Interaction and impacts of two introduced species on a soft-sediment marine assemblage in SE Tasmania

Introduced species are having major impacts in terrestrial, freshwater and marine ecosystems world-wide. It is increasingly recognised that effects of multiple species often cannot be predicted from the effect of each species alone, due to complex interactions, but most investigations of invasion impacts have examined only one non-native species at a time and have not addressed the interactive effects of multiple species. We conducted a field experiment to compare the individual and combined effects of two introduced marine predators, the northern Pacific seastar Asterias amurensis and the European green crab Carcinus maenas, on a soft-sediment invertebrate assemblage in Tasmania. Spatial overlap in the distribution of these invaders is just beginning in Tasmania, and appears imminent as their respective ranges expand, suggesting a strong overlap in food resources will result from the shared proclivity for bivalve prey. A. amurensis and C. maenas provide good models to test the interaction between multiple introduced predators, because they leave clear predator-specific traces of their predatory activity for a number of common prey taxa (bivalves and gastropods). Our experiments demonstrate that both predators had a major effect on the abundance of bivalves, reducing populations of the commercial bivalves Fulvia tenuicostata and Katelysia rhytiphora. The interaction between C. maenas and A. amurensis appears to be one of resource competition, resulting in partitioning of bivalves according to size between predators, with A. amurensis consuming the large and C. maenas the small bivalves. At a large spatial scale, we predict that the combined effect on bivalves may be greater than that due to each predator alone simply because their combined distribution is likely to cover a broader range of habitats. At a smaller scale, in the shallow subtidal, where spatial overlap is expected to be most extensive, our results indicate the individual effects of each predator are likely to be modified in the presence of the other as densities increase. These results further highlight the need to consider the interactive effects of introduced species, especially with continued increases in the number of established invasions.Communicated by M.S. Johnson, Crawley     

Growth,food utilization and effect of social interaction in the European bass Dicentrarchus labrax

Fingerling and juvenile bass, Dicentrarchus labrax (L.) fed on filleted saithe, Gadus virens, exhibited much higher K ₁ energy and nitrogen utilization efficiencies (growth as % energy or nitrogen intake) than those fed on trout pellets or shore crabs, Carcinus maenas, although the efficiency declined during prolonged feeding. Mean organic absorption efficiency for saithe was 99.5% and was independent of food intake but the efficiency for pellets and crab declined with increasing food intake from 99 to 92%. For each diet the regression of relative growth rate on food intake was linear up to the regression of relative growth rate on food intake was linear up to the highest feeding level and independent of body weight, enabling construction of a simple model for food transformation. The regression coefficient (equivalent to K ₃, the ratio of growth to food consumed in excess of maintenance) for saithe was about 3 times that for pellets in terms of energy and twice that for pellets in terms of nitrogen. The more efficient utilization of saithe for growth and of pellets for maintenance were related to differences in their nutritional value. With each diet, K ₃ and maximum K ₁ for energy were about twice those for nitrogen. Social interaction had a significant effect on food utilization in juvenile bass. Isolated fish showed poor growth and mean K ₁ efficiency in comparison with bass in pairs or groups. K ₃ efficiency and mainternance requirement were similar in individuals and groups, but were significantly greater in pairs. These results are related to behavioural observeations and environmental stress, and the limitations of extrapolating the results of laboratory feeding experiments on fish to other situations is discussed.     

Feeding and non-feeding aggression can be induced in invasive shore crabs by altering food distribution

Human activity can have a large impact on surrounding ecosystems. For example, humans alter resource distributions for other species, potentially modifying these species competitive dynamics. These changes in local competitive processes are frequently associated with species invasions. Here, we investigate how differences in resource distribution affect competitive behaviour using the highly invasive European shore crab (Carcinus maenas). Using a controlled laboratory experiment in combination with behaviour assays and social network analysis, we show that individuals feeding in habitats with clumped food distributions are more aggressive than individuals feeding in habitats where food is evenly dispersed, and this aggression is present even on days where crabs are not feeding. Additionally, this persistent aggression can be induced, suggesting that individuals of this invasive species possess the flexibility to modify their competitive behaviours in response to differences in food distributions. Furthermore, we show how these individual responses can lead to changes in overall organisation of aggressive interactions within a population. We discuss these results in relation to how human impacts can have long-term effects on competitive behavioural strategies, and how behavioural flexibility can allow invasive species to colonise and persist in highly impacted sites such as urban ecosystems.     

Foraging behavior of juvenile Carcinus maenas (L.) and Cancer pagurus L.

Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.     

Impact of epibenthic predation on estuarine intertidal harpacticoid copepod populations

The extent to which energy is transferred directly from benthic meiofauna to epibenthic predators was investigated on an intertidal sand-flat in the Exe estuary, southwest England, during 1981–1982 and compared with data obtained from an intertidal mud-flat in the Lyhner estuary, also in south-west England, between 1978 and 1981. Two species of flatfish (Pleuronectes platessa L. and Platichthys flesus L.), two species of goby [Pomatoschistus microps (Krøyer) and P. minutus (Pallas)], brown shrimp (Crangon crangon L.) and shore crabs (Carcinus maenas L.) are the most common epibenthic predators feeding on the benthic invertebrates in these locatites. Harpacticoid copepods are the only component of the meiofauna to form a significant part of the diet of early juvenile stages of these predators, particularly the invertebrates. Harpacticoids are a more important source of food for predators feeding over the sand-flat than for those feeding on the mud-flat because in the sand-flat alternative prey of suitable size, such as small annelids, are absent. Moreover, the impact of predation on the mud-flat is spread over the whole harpacticoid species spectrum whereas on the sandflat it is confined almost entirely to a single species, Asellopsis intermedia (T. Scott). Flatfish, gobies and shrimp consume daily an estimated 0.01 to 0.1% of the standing stock of A. intermedia and account for between 12 and 22% of the observed reduction in the population of this species between July and October. Therefore, only a very small proportion of total meiofauna biomass is transferred directly to higher trophic levels.     

Effects of algal and larval densities on development and survival of asteroid larvae

Effects of larval and algal culture density and diet composition on development and survival of temperate asteroid larvae were studied in the laboratory at Santa Cruz, California, USA, during summer and fall of 1990. Larvae of Asterina miniata were reared at two densities, 0.5 or 1.0 ml^-1, and fed one or two species of cultured phytoflagellates — Dunaliella tertiolecta alone or mixed with Rhodomonas sp. — at three concentrations of 5x10², 5x10³, and 5x10⁴ total cells ml^-1. Algal concentration strongly influenced larval development; however, larval density also had a marked effect. Development progressed further with increasing algal concentration. Larval growth and differentiation were sometimes uncoupled; i.e., growth measures were directly related to food level, while differentiation indicators were less so. At the lowest food level, growth was negative and differentiation was arrested at early precompetent stages; these larvae never formed juvenile rudiments or brachiolar attachment structures. Development times of larvae given more food ranged from 26 to 50 d and depended directly on food availability. Development time to metamorphosis at the highest food concentration was similar for siblings fed D. tertiolecta alone or mixed with Rhodomonas sp. In contrast, when food level was an order of magnitude lower, larvae fed the algal mixture metamorphosed significantly earlier than larvae fed the unialgal diet. This suggests interactive effects of food quantity and food quality. Survival was little affected by larval or food density, except at the lowest ration. Feeding experiments in well-controlled laboratory conditions are useful to predict and compare the physiological or developmental scope of response of larvae to defined environmental factors; however, results from such studies should not be extrapolated to predict rates and processes of larval development in nature.     

Studies to establish an optimal diet for Carcinus maenas

Carcinus maenas was maintained on synthetic foods containing various amounts of vitamins, cod liver oil and cholesterol. Food quality was evaluated in terms of growth, duration of intermoult periods and time of survival of the crabs. Vitamins are necessary for their growth and survival and cholesterol is also an essential nutrient. The best growth rates were obtained with 1.4 and 2.1% cholesterol in the food. At 6 and 9% cod liver oil in the food, we observed the best growth, while too high lipid levels slightly reduced the growth of the crustaceans, and definitely reduced their survival time.     

The diatom Thalassiosira rotula affects reproductive success in the copepod Acartia clausi

Fecundity, egg viability and fecal pellet production are reported for Acartia clausi females collected in the Bay of Naples, Italy, from April to October 1992 and fed either with a diatom (Thalassiosira rotula) or dinoflagellate (Prorocentrum minimum) diet, at food saturated conditions. The diatom diet significantly reduced both egg and fecal pellet production as well as hatching success. Blockage of egg development occurred with both axenic and non-axenic cultures of T. rotula, suggesting that inhibitors were provided by the diatoms and not by the bacteria associated with diatom cultures. Low hatching success was also artificially induced by exposing newly spawned A. clausi eggs to high concentrations of diatom extracts, indicating the presence of deleterious, inhibitory compounds blocking copepod embryogenesis. Fecundity and hatching success diminished significantly with female age. In contrast, female longevity was not significantly modified by food type. The presence of males did not significantly alter fecundity or egg viability. Females continued to produce viable eggs throughout the period of incubation, with and without males, in both food conditions, indicating that remating is infrequent and not necessary to sustain viable egg production in this species. The succession in low and high population densities may therefore be the outcome of variations in survival rates of eggs, rather than reproductive protential perse; such variations may strongly depend on the adult copepod diet.     

Spread and potential impact of the recently introduced European green crab,Carcinus maenas,in central California

Our study examines the potential impact of the European green crab Carcinus maenas on communities of coastal embayments of western North America. We document the current distribution and range expansion of this species beyond San Francisco Bay, where C. maenas first became established along this coast in 1989–1990, and we test the effect of C. maenas predation on different species and sizes of infaunal invertebrates in field and laboratory experiments. In our samples from eight coastal locations in central California collected between June 1993 and May 1994, we found no green crabs at the two closest embayments south of San Francisco Bay and found the crabs in all four embayments sampled within 120 km north of San Francisco Bay, up to and including Bodega Harbor. C. maenas was not present in samples from sites farther north. This northward range expansion is apparently the result of larval recruitment by a single cohort, corresponding to the predominant northern transport of surface waters and the approximate distance water moves during larval green crab development. At Bodega Harbor, the current northern range limit, the C. maenas population is now well established and reproducing. Females and males became sexually mature within their first year at 40 mm carapace width, molting approximately monthly from summer through fall, and females were ovigerous in late fall of their first year at 50 mm. We expect larvae from this population to recruit locally and to the north, promoting episodic range extensions as new populations are established and reproduce. Enclosure experiments conducted during the summer of 1993 at the intertidal sandflats of Bodega Harbor showed that C. maenas significantly reduced densities of the most abundant taxa, including the bivalves Transennella confusa and T. tantilla, the cumacean Cumella vulgaris, and the amphipod Corophium sp. Furthermore, Carcinus maenas selectively removed larger (>3 mm) rather than smaller (<1 mm) Transennella spp. in both field and laboratory experiments. Based on the available data from this and other studies of green crabs, and our 10 yr study of community dynamics at Bodega Harbor, we predict C. maenas will significantly alter community structure, ecological interactions, and evolutionary processes in embayments of western North America.     

Studies to establish an optimal diet for Carcinus maenas II. Protein and lipid requirements

Carcinus maenas (L.) was maintained on synthetic foods containing various proteins and lipids. Food quality was evaluated in terms of the crustacean's growth, duration of intermoult periods, and time of survival. Casein was found to be an adequate source of protein. Substituting amino acids or hydrolysates for casein results in reduced growth and delayed moults or absence of moulting. Animal proteins like casein are better suited than vegetable proteins. Correspondingly cod liver oil is more adequate than the vegetable lipid maize oil. The amount of salts and the pH-value also influence the quality of food-stuff for C. maenas.     

Use of kappa-carrageenan microbound diet (C-MBD) as feed for Penaeus monodon larvae

The performance of an artificial practical diet, kappacarrageenan microbound diet (C-MBD) was assessed on Penaeus monodon larvae at the SEAFDEC Broodstock and Maturation Experimental Laboratory in March 1986. Shrimps were reared from zoea₁ to post-larvae₁ using five dietary treatments: (a) natural food — Chaetoceros calicitrans and Artemia salina (b) C-MBD; (c) combination of natural food and C-MBD; (d) commercial diet (microencapsulated, MED); (e) combination of natural food and commercial diet. Results showed slow development with larvae fed the commercial diet. Feeding with C-MBD in combination with natural food resulted in the highest % survival among treatments (69.6), but this was not significantly different (P>0.05) from those obtained with larvae fed natural food alone, C-MBD alone or their combination. While mean values for survival of larvae fed the commercial diet, either alone or in combination, was significantly lower (p<0.05) than all other treatments, their mean growth indices were comparable with larvae fed C-MBD alone or in combination. The low levels of protein, lipid and essentially fatty acids (which are considered important nutrients during larval development) contained in the commercial diet may well justify the results on metamorphosis, survival and growth of the larvae fed this diet. The good performance of C-MBD in this experiment suggests that this kind of diet can be used as partial or total replacement to the traditional algal food.     

Feeding,starvation and metabolic rate in the shore crab Carcinus maenas

The relationship between food intake and metabolic rate (as measured by oxygen consumption) of Carcinus maenas (L.) was studied. The metabolic rate of feeding crabs which had been starved for a short period increased, and several days were required for it to return to its original level. During prolonged starvation, the metabolic rate of C. maenas fell in two stages. The first reduction was to about 60% of the feeding level; this occurred during the first week of starvation. There was then a further reduction to about 40% of the feeding level and, at this level of metabolism, the crabs were able to survive for 3 months, with only 50% mortality; most of this occurred in the last 2 weeks. Metabolic rate was found to affect food intake; crabs acclimated to 24°C took 2.4 times as much food as crabs acclimated to 10°C, although the metabolic levels of the two groups differed by a factor of only 1.4. From the results obtained when the crabs were starved and when starved crabs were fed, it is suggested that, during starvation, the metabolic rate of C. maenas first drops from the elevated feeding level to a level at which carbohydrate reserves are utilised, and subsequently to a minimum level at which lipid reserves and proteins are used.     

Toxicity and palatability of some British ascidians

Bioassays were conducted to test crude extracts of nine temperate-water ascidians for bioactivity against a range of marine organisms. The extracts generally produced moderate to weak effects, with the exception of those from the ascidian Clavelina lepadiformis, which consistently produced high mortalities against invertebrates. Some of these were then tested in simple palatability assays for feeding-deterrent effects. In general, no strong effects were detected for assays with shore crabs (Carcinus maenas) and fishes (the goby Pomatoschistus minutus and the grey mullet Chelon labrosus). Feeding-deterrent effects were observed with Carcinus maenas for the extracts of Aplidium proliferum, Botryllus schlosseri and Dendrodoa grossularia. It is postulated that these ascidians are unpalatable, but no toxic effects are evident.     

Trophic diversity of idoteids (Crustacea, Isopoda) inhabiting the Posidonia oceanica litter

The coexistence of three idoteid species in Posidonia oceanica litter raises the question of trophic diversity and their role in the litter degradation process. Hence, diet composition of Idotea balthica, Idotea hectica and Cleantis prismatica was studied using a combination of gut contents and stable isotopes analysis. Gut content observations indicate that P. oceanica dead leaves are an important part of the ingested food for the three species, although their tissues are constituted of only a small to medium fraction of P. oceanica carbon. Our results also underlined the potential role of these species in the degradation of P. oceanica litter by mechanically fragmenting the litter and by assimilating a small to medium fraction of carbon. Moreover, we showed that there were considerable inter- and intra-specific differences in diet composition. Diet differed between juveniles and adults for I. balthica. Crustaceans are an important food source for adults of I. balthica, while I. hectica indicated a major contribution of algal material. C. prismatica showed an intermediate diet. This trophic diversity is probably one of the factors allowing these species to coexist in the same biotope.     

Introduction,dispersal and potential impacts of the green crab Carcinus maenas in San Francisco Bay,California

The North Atlantic portunid crab Carcinus maenas (Linnaeus, 1758) has invaded the North Pacific Ocean following more than two centuries of global dispersal due to human activities. C. maenas was first collected in San Francisco Bay, California, in 1989–1990, where its distribution and prey selectivity were investigated in 1992–1994. It has become abundant in shallow, warm lagoons (which as favorable and retentive microhabitats may have served as invasion incubators) and spread throughout the north, central and south bays. It may have arrived in ballast water, on fouled ships, amongst algae with imported live bait or lobsters, or by intentional release; genetic comparisons of the Bay population with possible source populations may aid in defining the transport mechanism. C. maenas' eurytopic nature, its high breeding potential, and its diet and feeding behavior suggest the potential for extensive ecosystem alterations through predator-prey interactions, competition, disturbance, and indirect effects. Although both negative economic impacts through reduction or disruption of fisheries and positive impacts of providing bait and human-food fisheries have been documented in a few regions, the potential economic impacts in San Francisco Bay remain largely unknown.     

Feeding dynamics of Octopus mimus (Mollusca: Cephalopoda) in northern Chile waters

The natural feeding of 485 Octopus mimus (164 to 3088 g) was studied in relation to the species' life cycle and environmental seasonal variations off the north of Chile from autumn 1991 to summer 1992. Analysis of digestive-tract contents revealed that O. mimus preyed upon 25 different prey items belonging to five zoological groups (Teleostei, Mollusca, Crustacea, Echinodermata and Polychaeta). Cannibalism was only occasional. The results indicate that the diet and food intake of this species are significantly affected by sex and maturation. Senescent individuals ingest a small amount of food, and their diet is mainly based on small, not very motile prey. The food intake, expressed as body weight, of non-senescent individuals is higher in females than in males. Seasonal changes in sea-water temperature seem to be followed by adjustments in food intake. Like other Octopus species, O. mimus appears to be an opportunistic predator.     

Is benthic trophic structure affected by food quality? The Dogger Bank example

The aim of this study was to evaluate the relationship between food quality and trophic structure of macrobenthic communities of the Dogger Bank. Therefore, 21 stations along different depth gradients (a transect across the central Dogger Bank and at a station cross in the north-eastern part) were sampled and analysed for macrofauna trophic structure and food supply, indicated by phytopigment composition and organic matter contents of the sediment. The study revealed differences in the trophic structure of the benthic community in areas with contrasting food supply. The trophic structure reflected differences in relative quality of organic matter. The sandy sediments on top of the bank receiving intermediate quantities of high food quality were inhabited by high abundances of interface feeders and sand-licking amphipods of the genus Bathyporeia. The southern stations characterised by a comparatively high amount of intermediate food quality were dominated by interface feeders such as Amphiura filiformis. The northern and north-eastern stations showing the highest diversity of feeding types were characterised by a comparatively low, but highly variable input of intermediate-quality organic matter consisting of both fresh and degraded organic matter. Results are discussed with a special emphasis on specific Dogger Bank features such as benthic primary production and frontal system characteristics.Communicated by O. Kinne, Oldendorf/Luhe     

Dietary-induced responses in the phagotrophic flagellate Oxyrrhis marina

Primary producers may be limited by different nutrients as well as by light availability, which in turn affects their quality as food for higher trophic levels. Typically, algae with high C:N and/or C:P ratios are low-quality food for consumers. Heterotrophic protists are important grazers on these autotrophes, but despite their importance as grazers, knowledge of food quality effects on heterotrophic protists is sparse. In the present study, we examined how differently grown Rhodomonas salina (nutrient replete, N-limited and P-limited) affected the phagotrophic flagellate Oxyrrhis marina. The functional response of O. marina (based on ingested biovolume) did not show significant differences between food sources, thus food uptake was independent of food quality. O. marina was weakly homoeostatic which means that its C:N:P ratio still reflected the elemental composition of its food to some extent. Food quality had a significantly negative effect on the numerical response of O. marina. Whereas N-limited R. salina and nutrient replete R. salina resulted in similar growth rates, P-limited algae had a significantly negative effect on the specific growth rate of O. marina. Hence, the lack of elemental phosphorus of O. marina feeding on P-limited algae caused a reduction in growth. Thus, despite their weaker homoeostasis, heterotrophic protists are also affected by high C:P food in a similar way to crustacean zooplankton.     

Relative impact of a seagrass bed and its adjacent epilithic algal community in consumer diets

The aim of this work was to identify and compare, using nitrogen and carbon stable isotope data, the food sources supporting consumer communities in a Mediterranean seagrass bed (Gulf of Calvi, Corsica) with those in an adjacent epilithic alga-dominated community. Isotopic data for consumers are not significantly different in the two communities. Particulate matter and algal material (seagrass epiflora and dominant epilithic macroalgae) appear to be the main food sources in both communities. Generally, the δ¹³C of animals suggests that the seagrass Posidoniaoceanica (L.) Delile represents only a minor component of their diet or of the diet of their prey, but the occurrence of a mixed diet is not excluded. P. oceanica dominates the diet of only of few species, among which holothurians appear as key components in the cycling of seagrass material. Received: 30 July 1999 / Accepted: 17 January 2000     

Supplanting by olive baboons: dominance rank difference and resource value

Summary Interactions in which one animal takes over a food resource from another (feeding supplants) are usually interpreted as evidence of feeding competition, and the level of competition is predicted to be proportionate to the value of the resource. However, in this study of juvenile olive baboons (Papio anubis) living in a troop near Gilgil, Kenya, individuals were less selective about the quality of the supplant food under certain social circumstances. Two major food classes, Grass and Corm, were compared. Corms were less common in the diet, more difficult to locate as a replacement food after a supplant, and then at greater distances than Grass: in general, corms were the favoured supplant food, as predicted. But in interactions between animals who were close in rank, supplants over Grass occurred more often than expected. It is suggested here that, in these particular interactions, the winner benefitted socially, even at some cost of time or energy. Further, such interactions which may serve to reinforce the winner's status will be most productively directed to those close in rank, where relationships are potentially less stable. These data provide evidence that baboons can discriminate the relative ranks of others, rather than simply distinguishing between individuals who are higher- or lower-ranking than themselves.