Activity budgets in two species of bee flies (Lordotus: Bombyliidae,Diptera): a comparison of species and sexes

Summary This study compares time budgets of males and females of two sympatric species of bee flies, Lordotus pulchrissimus and L. miscellus. These species occur synchronously in sand dune habitat on the north shore of Mono Lake, Mono Co., California, and feed almost exclusively on the flowers of a composite shrub, Chrysothamnus nauseosus. The two closely related species, with similar ecological requirements and in an identical environment, allocate time and energy in very different ways, even when accomplishing the same ecological goals. L. pulchrissimus males and females engage respectively in aggressive interactions and feeding primarily in the morning. In contrast, L. miscellus males and females intersperse brief periods of resting with flying when defending territories and feeding, and they keep up these activities until later in the day. Similarly within species, males are involved in aggressive interactions for a shorter period each day, and females feed over a longer period. Flies in all sex-species classes but male pulchrissimus allocate their time energetically in similar ways. Male pulchrissimus spend more time each day in energetically costly activities; they engage in continuous hovering flight and intense interactions in aerial aggregations. While male miscellus feed little, male pulchrissimus spend a large portion of their time feeding, approximately as much as females, contrary to the expectation that males should be foraging-time minimizers. This study corroborates the conclusions of previous studies on bee-fly communities by showing that nectar and pollen are important resources for adult bee flies, at least for some species: Individuals of these two species spend a large proportion of their adult lifetimes feeding.     

Height-growth response to climatic changes differs among populations of Douglas-fir: a novel analysis of historic data

Projected climate change will affect existing forests, as substantial changes are predicted to occur during their life spans. Species that have ample intraspecific genetic differentiation, such as Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), are expected to display population-specific growth responses to climate change. Using a mixed-effects modeling approach, we describe three-year height (HT) growth response to changes in climate of interior Douglas-fir populations. We incorporate climate information at the population level, yielding a model that is specific to both species and population. We use data from provenance tests from previous studies that comprised 236 populations from Idaho, Montana, and eastern Washington, USA. The most sensitive indicator of climate was the mean temperature of the coldest month. Population maximum HT and HT growth response to changes in climate were dependent on seed source climate. All populations had optimum HT growth when transferred to climates with warmer winters; those originating in sites with the warmest winters were taller across sites and had highest HT growth at transfer distances closest to zero; those from colder climates were shortest and had optimum HT growth when transferred the farthest. Although this differential response damped the height growth differences among populations, cold-climate populations still achieved their maximum growth at lower temperatures than warm-climate populations. The results highlight the relevance of understanding climate change impacts at the population level, particularly in a species with ample genetic variation among populations.     

Relationship among the species richness of different taxa

Spatially explicit forecasting of changes in species richness is key to designing informative scenarios on the development of diversity on our planet. It might be possible to predict changes in the richness of inadequately investigated groups from that of groups for which enough information is available. Here we evaluate the reliability of this approach by reviewing 237 richness correlations extracted from the recent literature. Of the 43 taxa covered, beetles, vascular plants, butterflies, birds, ants, and mammals (in that order) were the most common ones examined. Forests and grasslands strongly dominated the ecosystem types studied. The variance explanation (R2) could be calculated for 152 cases, but only 53 of these were significant. An average correlation effect size of 0.374 (95% CI = +/- 0.0678) indicates positive but weak correlations between taxa within the very heterogeneous data set; None of the examined explanatory variables (spatial scale, taxonomic distance, trophic position, biome) could account for this heterogeneity. However, studies focusing on 10-km2 grid cells had the highest variance explanation. Moreover, within-phylum between-class comparisons had marginally significantly lower correlations than between-phylum comparisons. And finally, the explanatory power of studies conducted in the tropics was significantly higher than that of studies conducted in temperate regions. It is concluded that the potential of a correlative approach to species richness is strongly diminished by the overall low level of variance explanation. So far, no taxon has proved to be a universal or even particularly good predictor for the richness of other taxa. Some suggestions for future research are inclusion of several taxa in models aiming at regional richness predictions, improvement of knowledge on species correlations in human dominated systems, and a better understanding of mechanisms underlying richness correlations.     

Preponderance of a few diatom species among the highly diverse microphytoplankton assemblages in the Bay of Bengal

The microphytoplankton assemblages were studied from water samples collected at eight discrete depths in the top 120 m at five central (open ocean) and four western (shelf/slope region) locations in the Bay of Bengal. The Bay is a low-productive warm pool regime with poor inorganic nutrient inputs to its intensely stratified surface layer despite the very large riverine influx. In addition, the prolonged cloud cover has an adverse effect on the top 25–40 m, on primary production, chlorophyll concentration and phytoplankton assemblages. Microphytoplankton were the most abundant in the northern area of the Bay. A total of 153 phytoplankton species were identified during this study. The most abundant species (at least 1,800 individual cells belonging to a given species or ≥2% of the total counts of identified specimens) during this study were: Thalassiothrix longissima, Thalassiothrix fauenfeldii, Rhizosolenia styliformis, Nitzschia angularis, Thalassionema nitzschioides, Coscinodiscus radiatus, Chaetoceros eibenii, Skeletonema costatum, Coscinodiscus concinnus and Chaetoceros coarctatus. Similarly, there were 20 moderately abundant (≥0.5 but <2%) species. Thirty-three of the least abundant species (<0.5%) occurred at one station. Diatom species, T. fauenfeldii and T. longissima were ubiquitous in the study area. The exclusive occurrence of S. costatum in the northernmost stations suggests that it proliferates only in the low-salinity regions with adequate silica from the land inputs. Abundance of pennate diatoms was higher in the open Bay compared to that of centric diatoms in the more productive northern locations and the western Bay. There appears to be a basic difference between near shore flora and offshore flora. This first analysis of phytoplankton assemblages from the offshore Bay suggests that while there is predominance of only a few species, the Bay harbors very diverse diatom communities that seem to be syntrophic, non-competitive and co-habiting in the generally low nutrient, stratified surface waters.     

Temperature as a key driver of ecological sorting among invasive pest species in the tropical Andes

Invasive species are a major threat to the sustainable provision of ecosystem products and services, both in natural and agricultural ecosystems. To understand the spatial arrangement of species successively introduced into the same ecosystem, we examined the tolerance to temperature and analyzed the field distribution of three potato tuber moths (PTM, Lepidoptera: Gelechiidae), that were introduced in Ecuador since the 1980s. We studied physiological responses to constant temperatures of the three PTM species under laboratory conditions and modeled consequences for their overall population dynamics. We then compared our predictions to field abundances of PTM adults collected in 42 sites throughout central Ecuador. Results showed that the three PTM species differed with respect to their physiological response to temperature. Symmetrischema tangolias was more cold tolerant while Tecia solanivora had the highest growth rates at warmer temperatures. Phthorimaea operculella showed the poorest physiological performance across the range of tested temperatures. Overall, field distributions agree with predictions based on physiological experiments and life table analyses. At elevations >3000 m, the most cold-tolerant species, S. tangolias, was typically dominant and often the only species present. This species may therefore represent a biological sensor of climate change. At low elevations (<2700 m), T. solanivora was generally the most abundant species, probably due to its high fecundity at high temperatures. At mid elevations, the three species co-occurred, but P. operculella was generally the least abundant species. Consistent with these qualitative results, significant regression analyses found that the best predictors of field abundance were temperature and a species x temperature interaction term. Our results suggest that the climatic diversity in agricultural landscapes can directly affect the community composition following sequential invasions. In the tropical Andes, as in other mountain ecosystems, the wide range of thermal environments found along elevational gradients may be one reason why the risks of invasion by successively introduced pest species could increase in the near future. More data on potential biological risks associated with climatic warming trends in mountain systems are therefore urgently needed, especially in developing nations where such studies are lacking.     

Spatial and covariate-varying relationships among dominant tree species in Utah

Environmental and Ecological Statistics - The presence and establishment of a tree species at a particular spatial location is influenced by multiple physiological and environmental filters such as...     

Degradation of Benzo[a]Pyrene in Soil with Arbuscular Mycorrhizal Alfalfa

Mycorrhizal and non-mycorrhizal alfalfa (Medicago sativa) was grown in pots containing soil artificially contaminated with various levels of benzo[a]pyrene (B[a]P)(0, 1, 10 and 100 mg kg^–1). Soil and plants were sampled after 30, 40, 50, 60 and 90 days and compared with unlanted pots. The percentage of mycorrhizal root length colonized by Glomus caledoniun was not significantly affected by the addition of B[a]P up to 10 mg kg^–1 but was significantly lower at 100 mg kg^–1B[a]P compared with low concentrations (p < 0.05). There was no difference in soil polyphenol oxidase and dehydrogenase activity among the controls and applications of 1 and 10 mg kg^–1 of B[a]P. However, enzyme activities were significantly higher at 100 mg kg^–1B[a]P compared with the other three treatments, and there was no mycorrhizal effect. Over a period of 90 days the concentration of B[a]P in soil in which alfalfa was grown was significantly lower than in unplanted soil (p < 0.05). Degradation rates of B[a]P added at 1, 10 and 100 mg kg^–1 without G. caledonium were 76, 78 and 53%, and with mycorrhizal inoculation were 86, 87 and 57%. The degradation rate in unplanted soil was significantly lower than in planted soil, and was significantly higher in medium- and low-B[a]P treatments than in the high B[a]P concentration tested. There is a possibility of enhancement phytoremediation of PAHs in rhizosphere soil with arbuscular mycorrhizal fungi.     

Potential of bivalves' secondary settlement differs with species: a comparison between cockle (Cerastoderma edule) and clam (Ruditapes philippinarum) juvenile resuspension

Juvenile bivalves may be dispersed by entering a bysso-pelagic phase where they drift through the water mass aided by a long thread. The ability to resuspend and control the specific weight in two bivalve species, the cockle Cerastoderma edule (L.) and the Japanese clam Ruditapes philippinarum (Adams and Reeves), was documented with juveniles through flume and still-water experiments. Cockle juveniles initially placed on an unsuitable substratum were exposed to two shear velocities (u _*). At the end of the experiment, 42 (±15)% (for u _*=0.51 cm s⁻¹) and 79 (±9)% (for u _*=0.99 cm s⁻¹) of individuals were retrieved from the sand area which represents only 8% of the total flume surface. Most juveniles (70.5%) with shell lengths <2.5 mm migrated from the unsuitable Plexiglas substratum to the sand array by resuspension in the water column. The percentage was lower (21.5%) for larger individuals. The same experimental design was applied to clams, which immediately adhered to the Plexiglas substratum and remained attach to it. Sinking rates of live and dead specimens of both species were measured in a 1 m long transparent PVC tube. Cockle fall velocities showed severe deceleration, probably due to byssus secretion (up to 15-fold slower than dead cockles), sometimes interrupted by brutal acceleration probably due to byssal rupture. Cockles were able to reduce their sinking rate for shell lengths up to 4.25 mm. By contrast, clam sinking rates were constant, and similar to dead clam sinking rates. Specific weights of all experimental juveniles were calculated in relation to their lengths, and their passive motion into the boundary layer was theoretically assessed with Shields curve. In short, C. edule and R. philippinarum can both exhibit dense populations in the field with a good capacity to colonize, although juveniles display different abilities to resuspend in the water column. Received: 27 January 1997 / Accepted: 13 February 1997     

Similar cation exchange capacities among bryophyte species refute a presumed mechanism of peatland acidification

Fen-bog succession is accompanied by strong increases of carbon accumulation rates. We tested the prevailing hypothesis that living Sphagna have extraordinarily high cation exchange capacity (CEC) and therefore acidify their environment by exchanging tissue-bound protons for basic cations in soil water. As Sphagnum invasion in a peatland usually coincides with succession from a brown moss-dominated alkaline fen to an acidic bog, the CEC of Sphagna is widely believed to play an important role in this acidification process. However, Sphagnum CEC has never been compared explicitly to that of a wide range of other bryophyte taxa. Whether high CEC directly leads to the ability to acidify the environment in situ also remains to be tested. We screened 20 predominant subarctic bryophyte species, including fen brown mosses and bog Sphagna for CEC, in situ soil water acidification capacity (AC), and peat acid neutralizing capacity (ANC). All these bryophyte species possessed substantial CEC, which was remarkably similar for brown mosses and Sphagna. This refutes the commonly accepted idea of living Sphagnum CEC being responsible for peatland acidification, as Sphagnum's ecological predecessors, brown mosses, can do the same job. Sphagnum AC was several times higher than that of other bryophytes, suggesting that CE (cation exchange) sites of Sphagna in situ are not saturated with basic cations, probably due to the virtual absence of these cations in the bog water. Together, these results suggest that Sphagna can not realize their CEC in bogs, while fen mosses can do so in fens. The fen peat ANC was 65% higher than bog ANC, indicating that acidity released by brown mosses in the CE process was neutralized, maintaining an alkaline environment. We propose two successional pathways indicating boundaries for a fen-bog shift with respect to bryophyte CEC. In neither of them is Sphagnum CE an important factor. We conclude that living Sphagnum CEC does not play any considerable role in the fen-bog shift. Alternatively, we propose that exclusively indirect effects of Sphagnum expansion such as peat accumulation and subsequent blocking of upward alkaline soil water transport are keys to the fen-bog succession and therefore for bog-associated carbon accumulation.     

Strategic exploitation in a socially parasitic bee: a benefit in waiting?

Social parasitism has evolved at least ten times in the allodapine bees but studies that explore the parasite’s integration and exploitation of host colonies are lacking. Using colony content and dissection data, we examine how Inquilina schwarzi affects the social organisation of its host Exoneura robusta. Our samples include three critical periods in the host life cycle: initial formation of dominance hierarchies in late autumn, commencement of oviposition by host queens in late winter, and development of secondary reproductives in late spring. I. schwarzi preferentially parasitises larger host colonies in autumn, but during autumn and winter, the parasite appears to be socially invisible, living in the nest without disrupting the normal functioning of these colonies. Inquilines begin egg laying much later than their hosts, and by late spring, they have disrupted host reproductive hierarchies, leading to lower skew in ovarian sizes of their host nestmates. Living invisibly within the host nest for the first 6 months and waiting until well after host reproduction has begun before disrupting their social organisation appear to be unique among social insects. Such a change in strategy may be facilitated by the different social systems found in allodapine bees, with the social parasites possibly disrupting the reproductive hierarchies during spring to prevent or reduce the normal dispersal of some host females from their natal nests.     

Female aggression and male peace-keeping in a cichlid fish harem: conflict between and within the sexes in Lamprologus ocellatus

Summary Conflicts of interest within and between the sexes are important processes leading to variability in mating systems. The behavioral interactions mediating conflict are little documented. We studied pairs and harems of the snail-shell inhabiting cichlid fish Lamprologus ocellatus in the laboratory. Due to their larger size, males controlled the resource that limited breeding: snail shells. Males were able to choose among females ready to spawn. Females were only accepted if they produced a clutch within a few days of settling. When several females attempted to settle simultaneously the larger female settled first. Females were least aggressive when guarding eggs. Secondary females were more likely to settle when the primary female was guarding eggs. In established harems females continued to be aggressive against each other. The male intervened in about 80% of female aggressive interactions. Male intervention activity correlated with the frequency of aggression among the females in his harem. The male usually attacked the aggressor and chased her back to her own snail shell. When a male was removed from his harem, aggression between females increased immediately and usually the secondary female was expelled by the primary female within a few days. Time to harem break-up was shorter the more mobile the primary females' young were and did not correlate with the size difference between harem females. Male L. ocellatus interfere actively in female conflict and keep the harem together against female interests. Female conflict presumably relates to the cost of sharing male parental investment and to the potential of predation by another female's large juveniles on a female's own small juveniles.Correspondence to: F. Trillmich     

Intraspecific brood adoption in convict cichlids : a mutual benefit

Summary Convict cichlids (Cichlasoma nigrofasciatum) are biparental, substrate brooding cichlid fish which have extended care of their eggs, larvae (wrigglers) and free-swimming young (fry). Field observations indicate that intraspecific brood adoption of fry occurs frequently under natural conditions. Of 232 broods 29% contained foreign fry and of 232 broods 15% were identified as fry donors. Foreign fry were usually of similar size to or smaller than the host brood fry. Experimental fry transfers showed that parents accept foreign fry smaller than their own but immediately reject or eat foreign fry larger than their own. A predation experiment showed that within a mixed brood, smaller fry are eaten more often than larger fry. Thus, accepting foreign fry may benefit parents through the dilution effect and through a differential predation effect. In comparison with intact families, broods from which the male parent was removed were less likely to reach independence, had fewer fry surviving to independence, and had a greater probability of having some fry transferred to neighbouring broods. We propose that intraspecific brood adoption in convict cichlids is mutually beneficial to the adult donors and recipients. Correspondence to: B.D. Wisenden     

Maternal investment in Antarctic fur seals: evidence for equality in the sexes?

Studies of the otariids (fur seals and sea lions), a highly sexually dimorphic group, have provided conflicting evidence of differential maternal expenditure in male and female offspring and, thus, suggestions that they conform to predictions of investment theory are equivocal. Since the mid-1970s, a diversity of research on Antarctic fur seals (Arctocephalus gazella) including studies of their reproductive ecology, lactation energetics, and foraging behaviour have been conducted at Bird Island, South Georgia that have resulted in one of the more complete and diverse data sets for any species of otariid. These long-term data were reviewed to determine whether there was any evidence to support that differential maternal expenditure occurred in Antarctic fur seals. Most of the data examined were collected during five consecutive austral summers from 1988 through 1992 and included years in which local food resources were abundant and scarce. We were unable to detect differences in the sex ratios of pups at birth or sex-biased differences in growth rates estimated from serial data, the number of foraging trips made, the duration of attendance ashore, diving behaviour, suckling behaviour, or milk consumption in any year and in the duration of foraging trips or age at weaning in 2 of 3 years. In addition, we found no evidence of greater reproductive costs between mothers with sons or daughters relative to their reproductive performance the following year. In contrast, sex-biased differences were only found in the duration of foraging trips in 1990, the age at weaning in 1988, and consistently in growth rates estimated from cross-sectional data. We suggest that differential maternal expenditure does not occur in Antarctic fur seals because male pups probably do not gain greater benefit from additional maternal expenditure than female pups. After weaning, males experience a period of rapid juvenile growth over 3–4 years during which time body mass nearly trebles. This growth will almost certainly be dependent upon available food resources then rather than on any maternal expenditure received over the first 4 months of life and, thus, the assumptions of the Trivers and Willard hypothesis are probably invalid for Antarctic fur seals. Received: 10 July 1996 / Accepted after revision: 3 March 1997     

Habitat partitioning among four elasmobranch species in nearshore,shallow waters of a subtropical embayment in Western Australia

There is a need to explore, in an integrated and statistical manner, how the number of species, relative abundance, species composition and life-cycle stages of elasmobranchs in nearshore waters vary among habitat types and during the year. Therefore, four sites in a large marine embayment, each representing a different habitat type, were sampled at regular intervals. These sites were: (1) unvegetated, with no vegetation within at least 200 m; (2) unvegetated, immediately adjacent to sparse mangroves; (3) unvegetated, immediately adjacent to dense mangroves; and (4) vegetated, with seagrass (Posidonia australis) throughout and in the vicinity. Gill netting caught 10 shark species (5 families), 5 ray species (4 families) and 12 teleost species (10 families). Carcharhinus cautus, which contributed approximately 60% to the numbers of elasmobranchs caught, completed its life cycle in nearshore, shallow waters. Negaprion acutidens, Carcharhinus brevipinna, Carcharhinus limbatus and Rhizoprionodon acutus used these waters as a nursery area. C. cautus was caught mainly in the unvegetated sites, particularly in those near mangroves. N. acutidens was caught entirely in unvegetated sites, while R. acutus, C. brevipinna and Chiloscyllium punctatum were caught predominantly or exclusively in seagrass. The mean number of species and mean catch rate of elasmobranchs were greatest for the seagrass site and least for the unvegetated site with no vegetation within at least 200 m and were significantly less for the latter site than for the unvegetated site immediately adjacent to dense mangroves (P<0.05). The numbers of species and catch rates of elasmobranchs were significantly greater in summer and autumn than in winter (P<0.05) and, in the case of number of species, also than in spring (P<0.05). We conclude that the spatial and food resources in the nearshore, shallow waters of Shark Bay are partitioned among elasmobranch species, thus reducing the potential for competition among these species for the resources in those waters.Communicated by G.F. Humphrey, Sydney     

Trophic segregation between sexes in the Black Skimmer revealed through the analysis of stable isotopes

The degree of individual or gender variation when exploiting food resources is an important aspect in the study of foraging ecology within a population. Previous information on non-breeding skimmers obtained through conventional methodologies suggested sex-related differences in prey species. In this study, stable isotope techniques were used to investigate the intraspecific segregation in diet and foraging habits of the Black Skimmer (Rynchops niger intercedens) at Mar Chiquita Coastal Lagoon (37°40′S, 57°22′W), Argentina. These results were compared with contemporary data on the trophic composition obtained by conventional methodologies. Blood samples were taken from birds captured with mist-nets during their non-breeding season. The isotopic signatures of skimmers showed a diet mainly composed of marine prey with some degree of estuarine fish intake. When comparing diet between sexes, males showed enrichment in ¹⁵N compared to females, while no differences were observed in ¹³C. The use of mixing models revealed differences in the relative composition of prey in the diet of male and female skimmers. This study highlights stable isotope analysis as a valuable tool to test inter-individual differences and sexual segregation in trophic ecology of Black Skimmers as compared to conventional methodologies. The results show a trophic segregation in the Black Skimmer during the non-breeding season that can be explained by differences in prey species and larger prey sizes of male skimmers. Our findings have significant implications for conservation since any environmental change occurring at wintering areas might have profound effects on several avian life-history traits, and could be different for males and females due to trophic segregation.     

Contrasting feeding patterns among species of fish larvae from the tropical Andaman Sea

Feeding habits of tropical fish larvae were analysed in a comparative study of four species (Scorpaenodes sp., Carangoides sp., Acanthocepola sp. and Cynoglossus sp.) from the Andaman Sea. We investigated morphological characteristics and their potential influence on larval feeding, and looked for common patterns in larval prey preference. Gut contents of a total of 300 larvae were examined and compared with local zooplankton composition. The feeding habits of the investigated larvae shared a number of characteristics. During ontogeny both the preferred prey size and the number of prey in the gut increased, and across all larval size classes the relative prey size spectrum stayed constant, of approximately the same magnitude for all four species. On the other hand, larval feeding also differed in a number of aspects, especially differences in the taxonomic composition of preferred prey were apparent. Scorpaenodes sp. preferred abundant and large prey taxa, Acanthocepola sp. and Carangoides sp. preferred large, but less common prey taxa, while Cynoglossus sp., which had the relatively smallest mouth size, preferred smaller sized prey groups. Hence, the findings indicate that from an offset of common characteristics, especially related to prey size preference, larvae have their individual feeding patterns related to specific morphology and patterns of distribution.Communicated by M. Kühl, Helsingør     

Discrimination among and variation within species of Pseudocalanus based on the GPI locus

Patterns of allozyme variation at the GPI (glucose phosphate isomerase) locus showed that the recently described or clarified Pseudocalanus newmani, P. acuspes, P. moultoni and P. minutus are genetically isolated from each other even though morphologically they are very similar, and in places sympatric. This genetic isolation was inferred from the presence of private alleles in some species, and from differences in their degrees of observed heterozygosity and in the observed frequencies of shared alleles in all the species examined. Temporal variations were not detected in P. newmani, P. acuspes and P. minutus. Populations of P. newmani and P. moultoni from the Atlantic and Pacific coasts of North America have apparently diverged. No such divergence was detected among samples of P. acuspes from Nova Scotia, the Canadian Arctic and the Baltic sea, and of P. minutus from Nova Scotia and Hudson Bay. P. elongatus collected in the English Channel differed from P. acuspes from the Baltic Sea.     

Schooling tendency and foraging benefit in the ocean surgeonfish

Summary Field observations of individuallytagged ocean surgeonfish (Acanthurus bahianus) indicated that schooling provided a foraging benefit: an increase in relative forging time. The magnitude of this increase differed among individuals, primarily due to time budgets of the fish when they were not in schools. The proportion of nonschooling time devoted to foraging was positively correlated with fish size, while the proportion of schooling time that was spent forging was similar for all individuals. Schooling tendency (percentage of time in schools) was positively related to magnitude of foraging benefit and negatively related to fish size. When fish size was held constant by partial correlation, schooling tendency remained positively related to foraging benefit. These results provide evidence that fish which derive a greater foraging benefit from school membership spend more time in schools.     

Small-scale spatial and temporal interactions among benthic crustaceans and one fish species in the Bay of Biscay

In the summer of 2004, a video survey was carried out in the northern part of the central mud bank (Grande Vasière) of the Bay of Biscay to study the small scale relationship between the dominant crustacean megafauna Nephrops norvegicus, Munida rugosa and Goneplax rhomboides and juvenile hake (Merluccius merluccius). Using a towed body, high-resolution videos were recorded in six sampling sites. Statistical modelling using generalised additive models (GAM) revealed variations in activity patterns for two species. More N. norvegicus were observed outside their burrows at dawn and somewhat at dusk (no observations during night) while G. rhomboides was less observed in the morning. In addition, reduced spatial overlap between G. rhomboides and N. norvegicus suggested reduced competition for food but also space as both are burrowing species. The observed temporal and spatial activity patterns may contribute to regulating assemblage structure as competing species may be actively foraging at different times and locations thus reducing direct competition.