Fische als Feinde tropischer Seeigel

The techniques used by 9 species of fish (representing 3 families) to capture Diadema sea urchins were observed under water in the Red Sea at Eilat, Israel. Different species displayed different techniques. Fish belonging to the Lethrinidae and Labridae families devoured the entire urchin after capture; Labridae alone were also capable of breaking up large sea urchins by banging them on rocks. The fish did this by shaking its head while holding the urchin in its mouth. Under experimental conditions, fish can distinguish between large and small urchins, and show a preference for the small urchin. The fish also shun contact with some other species of urchins. The trigger fish Balistes fuscus exposed to a dummy sea urchin responds to optical stimuli (body shape and spines); when exposed to a living sea urchin it responds to both optical and behavioural stimuli (movements of the spines). Morphological and ethological preadaptation of the fish to alter its preference for other food is essential before it can develop differentiated capture techniques. The ecological significance of the behaviour of fish in the coral reef ecosystem and the alternating influences in the predator-prey-relationship are discussed.     

Ein mimetisches Kollektiv — Beobachtungen an Fischschwärmen,die Seeigel nachahmen

In the Indian Ocean, the fish Siphamia argentea usually lives in small groups within the interradial spaces of the sea urchin Astropyga radiata. The fish has a colouration similar to that of the sea urchin and is thus protected from predators both by camouflage and the physical barrier of the sea urchin's spines. If the fish do not keep to the interradial spaces, the urchin can prick them with its spines. When the fish groups become too large, there is not enough space for them in these protected interradial spaces. Individuals which cannot be accomodated, form a dense group around the sea urchin which resembles a large urchin. Such dense groups are also known to occur in the fish Plotosus anguillaris where the individuals actually touch each other. The relationship between fish and sea urchin is described and discussed as an example of collective mimetic behaviour. This is the first record of collective mimetic behaviour in a marine environment. It is difficult to decide whether this is mimicry or mimese, as detailed research on the predators of both the sea urchin and the fish has not been carried out.     

Host preference,site fidelity,and homing behavior of the symbiotically luminous cardinalfish,Siphamia tubifer (Perciformes: Apogonidae)

The sea urchin cardinalfish, Siphamia tubifer (Perciformes: Apogonidae), is unusual among coral reef fishes for its use of bioluminescence, produced by symbiotic bacteria, while foraging at night. As a foundation for understanding the relationship between the symbiosis and the ecology of the fish, this study examined the diel behavior, host urchin preference, site fidelity, and homing of S. tubifer in June and July of 2012 and 2013 at reefs near Sesoko Island, Okinawa, Japan (26°38′N, 127°52′E). After foraging, S. tubifer aggregated in groups among the spines of the longspine sea urchin, Diadema setosum, and the banded sea urchin, Echinothrix calamaris. A preference for D. setosum was evident (P < 0.001), especially by larger individuals (>25 mm standard length, P < 0.01), and choice experiments demonstrated the ability of S. tubifer to recognize and orient to a host urchin and to conspecifics. Tagging studies revealed that S. tubifer exhibits daily fidelity to a host urchin; 43–50 and 26–37 % of tagged individuals were associated with the same urchin after 3 and 7 days. Tagged fish also returned to their site of origin after displacement; by day two, 23–43 and 27–33 % of tagged individuals returned from displacement distances of 1 and 2 km. These results suggest that S. tubifer uses various environmental cues for homing and site fidelity; similar behaviors and cues might be used by larvae for recruitment to settlement sites and for the acquisition of luminous symbiotic bacteria.     

Die Bedeckungsreaktion von Seeigeln. Neue Versuche und Deutungen

The conditions under which covering (“decorating”, “masking”) takes place have been studied in the sea urchins Paracentrotus lividus, Psammechinus miliaris and 2 other species. Covering occurs equally in darkness or light. It requires suitable objects and locomotion or searching activity of the tube feet. The covering reaction which may follow chemical, mechanical or optical stimuli may be purely the result of an increase in locomotory or general activity. Initial selection of different covering objects depends on the tube feet reflexes; size, form and weight of the object are important. Whether an object is accepted or not, depends on its surface and structure, the amount of water movement and the general activity level of the sea urchin. Transparency and colour of an object do not appear to be important. It is suggested that the covering process can be explained in terms of local tube feet and spine reflexes. The loading-up of objects may be understood as “relative walking”: the same reflexes which move the urchin on stable ground, draw loose particles towards the animal and then upwards.     

Prey defense,predator preference,and nonrandom diet: The interactions between Pycnopodia helianthoides and two species of sea urchins

Interactions between the predatory sea star Pycnopodia helianthoides (Brandt, 1835) and two of its natural prey, the sea urchins Strongylocentrotus purpuratus (Stimpson, 1857) and S. franciscanus (Agassiz, 1863), are examined with regard to predator preference, predator diet, and prey defenses. The sea star is able to detect both species of sea urchin upstream in a Y-trough, but does not consistently choose one over the other (i.e., no preference). However, when the sea star is presented with equal numbers of similar-sized specimens of the two species of sea urchin, its diet is markedly nonrandom, since S. purpuratus is eaten almost 98% of the time. The defensive responses of the two species of sea urchin differ in form and effectiveness. S. franciscanus employs its long spines as defensive weapons, pinching the rays of an attacking sea star. This defensive response is more effective than the pedicellarial response used by S. purpuratus. The nonrandom diet of the predator seems to result primarily from prey defensive responses that differ in effectiveness, rather than from an intrinsic, behavioral preference of the predator at an earlier stage in the predator/prey interaction.     

Food webs and fishing affect parasitism of the sea urchin Eucidaris galapagensis in the Galápagos

In the Galápagos Islands, two eulimid snails parasitize the common pencil sea urchin, Eucidaris galapagensis. Past work in the Galápagos suggests that fishing reduces lobster and fish densities and, due to this relaxation of predation pressure, indirectly increases urchin densities, creating the potential for complex indirect interactions between fishing and parasitic snails. To measure indirect effects of fishing on these parasitic snails, we investigated the spatial relationships among urchins, parasitic snails, commensal crabs, and large urchin predators (hogfish and lobsters). Parasitic snails had higher densities at sites where urchins were abundant, probably due to increased resource availability. Commensal crabs that shelter under urchin spines, particularly the endemic Mithrax nodosus, preyed on the parasitic snails in aquaria, and snails were less abundant at field sites where these crabs were common. In aquaria, hogfish and lobsters readily ate crabs, but crabs were protected from predation under urchin spines, leading to a facultative mutualism between commensal crabs and urchins. In the field, fishing appeared to indirectly increase the abundance of urchins and their commensal crabs by reducing predation pressure from fish and lobsters. Fished sites had fewer snails per urchin, probably due to increased predation from commensal crabs. However, because fished sites also tended to have more urchins, there was no significant net effect of fishing on the number of snails per square meter. These results suggest that fishing can have complex indirect effects on parasites by altering food webs.     

Effects of predators on sea urchin density and habitat use in a southern California kelp forest

Grazing sea urchins can reduce kelp abundance and therefore strongly affect kelp forest community structure. Despite the ecological importance of sea urchins, direct field studies on the role that urchin predators play in shaping urchin populations are rare for southern California. We conducted surveys and manipulative experiments within kelp forests near San Diego, CA, (32–51′28″N, 117–16′00″W) from 2006 to 2009 to determine whether predators such as sheephead (Semicossyphus pulcher) and spiny lobsters (Panulirus interruptus) may be linked to purple urchin (Strongylocentrotus purpuratus) and red urchin (Strongylocentrotus franciscanus) distribution and habitat use, as well as purple urchin density-dependent mortality. Purple urchins were less dense and more cryptic inside a local marine protected area (MPA) that contained high predator abundance than in nearby heavily fished areas, whereas red urchins rarely were found outside the MPA. Urchin proportional mortality was inversely density dependent during the day when sheephead were active, despite fish aggregations in plots of high urchin density, but was density independent during the night when lobsters were active. Urchin mortality was reduced under understory algal cover during the day, but not during the night. Examining whether urchin mortality from predation is density dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.     

Patterns of distribution and composition of sea urchin assemblages on Brazilian subtropical rocky reefs

Sea urchins are a key group of herbivores in both temperate and tropical food webs because they control macroalgal cover, and consequently influence primary productivity and phase shifts on reefs. Despite being abundant on southwestern Atlantic reefs, sea urchin distributions, and their association with abiotic and biotic variables, are poorly known. In this study, sea urchin assemblages were surveyed in 2011 at multiple depths at eight sites in Arraial do Cabo (Brazil, 22°57′S/41°01′W), with sites split between a colder, more wave-exposed location, and a warmer, more sheltered location. The influence of this large-scale physical gradient, along with changes in depth and substrate complexity, on sea urchin densities was then investigated. Predator biomass was low and did not vary significantly among sites. Among the seven species recorded, Paracentrotus gaimardi, Echinometra lucunter and Arbacia lixula were dominant. Linear mixed-effects models indicated that location was important, with mid-sized P. gaimardi individuals and A. lixula more common at cooler, exposed sites and E. lucunter more abundant at warmer, sheltered sites. Sea urchin densities typically decreased with increasing depth, probably caused by changes in factors such as light, wave exposure, and sedimentation. Substrate complexity had a positive effect on the abundance of all species, presumably because of the increased availability of refuges. Physical gradients have important consequences for urchin distributions and their ecological functions at relatively small spatial scales on these reefs, and should be incorporated into herbivore monitoring programmes. Research is also required to examine how differential sea urchin distributions affect benthic dynamics.     

Influence of increased cod abundance and temperature on recruitment of northern shrimp (Pandalus borealis)

Despite an increase in northern shrimp (Pandalus borealis) female biomass in the past years, the recruitment of the offshore population north and northeast of Iceland has remained very low. In this study, the influence of abiotic and biotic factors was studied in relation to shrimp recruitment. Two factors, cod (Gadus morhua) abundance and summer sea surface temperature (SST), were found to have a negative effect on offshore shrimp recruitment, explaining 71 % of the observed variation. Both cod abundance and temperature on the offshore shrimp grounds have increased in the past years, while recruitment has decreased and been at historically low levels since 2005. No significant relationship was found between recruitment and spawning biomass, indicating that recruitment variability is mainly driven by other factors. Cod abundance and summer SST are likely to affect different life stages of shrimp, as SST influences shrimp during its planktonic phase while cod abundance influences the demersal stage.     

Natural growth bands and growth variability in the sea urchin Echinus esculentus: results from tetracycline tagging

Growth of the European edible sea urchin Echinus esculentus L. was studied in a population held for 2 yr in cages on the sea bed, after labelling with the skeletal growth marker tetracycline. The final position of the tetracycline tag on the genital plates agreed with an annual periodicity in natural growth zones; two such growth zones appeared beyond the position of the tag on the ground surface of the plate as light-reflecting bands separated by narrow dark lines in the middle layer. Individual and group (pooled data) growth parameters were estimated from the growth increment shown in the genital plate, whose lateral growth displayed a linear relationship to the diameter of the urchin test within the size range of these measurements. Von Bertalanffy growth parameters (asymptotic size and growth-rate function K) fitted to the growth increment on each individual were highly significantly correlated to those fitted to the natural growth lines, assuming an annual periodicity. The inferred growth pattern agrees well with conclusions based on H. B. Moore's growth-band data. The caged urchins can be assumed to have experienced exactly similar conditions, yet the growth curves fitted to individuals showed considerable variability. The good agreement between estimated growth function parameters of individuals obtained by the two methods indicate that this reflects real variability in growth between individuals that probably is genetically rather than environmentally determined. The growth of E. esculentus, and the adaptational significance of high growth variability in the population is briefly discussed.     

Effects of sea urchin disease on coastal marine ecosystems

Outbreaks of disease in herbivorous sea urchins have led to ecosystem phase shifts from urchin barrens to kelp beds (forests) on temperate rocky reefs, and from coral to macroalgal-dominated reefs in the tropics. We analyzed temporal patterns in epizootics that cause mass mortality of sea urchins, and consequent phase shifts, based on published records over a 42-year period (1970–2012). We found no evidence for a general increase in disease outbreaks among seven species of ecologically important and intensively studied sea urchins. Periodic waves of recurrent amoebic disease of Strongylocentrotus droebachiensis in Nova Scotia coincide with periods when the system was in a barrens state and appear to have increased in frequency. In contrast, following a major epizootic that decimated Diadema antillarum throughout the Caribbean in 1983, subsequent outbreaks of disease were highly localized and none have been reported since 1991. Epizootics of Strongylocentrotus in the NW Atlantic and NE Pacific, and Paracentrotus and Diadema in the eastern Atlantic, have been linked to climate change and overfishing of sea urchin predators. The spatial extent of recurrent disease outbreaks in these species, and the frequency of phase shifts associated with these epizootics, has decreased over time due to the expansion of the macroalgal state and its stabilization through positive feedback mechanisms. Longitudinal studies to monitor disease outbreaks in sea urchin populations and improved techniques to identify causative agents are needed to assess changes in the frequency and extent of epizootics, which can profoundly affect the structure and functioning of coastal marine ecosystems.     

Patterns of abundance, population size structure and microhabitat usage of Paracentrotus lividus (Echinodermata: Echinoidea) in SW Portugal and NW Italy

In coastal habitats, wave exposure influences several aspects of the life history of marine organisms. Here, we assess how hydrodynamic conditions can generate variation in density, size structure and microhabitat usage of Paracentrotus lividus and whether these effects are consistent between regions that are markedly different for oceanic climate, such as the coasts of SW Portugal and NW Italy. The abundance of P. lividus was ~4 times higher in SW Portugal than in NW Italy, but within each region, there was no effect of wave exposure. In SW Portugal, higher urchin abundances were found at shallower depths, while no effect of depth on urchin abundance emerged in NW Italy. Most of the variation in urchin abundance occurred at small spatial scales (metres), and our results suggest that habitat complexity, that is, the presence of cracks and crevices, is an important determinant of patterns of distribution of this species. The population in NW Italy was characterized by a unimodal size distribution, with a higher proportion of medium-sized individuals. In contrast, in SW Portugal, smaller individuals represented a large proportion of the populations. Size structure varied between exposed and sheltered habitats in SW Portugal, suggesting that the proportion of individuals from different size cohorts may vary along wave-exposure gradients as a result of direct or indirect effects of hydrodynamic forces. In SW Portugal, most urchins occurred in burrows, while in NW Italy, urchins were mainly observed in crevices. These results suggest that creating/occupying burrows might be an adaptive behaviour that allows sea urchins to better withstand stressful hydrodynamic conditions and, therefore, are more common on exposed Atlantic coasts. Overall, our study suggests that the effects of hydrodynamic forces on sea urchin populations are context dependent and vary according to background oceanic climate.     

Profitability and sustainability of edible sea urchin fishery in Sardinia (Italy)

Sea urchin (Paracentrotus lividus) fishery is intensively practiced in several areas of the Mediterranean basin. In Sardinia, as well as other Mediterranean countries, sea urchin is a basic ingredient for several dishes due to the delicacy of its gonads (roe), and demand is constantly increasing. Restrictions have been implemented in order to minimise the risk of overexploitation, however, these measures might jeopardize economic convenience in sea urchin harvesting. This paper estimates economic convenience within the edible sea urchin fishery in Sardinia. It aims to determine whether both profitability and sustainability, in terms of the preservation of sea urchin stock, can be guaranteed by actual policy regulation. We found high variability in terms of captures and profitability among firms, and a considerable degree of this variability is the result of technological differences between fishing methods. Analysis also suggests that a slight increase in allowed captures should generate a more than proportional increase in profits. This evidence gives some useful suggestions for improving the efficacy of policies in affecting the economic and environmental sustainability of the Mediterranean sea urchin fishery.     

Comparative feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for the invasive seaweed Codium fragile ssp. tomentosoides (Chlorophyceae) and four other seaweeds

Laboratory experiments conducted during 1987 on Appledore Island, Maine, USA, tested whether feeding preference or the absence of an attractant was the cause for the occurrence of beds of Codium fragile ssp. tomentosoides (herein referred to as Codium fragile) within rocky barrens grazed clear of kelp by the sea urchin Strongylocentrotus droebachiensis. Consumption of C. fragile in single-diet experiments (1 seaweed/sea urchin) was highly variable and was not significantly different from that for several other seaweeds (Agarum cribrosum, Ascophyllum nodosum, Chondrus crispus, and Laminana saccharina) important in the field diet of the green sea urchin. In multiple-diet experiments (5 seaweeds/sea urchin) significantly less Codium fragile was eaten than Chondrus crispus, but significantly more Codium fragile was eaten than A. cribrosum. Chemosensory experiments suggest that C. fragile does not attract the sea urchin. Sea urchins are unable to detect C. fragile but will eat it when they come in contact with it.     

Interactions among sea otters, sea stars, and suspension-feeding invertebrates in the western Aleutian archipelago

The structuring and organizing effects of apex predators on ecosystems are becoming increasingly well documented. The enhancement of kelp forests via sea otter predation on herbivorous sea urchins is among the earliest and best known examples. This study provides evidence for direct and indirect trophic interactions among sea otters, predatory sea stars, and filter-feeding mussels (Mytilus trossulus) and barnacles (Semibalanus cariosis). In western Massacre Bay at Attu Island (173°E, 53°N), subtidal transects showed sea star body size and biomass density declined markedly between 1983 and 1994 as sea otters reinhabited this area. Mussels and barnacles translocated from the rocky intertidal zone to shallow subtidal habitats to assess loss rates from sea star predation showed lower mortality rates after the arrival of sea otters. Prey mortality rates in subtidal caged controls were consistently low and similar to those of intertidal controls in both years. These findings elucidate a trophic pathway by which sea otters can influence ecosystems separate from the well-known sea otter/sea urchin/macroalgae cascade.     

Moderate stoichiometric homeostasis in the sea urchin Lytechinus variegatus: effects of diet and growth on C:N:P ratios

The influence of dietary elemental contents on consumer stoichiometry was investigated in selected and combined soft tissues (as a proxy of the whole individual) of the omnivorous sea urchin, Lytechinus variegatus. We raised urchins for 4 months in controlled seawater tanks using three different diets with different nutritional contents (from lower to higher: seagrass, red macroalgae, and a formulated diet). Individuals fed the different diets varied an average of 19.7, 19.4, and 38 % in C:N, C:P, and N:P ratios, respectively, with stronger temporal variability for C:P and N:P ratios across tissues and whole individuals. This resulted in homeostasis parameters (1/H) of ?0.45, 0.09, and 0.38, respectively, for C:N, C:P, and N:P, indicative of homeostatic to weakly homeostatic organisms, at least for C:P and N:P ratios. Individuals fed the nutrient-rich formulated diet had higher growth rates (14 ± 0.83 g WW month^?1) than those fed macroalgae or seagrass (9.3 ± 0.57 and 3.4 ± 0.33 g WW month^?1, respectively). However, rapid body increments in more nutritional diets caused both a decrease in the %N and an increase in the %P of soft tissues, which resulted in significant but opposite effects of diet stoichiometry and growth in sea urchin C:N (R = ?0.74 and R = 0.93, for diet and growth effects, respectively) and N:P ratios (R = 0.60 and R = ?0.63, also, respectively, for diet and growth effects). Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P < 0.01), and moderate variability in C:N:P ratios of sea urchins possibly arise from differences in the allocation of proteins and RNA to body components, similarly to what has been proposed by the growth rate hypothesis.     

The distribution,metabolism and toxicity of 14c-DDT in model aquarium tanks with fish and sediment simulating a tropical marine environment

Studies conducted on the distribution, fate and metabolism of DDT in a model ecosystem simulating a tropical marine environment of fish, Gobious nudiceps, Lethrinus harak, Gobious keinesis, Gobious nebulosis and white shrimp (Panaeus setiferus), show that DDT concentration in the water decreases rapidly within the first 24?h. Rapid accumulation of the pesticide in the biota also reaches a maximum level in 24?h before gradually declining. The bioaccumulation factors calculated for the fish species (G. keinesis) and white shrimp (P. Setiferus) were 270 and 351, respectively, after 24?h. There was a steady build up of DDT residues in the sediment during the first 24?h which continued to a maximum concentration of 6.66?ng/g in the seawater/fish/sediment ecosystem after 3 weeks and 5.27?ng/g in the seawater/shrimps/sediment ecosystem after 2.7 days. The depuration of the accumulated pesticide was slow with only 54% lost in G. nudiceps within 3 days of exposure in fresh sea water. By contrast, depuration was fast in the white shrimp, which lost 97% of the accumulated pesticide under the same conditions. DDT was found to be toxic to two of the fish species (G. nebulosis and L. harak) and to white shrimp, and the degree of toxicity was dependent on the particular species. The 24?h LC₅₀ at room temperature for the fish species G. nebulosis and white shrimp was found to be 0.011 and 0.116?mg/kg, respectively. These levels are comparable to the ones recorded for the temperate organisms. Degradation of DDT to its primary metabolites, DDE and DDD, was found in all the compartments of the ecosystem with DDE being the major metabolite in the fish, shrimps and sediment, while in seawater, DDD dominated as the major metabolite.     

Crushing predation of the spiny star Marthasterias glacialis upon the sea urchin Paracentrotus lividus

Literature data report that only fish predators are able to crush sea urchin tests in Mediterranean rocky reefs. This experimental study showed that the spiny star Marthasterias glacialis is able to break Paracentrotus lividus tests and that the breaking event is more likely to occur for small-sized sea urchins than for big ones. Our results show that the role of M. glacialis in regulating P. lividus population density can be important in specific locations. They may have important implications, moreover, for the use of tethering techniques aimed at identifying predator types of sea urchins.     

Alarm responses of the green sea urchin, Strongylocentrotus droebachiensis, induced by chemically labelled durophagous predators and simulated acts of predation

The green sea urchin, Strongylocentrotus droebachiensis, exhibited immediate behavioural responses to waterborne chemosensory cues from two durophagous predators, the Atlantic wolffish, Anarhichas lupus, and the edible crab, Cancer pagurus; as well as from crushed conspecifics and crushed blue mussels, Mytilus edulis. The response patterns were dose dependent and diet dependent. Strong responses were elicited by water conditioned by echinivorous wolffish (97.5%), undiluted urchin extract (73%), undiluted mussel extract (52.5%), and by water conditioned by echinivorous crab (45%). In contrast, urchin extract diluted to 1%, mussel extract diluted to 10%, and water conditioned by predators on a mussel diet elicited weak responses (~20%). Infection by the endoparasitic nematode Echinomermella matsi had no significant effect on the response pattern of S. droebachiensis. There was no conclusive evidence of an alarm response to the predators per se, as the weak response to stimuli from non-echinivorous wolffish and crab, as well as from the seastar Asterias rubens, may have been caused by chemical cues transmitted from their mussel diet. The diet-dependent response to predators suggests that active predators were labelled by chemical cues from their echinoid prey. The chemical cue from echinivorous wolffish acted as both an arrestant and as a repellent, whereas the response to other cues was predominantly or exclusively repellent. The response to echinivorous wolffish was quantitatively stronger than, and qualitatively different from, the response to other stimuli, including undiluted urchin extract. The wolffish is apparently being labelled by a latent chemical cue which derives its potency from activation by, or interaction with, substances in the digestive system of the wolffish. We interpret this novel phenomenon as evidence of alarm signal magnification. The induced behavioural modifications demonstrate the green sea urchins' ability to detect chemical cues associated with active durophagous predators, and would therefore seem to have adaptive potential as a predator defence mechanism.     

Effects of small-scale habitat fragmentation on predator–prey interactions in a temperate sea grass system

During the last decades, fragmentation has become an important issue in ecological research. Habitat fragmentation operates on spatial scales ranging over several magnitudes from patches to landscapes. We focus on small-scale fragmentation effects relevant to animal foraging decision making that could ultimately generate distribution patterns. In a controlled experimental environment, we tested small-scale fragmentation effects in artificial sea grass on the feeding behaviour of juvenile cod (Gadus morhua). Moreover, we examined the influence of fragmentation on the distribution of one of the juvenile cod’s main prey resources, the grass shrimp (Palaemon elegans), in association with three levels of risk provided by cod (no cod, cod chemical cues and actively foraging cod). Time spent by cod within sea grass was lower in fragmented landscapes, but total shrimp consumption was not affected. Shrimp utilised vegetation to a greater extent in fragmented treatments in combination with active predation. We suggest that shrimp choose between sand and vegetation habitats to minimize risk of predation according to cod habitat-specific foraging capacities, while cod aim to maximize prey-dependent foraging rates, generating a habitat-choice game between predator and prey. Moreover, aggregating behaviour in grass shrimp was only found in treatments with active predation. Hence, we argue that both aggregation and vegetation use are anti-predator defence strategies applied by shrimp. We therefore stress the importance of considering small-scale behavioural mechanisms when evaluating consequences from habitat fragmentation on trophic processes in coastal environments.