed that, in the absence of fish predators, A. compressa selected seagrass as its preferred habitat over the other types of wrack (P < 0.001). When satiated or starved, between 68 and 83 and 79 and 98% of amphipods were found in Amphibolis and Posidonia, respectively. In contrast, field-cage experiments revealed that A. compressa preferred either mixed wrack, brown algae or red algae over seagrass as a habitat (P < 0.01). The contrasts between results from the laboratory and field suggest that other factors such as the presence of predators, water flow and light could influence habitat choice in the surf zone. This study shows that allochthonous material transported to surf zones from other habitats therefore play different roles in driving secondary production in this shoreline habitat.     

Comparative study of sympatric populations of two hyperiid amphipods,Primno johnsoni and P. evansi,from the eastern North Atlantic Ocean

Hyperiid amphipods are common components of oceanic plankton communities, and yet information is lacking on the ecology and biology of the majority of species. This study compares sympatric populations of two phrosinids, Primno johnsoni Bowman and P. evansi Sheader, using material collected on R.R.S. Discovery Cruises 121 and 140 in the eastern North Atlantic Ocean. The two species were essentially very similar in terms of their vertical distribution and diurnal migration, and in the timing and sequence of reproductive events (onset of maturity, ovary/oocyte development, marsupial development). Mature female and male size ranges were comparable for the two species, males maturing at a small body size, with little subsequent growth, and females producing a succession of 5 (P. evansi) or 6 (P. johnsoni) broods maximum. The species differed significantly in egg size and in total egg output. These dissimilarities were accentuated by differences in the patterns of female mortality in the two species, such that actual egg output per female was 1.8 times greater (2.4 times in terms of total egg volume) in P. johnsoni than in P. evansi. These differences were reflected in the relative size of populations in the field. Although both species are capable of consuming a wide range of planktonic prey, it is assumed that gelatinous species are important, especially as hosts for the parasitoid juvenile stages, and it is suggested that host-specificity might act to separate the niches.     

Individual- and condition-dependent effects on habitat choice and choosiness

Research on consistent individual differences in behavior, or “behavioral syndromes”, continues to grow rapidly, and yet, the aspects of behavior under consideration have remained remarkably limited. Here, we consider individual variation in consistency of choice (termed here “choosiness”), as expressed during habitat choice. We repeatedly tested the responses of female Western Black Widows, Latrodectus hesperus, to two cues of habitat quality: prey chemical cues and variation in web site illuminance. We estimated females’ response by the distance they positioned themselves from (1) the source of prey chemical cues and (2) the darkest edge of our test arena. Individuals with low variance in their responses are deemed more “choosy”, whereas individuals with high variance are deemed less “choosy”. Generally, most females initiated web construction near the source of the prey chemical cues and tended to place themselves in low-light conditions. However, we detected strong, repeatable differences in females’ intensity of response, and within-individual variance of response (i.e., choosiness) was correlated across situations: females with highly consistent responses towards cricket chemical cues also exhibited highly consistent responses towards variation in light conditions. When deprived of food for extended periods, females were indistinguishable in their responses towards prey chemical cues, but tended to initiate web construction in brighter lighting conditions. Food-deprived females universally exhibited higher variance and diminished consistency in their responses (i.e., they were less choosy). Additionally, higher choosiness was associated with greater mass loss during choice trials, suggesting choosiness is energetically costly. Our results demonstrate that consistency of response to environmental cues is yet another element of behavior that varies among individuals and variation in choosiness could beget speed/quality trade-offs during animal decision making.     

Phototaxis and cannibalism in gammaridean amphipods

Development changes in phototaxis were investigated in three species of estuarine amphipods: Gammarus lawrencianus. G. tigrinus, and G. mucronatus. In water of salinity 16 S, juveniles of G. tigrinus and G. mucronatus were initially photoneutral and became photonegative within two days after hatching. Juveniles of G. lawrencianus were photopositive on hatching and became photoneutral approximately 11 d later. In all three species, the change in phototactic behaviour coincided with the age at which juveniles became less vulnerable to cannibalism by adults. Evidence is presented showing genetic variation for phototactic behaviour in G. lawrencianus. Our data are consistent with the hypothesis that juvenile phototaxis is an evolutionary adaptation to minimise cannibalism by adults.     

Scavenging deep-sea amphipods: Effects of food odor on oxygen consumption and a proposed metabolic strategy

In situ respiration rates as a response to the odor of food were measured for two species of scavenging amphipods, Paralicella caperesca from 3 650 m in the western North Atlantic Ocean and Orchomene sp. B from 1 300 m in the Santa Catalina Basin off southern California (USA). In addition, complementary laboratory starvation/respiration rates for a shallow-water species, Orchomene sp. A, were determined. Initial elevated O₂ consumption rates were found for up to 8 h in all deep-sea amphipods exposed to bait odor, followed by a period of lowered respiration equivalent to rates observed in individuals not exposed to bait. Orchomene sp. A revealed a response similar to that observed in the deep-sea species. A metabolic strategy is proposed whereby scavenging amphipods efficiently utilize large episodic organic falls in the food-limited environment of the deep sea. This strategy involves (1) the ability to withstand long periods of starvation, (2) rapid response to an organic fall, (3) rapid location of the organic fall, (4) maximal rate of food consumption with maximal quantity ingested, and (5) efficient utilization of the consumed food. Each of these attributes are explored with the expected and observed mechanisms employed to achieve them.     

The evolution of leks through female choice: differential clustering and space utilization in six sympatric manakins

Summary The degree to which lekking and non-lekking male manakins select display sites in order to maximise proximity to females was examined by contrasting movements of females with male dispersion. Data on female visiting patterns, male courtship disruption, and mating skew were also collected over three successive breeding seasons. For the five lek-breeding species, female home-ranges were 3–7 times larger than those of adult males. Female movements were concentrated around leks, fruiting places and stream bathing sites. None of the females monitored by radio-tracking expanded her normal range in order to visit males on leks. On the contrary, feeding bouts of females frequently preceded a visit to potential mates at neighboring leks. Despite small sample sizes, significant correlations were found between female home-range size and male clustering (distances between neighboring leks and distances between neighboring males), as predicted by the female choice model and the hotspot model. Adult and immature male home-range sizes were not significantly correlated with male dispersion or female ranges. On the other hand, males and females of the only non-lekking species exhibited similar use of space and home-range size. Male settlement at sites with high levels of female traffic showed that the hotspot model is adequate to explain differences in male dispersion among sympatric lekking species. Comparisons with other studies suggest that apparent female choice could be overidden by past and present male-male interactions or female mate-comparison tactics. In fact, both the hotspot model and the attractiveness hypothesis appear to shape male dispersion on leks: males appear to settle under hotspot conditions with despotic rules generated through bias in female choice or male-male interference. It is proposed that the evolution of leks is ecologically motivated by the spatio-temporal distribution of trophic resources, initially leading to a dispersed male-advertisement polygyny. Following this, a foraging ecology that promotes high mobility by females and the magnetic effect of mating skew in particular males may have favored clustering on exploded leks. Later, the development of male-male interference and the increasing female home-range size could have led to the evolution of classical leks.     

Winter food utilisation by sympatric mysids in the Baltic Sea,studied by combined gut content and stable isotope analyses

We studied the winter dietary characteristics of two sympatric mysid species, Mysis mixta and M. relicta, which exploit both benthic and pelagic habitats during diel vertical migrations. Samples collected before and after the ice-covered period in the northern Baltic Sea were investigated using both stomach content analyses and stable isotope analyses of carbon and nitrogen. Both of the mysid species were omnivorous during winter and utilised both benthic and pelagic food sources. The main food source before the ice period was calanoid copepods (40 and 36% for M. mixta and M. relicta, respectively), and after ice-out calanoid copepods (23%) and zooplankton resting eggs (23%) for M. mixta and diatoms (44%) and calanoids (25%) for M. relicta. Their patterns of food utilisation broadly followed seasonal fluctuations in the abundance of the main prey groups. Although pelagic food availability is low in winter both mysid species utilised pelagic prey widely. We also show that when combining these different diet analysis methods it is important to take into account the time lag in isotopic signatures, otherwise the obtained results do not correspond but instead show the feeding history at different times.     

Meta-analysis: trophic level, habitat, and productivity shape the food web effects of resource subsidies

Studies of the effects of cross-habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat. Ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 data sets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, and functional group. Changes in consumer density or biomass in response to subsidies were inconsistent across habitats, trophic, and functional groups. Responses in stream cobble bar and coastline habitats were larger than in other habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity. The commonness of subsidy effects suggests that many ecosystems need to be studied as open systems.     

Thermal effects on swimming activity and habitat choice in juvenile Pacific cod (Gadus macrocephalus)

The behavioral responses of fishes to temperature variation have received less attention than physiological responses, despite their direct implications for predator–prey dynamics in aquatic ecosystems. In this paper, we describe the temperature dependence of swimming performance and behavioral characteristics of juvenile Pacific cod (Gadus macrocephalus; 75–125 mm total length). Maximum swimming speeds increased with temperature and body size. Routine swimming speeds of Pacific cod in small groups of similarly sized fish (N = 6) increased with body size and were 34 % faster at 9 °C than at 2 °C. The response to temperature was opposite that previously described for juvenile walleye pollock (Theragra chalcogramma), reflecting species-specific differences in behavioral responses. In a separate experiment, we demonstrated the effect of temperature on habitat selection of juvenile Pacific cod: Use of an artificial eelgrass patch in a 5-m-long laboratory tank was significantly greater at 9 °C than at 2 °C. These results illustrate that temperature affects a range of behavioral traits that play important roles in determining the frequency and outcomes of predator–prey interactions.     

Effects of nutrients, herbivory, and depth on the macroalgal community in the rocky sublittoral

We studied the interacting roles of nutrient availability and herbivory in determining the macroalgal community in a rocky littoral environment. We conducted a factorial field experiment where we manipulated nutrient levels and herbivory at two sublittoral depths and measured macroalgal colonization and the following young assemblage during the growing season. At the community level, grazing reduced algal colonization, though the effect varied with depth and its interaction with nutrient availability varied in time. In shallow water, the total density of macroalgae increased in response to nutrient enrichment, but the ability of grazers to reduce macroalgal density also increased with the nutrient enrichment, and thus, the community could not escape from the top-down control. In deep water, the algal density was lower, except in July when nutrient enrichment caused a very dense algal growth. Grazing at the greater depth, though effective, was generally of smaller magnitude, and in July it could not limit algal recruitment and growth. Species richness peaked at the intermediate nutrient level in deep but not in shallow water during most of the growing season. Grazing had no effect on diversity of the algal community at either depth and only a minor effect on species richness at the greater depth. Opportunistic and ephemeral algae benefited from the nutrient enrichment but were also grazed to very low densities. Slowly growing and/or perennial species colonized poorly in the nutrient enriched treatments, and depending on the species, either suffered or indirectly benefited from herbivory. For all species, effects of nutrients on colonization depended on depth; usually both nutrient and herbivory effects were more pronounced at the shallow depth. We conclude that grazers are able to reduce macroalgae over a large range of nutrient availabilities, up to 12-fold nutrient enrichment in the current experiment, and that the sublittoral depth gradient generates variation in the algal community control exerted by both herbivory and nutrient availability. Thus temporal and spatial variability in both top-down and bottom-up control and in their interaction, especially along the depth gradient, may be crucially important for producer diversity and for the successional dynamic in a rocky sublittoral environment.     

The indirect effects of eutrophication on habitat choice and survival of fish larvae in the Baltic Sea

The structure of the habitat is usually crucial for growth and survival of young life stages. Presently, some nursery areas of fish larvae are changing due to eutrophication, e.g. due to enhanced growth of ephemeral filamentous algae at the expense of perennial species. We studied the influence of two habitats, one with filamentous algae (Cladophora glomerata) and the other with bladder wrack (Fucus vesiculosus), on habitat choice of pike larvae (Esox lucius) in the absence/presence of a predator or a competitor. We further tested whether the habitat choice is adaptive in increasing survival under predation threat. In contrast to expectations, pike larvae preferred the habitat with ephemeral filamentous algae to the bladder wrack, thriving in clean waters, independent of the presence/absence of both predator/competitor. In addition, the survival of the larvae was higher in the filamentous algae in the presence of predators, which suggested that the habitat preference of the larvae was adaptive. The structure of the bladderwrack habitat was probably too open for newly hatched larvae, which implies that F. vesiculosus and other large brown algae are not as important refuges for young larvae as previously thought.     

Food and habitat choice in floating seaweed clumps: the obligate opportunistic nature of the associated macrofauna

The species composition of macrofauna associated with floating seaweed rafts is highly variable and influenced by many factors like spatial and temporal variation, period since detachment and probably also the seaweed species. The presence of seaweed preferences was assessed by a combination of in situ seaweed samplings and multiple-choice aquarium experiments in a controlled environment, using the seaweed-associated grazing organisms Idotea baltica and Gammarus crinicornis. Results from the sampling data confirm that the seaweed composition influences macrofaunal species composition and abundance: samples dominated by Sargassum muticum displayed higher densities but lower diversities compared to samples dominated by Ascophyllum nodosum and Fucus vesiculosus. Seaweed preference was also apparent from the multiple-choice experiments, but did not exactly match the results of the community analysis: (1) I. baltica had high densities in seaweed samples (SWS) dominated by F. vesiculosus and A. nodosum, while in the experiments, this isopod was most frequently associated with Enteromorpha sp. and F. vesiculosus, and fed mostly on S. muticum, A. nodosum and Enteromorpha sp.; (2) G. crinicornis had high densities in SWS dominated by F. vesiculosus, while in the experiments, this amphipod was most frequently associated with S. muticum, but fed most on A. nodosum and F. vesiculosus. It is clear from the laboratory experiments that preference for habitat (shelter) and food can differ among seaweed species. However, food and habitat preferences are hard to assess because grazer preference may change if choices are increased or decreased, if different sizes of grazers are used, or if predators or other grazers are added to the experiments. The effects of seaweed composition may also be blurred due to the obligate opportunistic nature of a lot of the associated macrofaunal species.