Artificial Seagrass Research Articles

The relative importance of food and shelter for seagrass-associated invertebrates: a latitudinal comparison of habitat choice by isopod grazers.

The generality of mechanisms affecting habitat choice and grazing in seagrass meadows was evaluated in a latitudinal comparison of seagrass grazers from the temperate (60°N) Baltic Sea and the subtropical (30°N) Gulf of Mexico. Using similar habitat choice experiment set-ups in Finland and the USA, the role of food type, habitat complexity and predation hazard on habitat choice of the isopods Idotea baltica (Pallas) and Erichsonella attenuata Harger were tested. When shelter was provided by both living and artificial seagrass, epiphytic food resources on artificial vegetation were clearly preferred by both species, although Idotea was attracted to epiphyte-free seagrass when no alternative food was present. When choosing between food and shelter, both species preferred epiphytic food over shelter. However, under predation hazard of fish, Erichsonella clearly switched to the habitat offering shelter, while the presence of a predatory fish produced no preference for shelter by Idotea. Food type may be considered as an universal mechanism that partly determines the presence of grazers in seagrass habitats and is, in the absence of a predator, more important than shelter. Predation risk affected the behaviour of the grazers, but the response varied between species possibly due to varying importance of fish predation in the areas studied.

Effect of habitat complexity and predatory style on the capture success of fish feeding on aggregated prey

When offered a choice between open water and a ‘vegetated’ area of an aquarium (artificial sea grass), mysid (Paramesopodopsis rufa) swarms always selected open water. In contrast, juvenile and adult sea horses, Hippocampus abdominalis, which feed by ambush predation, always selected vegetated areas even when their mysid prey occurred in the adjacent clear water. Lunging predators, Australian salmon (Arripis trutta), always preferred open water to vegetated regions of the tank. Increasing vegetation density, i.e. from low to medium habitat complexity had a positive effect on capture success of juvenile and adult sea horses feeding on mysid swarms. However, number of attacks by juvenile sea horses, but not adults, was significantly reduced in higher density vegetation. Number of attacks by Australian salmon decreased as vegetation density increased but capture success, nonetheless, increased. Density of vegetation did not significantly affect the number of unsuccessful attacks by adult sea horses. Whereas proportion of unsuccessful attacks increased with vegetation density for juvenile sea horses, it decreased for juvenile salmon. This result is thought to be due to the fact that high vegetation densities disrupt structure of the prey swarm, itself a defensive formation, leaving a large number of more vulnerable small groups. Cohesive escape responses of a large swarm are likely to be a more effective defence against a lunging predator than against an ambush predator whose strategy is to use stealth to avoid provoking escape responses. Dense vegetation will militate against this strategy.

Organism responses to habitat fragmentation and diversity: Habitat colonization by estuarine macrofauna

Ecologists increasingly recognize that their choice of spatial scales may influence greatly their interpretation of ecological systems, and that small changes in the patchiness of habitat resources can produce abrupt, sometimes dramatic shifts in distribution and abundance patterns of a species. Moreover, identification of scale- and habitat-dependent ecological patterns are central to management efforts aimed at predicting the response of organisms to the increasing threat of habitat fragmentation. We used habitat plots containing artificial seagrass, oyster shell, and a mixture of seagrass and shell, placed on unstructured seafloor for 14 days in Back Sound, North Carolina, USA to examine the interactive effects of patch size, habitat diversity and experimental site on colonization by assemblages of estuarine macrofauna. We tested three a priori predictions of the general hypothesis that macrofaunal colonization is scale- and habitat-dependent: (1) colonization (per unit area) will be higher in small patches than in large ones; (2) small macrofauna will show a stronger response to habitat patchiness at a given scale than large macrofauna; and (3) colonization by estuarine macrofauna will be higher in habitat plots containing a mixture of seagrass and oyster shell compared to monotypic plots. Macrofauna responded to habitat patchiness in a complex manner that varied according to habitat type, experimental site, species, taxon, functional group, and animal body size (small: 500 μm–2 mm; large: >2 mm). Of the five out of seven response variables where we observed a significant patch size effect, grass shrimp ( Palaemonidae sp.) and small, mobile crustaceans (i.e., amphipods and isopods) were the only taxonomic or functional groups whose densities were higher in small (0.25 m 2) than large (1 m 2) patches, as predicted. Moreover, there was a disproportionate reduction in macrofaunal abundance and diversity in small patches of oyster shell compared to seagrass and mixed habitat treatments; this pattern was significant for both the total density and numbers of small species but not for large macrofauna. The total density and number of macrofaunal species was not higher in the mixed habitat treatment compared to seagrass or oyster shell. Our study demonstrates that an organism's response to habitat patchiness is dependent upon species, taxa, functional group, and animal body size, and that an organism's response is further modified by habitat type. The patterns observed in this study highlight the importance of scale- and habitat-dependent responses by mobile organisms to complex benthic habitats, and, because of the disproportionate reduction in faunal density and diversity in small versus large patches of oyster shell, heightens concern over the negative impacts to biodiversity through large-scale fragmentation of subtidal oyster reefs in certain regions.

Abundance of fish and crustacean postlarvae on portable artificial seagrass units: daily sampling provides quantitative estimates of the settlement of new recruits

Artificial collectors and seagrass units have mainly provided qualitative samples of epifaunal abundance or have been difficult and time-consuming to sample. Consequently, they are useful for distinguishing temporal or spatial trends in abundance or they are deployed for several weeks and, as a result, the quantitative samples are cumulative. We developed a portable artificial seagrass unit (ASU) with buoyant plastic artificial seagrass (47 cm long by 15 mm wide strips) that can be retrieved, harvested and re-deployed with 98% catch efficiency in about 5 min by two people from a small boat. They can quickly and easily quantify settlement of crustacean and fish postlarvae over tidal or deil periods. When set for 24 h, postlarvae settled from the plankton during the night and their abundance is the result of a distinct settlement event. When set for longer periods, the numbers of postlarvae may represent several settlement events and post-settlement activities. Crustacean and fish postlarvae and juveniles used ASUs deployed within seagrass in a similar way to natural seagrass. Estimates of juvenile tiger prawn abundance from beam-trawl catches showed similar densities in natural seagrass (2.93 Penaeus semisulcatus de Haan m −2 day) to those in the ASUs (2.40 P. semisulcatus m −2 day −1); their density was significantly lower on bare trays (0.48 P. semisulcatus m −2 day −1). When deployed on bare areas, more epifaunal crustacean postlarvae were collected from the ASUs (e.g. Portunus pelagicus Linnaeus, 1.21 m −2 day −1; caridean shrimp 4.03 m −2 day −1) than from the bare trays (e.g. P. pelagicus, 0.46 m −2 day −1; caridean shrimp 0.78 m −2 day −1). However, greater abundances of the postlarvae of other crustacean taxa were collected from the bare trays than the ASUs (e.g. Sergestes spp. 1.21 and 0.31 m −2 day −1; tiger prawn postlarvae 0.15 and 0.06 m −2 day −1, respectively). Sampling with portable ASUs allows settlement to be assessed temporally (e.g., daily or tidally) or spatially (e.g., distinct areas affected by different current regimes). The strength of settlement can be used to evaluate the productivity of nursery habitat for fishery populations.

Distribution of the epiphytic organisms on Posidonia australis and P. sinuosa, two seagrasses with differing leaf morphology

ABSTRACT: The distribution of epiphytic algae and sessile invertebrates on the leaves of the seagrasses Posidonia australis Hooker f. and P. sinuosa Cambridge and Kuo is not random. Epiphyte load on the leaves of both species increases with increasing distance away from the basal meristem. There are approximately 3 times as many epiphytic algal species as invertebrate species, and many of these epiphytes grow at distinct locations on the leaves. Epiphytic invertebrates were found primarily on the basal sections of the leaves, whereas algae were most abundant near the leaf apex. Distribution of epiphyte load across the leaf surface was also non-random, with initial settlement of epiphyte propagules occurring at the margins of the leaves. The structure of the epiphytic community is strongly correlated with leaf age, with a greater abundance of epiphytic species occurring on the older leaves. It is clear that leaf morphology also plays a significant role in the distribution of the epiphytes. There is no apparent difference in the epiphytic community between the sides of the flat P. australis leaf whereas, in P. sinuosa, the concave side of the curved leaf supports a more diverse epiphytic community than the convex side. Similar patterns in the distribution of epiphytic organisms were observed on artificial seagrass leaves, indicating that, although a temporal component is involved, epiphyte distribution is influenced mainly by the relative position upon the leaf surface as well as leaf morphology, which affects the water flow pattern over the leaf.

The contributions of habitat structure and larval supply to broad-scale recruitment variability in a temperate zone, seagrass-associated fish

The contribution of habitat structure and larval supply to broad-scale spatial variability in recruitment of a temperate zone, seagrass associated fish, Sillaginodes punctata (Cuvier and Valenciennes), was investigated in Port Phillip Bay, Australia, from September to November, 1994. Replicate artificial seagrass beds were placed at five sites over a 50 km section of coastline, and artificial and adjacent natural seagrass were sampled approximately fortnightly for Sillaginodes punctata recruitment. Significant differences in recruitment amongst sites were apparent for both natural and artificial seagrass. A small but significant effect of habitat was detected where more recruits occurred in artificial relative to natural seagrass at sites with longer plant stems in the natural seagrass. The contribution of larval supply to spatial variability in recruitment was investigated by sampling natural seagrass, and concomitantly sampling the plankton immediately offshore for pre-settlement larvae. There was no significant correlation between larval abundances and recruitment, or between habitat structure and recruitment, over nine sites. We hypothesise that the high spatial variability in recruitment attributable to location is probably related to a combination of factors. These factors may include variation in larval supply, and also variation in the physical exposure of the location that influences mortality and movement of recruits in the early post-settlement stage.

Organism response to habitat patchiness: species and habitat-dependent recruitment of decapod crustaceans

Habitat fragmentation from natural or anthropogenic causes is a common phenomenon in shallow water marine habitats such as seagrass beds and oyster reefs throughout the world. Thus, habitat- and scale-dependent information on how organisms perceive and respond to changes in the landscape is critical to efforts aimed at predicting the effects of habitat change on population abundance. In estuarine systems, seagrass is considered one of the predominant nursery habitats for numerous ecologically and commercially important species; however, recent information suggests that alternative habitats such as oyster reefs may also be important sites for settlement and refuge. We used artificial seagrass- and shell-filled plastic trays placed on unstructured seafloor in Back Sound, North Carolina, USA to examine the interactive effects of Experiment, Patch size and Habitat type upon recruitment success of four species of decapod crustaceans: the grass shrimps, Palaemonetes intermedius Holthuis, P. pugio Holthuis and P. vulgaris (Say), and the blue crab, Callinectes sapidus Rathbun. Regardless of habitat type, all species of grass shrimp and J1 blue crab responded to habitat patchiness at scales as small as 0.25 to 4 m −2, whereas larger J2–J4 blue crab did not. Mean densities of all three species of grass shrimp responded in a similar manner to patch size, with significantly higher densities in the smallest patch size (0.25 m −2) compared to larger patches (0.5 to 4 m −2). Conversely, mean densities of J1 blue crabs were significantly higher in the largest patch size (4 m −2) compared to smaller patches (0.25 to 1 m −2). There was a negative and significant relationship between the density of grass shrimp predators and density of J1 prey, and no relationship between potential fish predators and J1–J4 prey. Thus, patterns observed for J1 may be dependent upon predators and not habitat patch size. Both species demonstrated different responses to habitat type depending upon relative densities and body size. Mean densities of P. intermedius and P. vulgaris were higher in seagrass than oyster shell during Experiment 1 (May) when shrimp were relatively abundant, but did not differ between habitat types during Experiment 2 (October), when densities were reduced 10-fold. Conversely, densities of P. pugio did not differ between seagrass and oyster shell. Mean densities of J1 blue crab were similar between seagrass and oyster shell, but higher in seagrass than oyster shell for later juvenile stages (J2–J4). Thus, blue crabs may rely equally on seagrass and oyster reefs as an initial settlement and refuge habitat for early juveniles. Our study demonstrates that an organism's response to habitat patchiness is species-specific, and that for a given species, the response is further modified by animal density and body size. The patterns observed in this study further highlight the importance of scale-dependent responses by mobile organisms to complex benthic habitats.

Intraspecific variation in home range exclusivity by female green razorfish, Xyrichtys splendens (family Labridae), in different habitats

Female green razorfish, Xyrichtys splendens, were studied in the field in three shallow tropical marine habitats: sandbed, Thalassia grassbed, and coral rubble bottom. Individuals showed varying degrees of home range overlap (one measure of territoriality) in each habitat, due to differences in predator pressure, the availability of refuges, and water turbidity. In a large scale field manipulation, a 68 m2 bed of artificial seagrass (300 'grass' blades per m2) was planted at one site with low turbidity, thereby increasing the availability of refuges. The females' home ranges overlapped more, whereas control fish did not change their home range use, indicating that home range exclusivity in this species is a function of refuge distribution. Female X. splendens apparently have the phenotypic plasticity to adapt their social behavior to local conditions; in this case, the availability of economically inexpensive refuges, and the risk of predation as a function of the turbidity of the water.

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish.

Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly.

Size and time of day affect the response of postlarvae and early juvenile grooved tiger prawns Penaeus semisulcatus De Haan (Decapoda: Penaeidae) to natural and artificial seagrass in the laboratory

Juvenile tiger prawns ( Penaeus semisulcatus De Haan and P. esculentus Haswell) show a strong association with vegetated habitats and are rarely caught on non-vegetated areas. This pattern of distribution may be caused by postlarvae selecting vegetation when they settle, or to differences in post-settlement mortality in different habitats. In this study, we examined whether the postlarvae and early juvenile stages of P. semisulcatus would distinguish between seagrass ( Zostera capricorni Aschers) without epiphytes, artificial seagrass and bare substratum in the laboratory. The responses of prawns reared from the egg to different stages of postlarval and juvenile development were tested to determine whether, and when, each size class showed a response to a particular habitat. Five size classes of postlarvae (average carapace lengths [CL] of 1.2, 1.4, 1.6, 1.7 and 2.0 mm) were offered a choice between Z. capricorni and bare sand. Small size classes of postlarvae either did not respond to Z. capricorni (1.2 and 1.6 mm CL), or were more abundant on bare substratum than Z. capricorni. In contrast, the largest size classes of postlarvae (1.7 and 2.0 mm CL) were more abundant on Z. capricorni during the day but not at night. The behaviour of postlarvae changed markedly at a size of 1.7 mm CL (22 days from the first nauplius): smaller postlarvae frequently swam in the water column; 1.7 and 2.0 mm CL postlarvae spent much more time resting on the substrate and perched on seagrass leaves. This size at which postlarvae first respond to seagrass during the day, and show mainly benthic behaviour, is similar to the size at which they are found on shallow seagrass beds in northern Australia. Large postlarvae (2.7 mm CL) and juveniles (4.1 mm CL) both were more abundant on artificial seagrass than bare sand during the day but not at night, indicating that they respond to structured habitats. When large postlarvae (2.4 mm CL) and juveniles (3.5 mm CL) were offered a choice between Z. capricorni without epiphytes and artificial seagrass, they were more abundant on the Z. capricorni, which suggests that chemical cues from seagrass may explain some of the responses of P. semisulcatus to seagrass.

Seagrass Patch Size and Survivorship of an Infaunal Bivalve

In nature, seagrass beds can occur as a mosaic of differently sized patches in a matrix of unvegetated sediments. While continuous, extensive seagrass meadows are recognized for their importance in providing refuge for a variety of organisms, the refuge potential of smaller (1-10 m across) vegetated patches has not been widely investigated. The degree of protection afforded by small seagrass patches may vary with patch size due to changes in edge to interior ratios and/or changes in plant characteristics. In mark-recapture experiments conducted at three separate sites, survivorship (i.e., those not lost to predation) of juvenile hard clams (Mercenaria mercenaria) varied among sites and increased with the areal extent of natural seagrass cover at all sites. At least 40% more clams were recovered live from one site than from the other two sites. Twice as many clams were recovered live from large patches of seagrass (5-10 m across) than from small (ca 1 m across), 18 times more were recovered live from large patches than from unvegetated sediments, and approximately eight times as many were recovered live from small patches than from unvegetated sediments. These patterns in clam survivorship were positively, but not significantly, correlated to patterns in below-ground biomass and shoot density among sites and seagrass patch-size treatments. When below-ground biomass and overall shoot density were held constant by using artificial seagrass mats there was no significant difference in the proportion of clams recovered live between 1 x 2 m and 4 x 4 m patches of seagrass. These results suggest that the refuge potential of natural patches of seagrass for infaunal prey species changes with patch size due to differences in the structural characteristics of the vegetation, not changes in the areal extent of the habitat.

Toward a landscape approach in seagrass beds: using macroalgal accumulation to address questions of scale.

An experimental investigation of drift macroalgal accumulation in seagrass beds was conducted to determine if the relationship between passively dispersed plant structure and the spatial arrangement of rooted macrophytes differed when examined across two spatial scales. Experiments were performed from December 1992 to April 1993 at four different sites in Tampa Bay, Florida, utilizing artificial seagrass units (ASUs) of uniform shoot length and density but with different areal dimensions [1 m2 (S) versus 4 m2 (L)]. Drift macroalgae were also collected from 1 m×1 m plots of natural seagrass at each of the experimental sites from November 1990 to May 1992 to determine the relationship between macroalgal abundance and structural characteristics of natural seagrass. Disproportionately higher amounts of macroalgae were captured in L compared to S plots suggesting that macroalgal accumulation does not scale up directly with the areal dimensions of ASU patches. Higher amounts of algae recovered in L plots is in accordance with patterns expected if algae accumulate in zones of attenuated water flow. Neither seagrass shoot density nor blade length could adequately describe the patterns of algal accumulation. These combined results suggest that explanations for trapping/retention of passively dispersed particles should extend beyond traditional measures of vegetation complexity.

Diel Variation in Immigration of Fishes and Decapod Crustaceans to Artificial Seagrass Habitat

Fish and decapod entry into small (1.5 m2) artificial seagrass habitats positioned on an open sand area in a New Jersey estuary was examined to determine if immigration varied between day and night. To encounter the structured habitats, colonizers had to cross an expanse of bare sand, with its presumably higher predation risk. Contrasts in abundance in the artificial seagrass plots between dawn and dusk indicated higher nighttime immigration for four species, including the fishesFundulus heteroclitus andMyoxocephalus aenaeus, and the caridean shrimpsPalaemonetes vulgaris andHippolyte pleuracanthus. Size-frequency distributions of colonizers varied between day and night for two fish species,Menidia menidia andSyngnathus fuscus, with a greater proportion of smaller individuals immigrating to the artificial seagrass at night.Callinectes sapidus also displayed a diel contrast in size distribution but, for this species, proportionately more small individuals colonized the plots during the day. We suggest that diel variability in predation risk and/or diel patterns in motor activity may be responsible for these patterns in immigration.

The effects of habitat complexity and light intensity on ambush predation within a simulated seagrass habitat

Laboratory studies were used to evaluate the effects of varying structural complexity (at densities of 0, 747, 1494, 2250 and 3032 blades/m 2 artificial seagrass) and light intensity (0, 1.67, 76.28 μEin/m 2/sec) on the foraging behavior and success rate of an ambush predator, the lined seahorse ( Hippocampus erectus Perry). In the presence of artificial seagrass, seahorses used “sit and wait” foraging behavior, but in the absence of structure, seahorses actively pursued prey. Even the highest seagrass densities did not significantly reduce seahorse captures of their shrimp [ Hippolyte zostericola (Smith)] prey, in contrast to the reduced capture rates often reported for actively foraging fish predators in dense vegetation. However, capture rates were significantly reduced at low light levels, even though seahorses were able to capture small numbers of shrimp in 0 light treatments. We conclude that variation in structural complexity appears to have little effect on the foraging success of this visual “sit and wait” predator, while variation in light level produces significant shifts in capture rates.

Single Large or Several Small Marine Reserves? An Experimental Approach with Seagrass Fauna

One of the most frequently debated theories in conservation biology is whether a Single Large area will preserve more species than Several Small areas of the same total area (the SLOSS debate). We tested this theory in the marine environment by comparing the diversity and abundance of fish and macroinvertebrates associated with small and large seagrass beds. In natural beds, several small beds had significantly more species than a single large bed. This occurred because a large proportion of species were not shared among beds. However, results of experiments using artificial seagrass beds to manipulate size of beds were equivocal. More species and more individuals were found in several small beds on only few occasions in experimental treatments. These results contradicted the patterns found in comparisons among natural beds. Our results stress the need to experimentally validate theories for reserve design and also indicate the danger of directly applying land-based principles to marine communities without critically examining them for each

The influence of predator presence on utilization of artificial seagrass habitats by juvenile walleye pollock,Theragra chalcogramma

Behavioral preference for a structured habitat (artificial seagrass) by juvenile walleye pollock,Theragra chalcogramma, was tested in controlled laboratory experiments. We monitored position of fish in 2000 1 tanks with and without artificial seagrass present in one half of the tank. In addition, we exposed walleye pollock to a predator model, assessing their response when a grass plot was available or unavailable as a potential refuge. In the absence of predators, the fish avoided the artificial seagrass, displaying a preference for the open water side of the experimental tanks. In the presence of a predator model, however, juvenile walleye pollock readily entered the artificial seagrass plots. In addition, they often remained in the grass canopy in proximity to the predator instead of moving out of the grass to avoid the predator (when no grass was present they consistently moved to the opposite side of the tank from the predator). The behavioral choices exhibited in this study suggest that juvenile walleye pollock modify habitat selection in response to perceived predation risk, and recognize the structure provided by artificial seagrass as a potential refuge.

The role of epiphytic periphyton and macroinvertebrate grazers in the trophic flux of a tropical seagrass community

Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae ( Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm −2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm −2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m 2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O 2 cm −2 h −1 and 2.0 μg O 2 cm −2 h −1, respectively. Daily net productivity was 14 μg C cm −2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m −2 day −1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Predation on postlarvae of the blue crab Callinectes sapidus Rathbun by sand shrimp Crangon septemspinosa Say and grass shrimp Palaemonetes pugio Holthuis

The megalopal stage ( = postlarva) of the blue crab Callinectes sapidus Rathbun enters Chesapeake Bay after larval development in coastal waters and settles in shallow areas of lower Chesapeake Bay. We examined the effects of two potential predators, two megalopal densities and two levels of habitat complexity on mortality of blue crab megalopae in laboratory experiments. Sand shrimp Crangon septemspinosa Say consumed > 99% of megalopae in each of two 40-h trials. Grass shrimp Palaemonetes pugio Holthuis consumed 37% of megalopae in the first trial, but only 5% in the second trial. No effect of habitat (sand or sand with artificial seagrass) or prey density on predation rates for either predator species was found. We then tested the hypothesis that megalopae undergoing metamorphosis to the first juvenile instar suffer higher mortality to grass shrimp predation than megalopae not in the process of metamorphosis. This hypothesis was rejected because predation rates did not differ between treatments with 0 or 95% metamorphosis. Higher predation rates (77.4%) by grass shrimp in the second set of experiments were associated with warmer water temperature, though temperature was not included formally as a factor in the design. These shrimps are seasonally abundant in Chesapeake Bay grass beds and capable predators of blue crab megalopae in the laboratory; field studies will be necessary to determine their impact on blue crab recruitment success.

Evidence for interaction between the seagrasses Zostera marina and Zostera japonica on the Pacific coast of Canada

Experiments were undertaken to differentiate between abiotic and biotic factors affecting seagrass growth. Monospecific patches of both Zostera marina and Zostera japonica were transplanted into one shallow subtidal and three intertidal sites at Roberts Bank, British Columbia. In transplanted patches, initiated in 1988, neither Z. marina nor Z. japonica showed any consistent differences in either population growth or mean shoot length among the sites. Abiotic environmental conditions could therefore not be considered responsible for the differences observed between the natural vegetation of the respective monospecific zones and the zone of naturally mixed seagrass vegetation. In a manipulation experiment, opaque and clear plastic seagrass canopies were imposed on Z. japonica vegetation and the resulting growth was compared with treatments of a natural Z. marina canopy and removal of the natural canopy. The artificial seagrasses combined with the patch layout of the experiment did not create the shade conditions intended and may have facilitated the loss of shoots. The results were not conclusive, but there were consistently higher densities of Z. japonica in the treatment where Z. marina had been removed. Interactions between these two seagrasses in their vegetative phase may contribute to the observed differences in population and morphological characters, but the dispersal and establishment phases remain to be studied. Key words: Zostera marina, Zostera japonica, seagrass, competition, transplants, artificial seagrass.

Fish larvae settling in seagrass: Effects of leaf density and an epiphytic alga

AbstractSettlement of fish to seagrass with low leaf densities was investigated by a field experiment in Botany Bay, NSW. Artificial seagrass units (ASU) were used to simulate isolated seagrass habitats of 0, 25, 50, 100, 200 and 400 leaves m‐2. A total of 369 fish of 21 species settled in the experimental units during a 6 week period. The labrid, Achoerodus viridis, made up almost 70% of these individuals. The abundance of recently settled A. viridis increased greatly between 0 and 25 leaves m ‐2, At more than 25 leaves m‐2, A. viridis settled in approximately equal numbers. Patterns of settlement for all other species combined were similar to those for A. viridis. During the experiment, an epiphytic alga. Giffordia mitchellae, colonized and grew on the ASU. The alga increased the structure associated with the experimental units. Treatment leaf densities could thus be regarded only as relative measures of shelter. At low leaf densities, fish settlement increased with increasing algal cover; at higher leaf densities, the opposite occurred. The rapid increase in abundance of recently settled fish over a relatively narrow range of leaf densities above zero implies that small increases in shelter can have large effects on settlement of larval fish.