Man-made structures on marine sediments: Effects on adjacent benthic communities

This study (1975–1977) examines the effect of man-made structures on natural sand bottom communities in shallow water in San Diego County, southern California, USA. While there were shallow scour effects to 15 m around some artificial reefs, the reefs had no measurable effect on sand ripple patterns, grain size, organic carbon or infauna beyond the scoured areas. Foraging by reef-associated fishes produced profound alterations in the epifauna populations of the sea pen Stylatula elongata. The sea pen densities were 4 to 10 m^-2 before the reefs were established, but within 5 mo were eliminated from distances greater than 200 m around the reefs. On the other hand, densities of the tube-building polychaetes Diopatra spp. seemed to be enhanced in the immediate vicinity of the artificial reef. Oil platforms and bridge pilings seem to have much more profound effects on the nearby sand communities than do the relatively small artificial reefs. In addition to the elimination of sea pens, Diopatra spp. densities increased from <1.0 m^-2 in control areas to as many as 73 m^-2 in the vicinity of oil platforms. Grain size and infauna were strongly affected by the oil platform.     

Influence of wave exposure on South African mussel beds and their associated infaunal communities

This study investigates effects of wave exposure on beds of the mussel Mytilus galloprovincialis and the infaunal communities associated with them. Nine sites of varying wave exposure were sampled near Groenrivier, on the west coast of South Africa. Wave exposure was measured using a Palumbi device, and exposure ranged from 7.17×10³ to 18.49×10³ N m^–2. Percentage mussel cover at each site was measured using 50×50 cm quadrats, and three 10×10 cm samples were removed from the mussel beds in the mid-intertidal zones of each site for examination of infauna. Percentage mussel cover, mean mussel length, mussel biomass and mussel bed depth all peaked at intermediate exposures, declining towards both the most sheltered and most exposed sites. Infaunal species diversity and richness both showed the reverse trend, peaking at the most sheltered and most exposed sites, and declining at intermediate exposures. Neither infaunal abundance, nor biomass, was significantly correlated to wave exposure, although abundance was highest at sheltered sites and biomass greatest at the most exposed site, which was dominated by limpets and large robust polychaetes.Communicated by J.P. Thorpe, Port Erin     

Influence of predation on infaunal abundance in Upper Chesapeake Bay,USA

The importance of predators in controlling the densities of infaunal (>0.5 mm) organisms was investigated in the mesohaline region of the Upper Chesapeake Bay (USA) using field experiments. The role of predators in controlling infaunal density and community characteristics varied with habitat type, season (i.e., predator abundance) and developmental or successional stage of the community. Few infaunal species were adversely affected by predator exclusion. Species that increased greatly in abundance in the absence of predators (e.g. Eteone heteropoda, Streblospio benedicti, Nereis succinea, and juvenile Macoma balthica and Mya arenaria) lived near the sediment-water interface and had major population pulses from fall through spring. Species whose abundances increased moderately or were not affected by predator exclusion were deeper burrowing organisms (e.g. Heteromastus filiformis and adult Mya arenaria), or were relatively small organisms (e.g. Paraprionospio pinnata, Scolecolepides viridis and Peloscolex gabriellae) whose principal predators could be other members of the infauna. Competition did not appear to be an important factor controlling infaunal density in these experiments.This work is Contribution No. 973 of the Center for Environmental and Estuarine Studies of the University of Maryland     

Predation on meiofaunal and macrofaunal invertebrates by western sandpipers (Calidris mauri): evidence for dual foraging modes

Western sandpiper (Calidris mauri) predation was examined by concurrent experiments and direct observations of foraging behaviour on high intertidal mudflats of the Fraser River estuary, British Columbia. Western sandpipers foraged by either “pecking” on the surface (64% of observational time) or probing into sediment (29%). The first experiment (probe-mark method) consisted of collecting small-volume cores (21.2 cm³) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min feeding period. The second experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collection of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal and meiofaunal size fractions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control sediments revealed significant differences, attributed to shorebird predation, for both experiments. The probe-mark experiment detected the removal of large infaunal polychaetes (∼ 20 mm), while the exclosure experiment showed depletion of epifaunal harpacticoid copepods (0.063–0.5 mm). Predation on macrofaunal cumaceans was detected in both experiments. Invertebrates selected by western sandpipers neither fell within traditional infaunal size classifications (macro- vs. meiofauna; 500 μm delineation) nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western sandpipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal size ranges and (2) selective probing for larger infauna, such as polychaetes. These findings were facilitated by the combination of methodologies employed. Received: 29 December 1999 / Accepted: 11 September 2000     

Detrital pathways in a coral reef lagoon

Coral reef lagoons are generally regarded as zones of net heterotrophy reliant on organic detritus generated in more productive parts of the reef system, such as the seaward reef flat. The abundance and biomass of sediment infauna were measured seasonally for one year (1986) within the lagoon of Davies Reef, central Great Barrier Reef, to test the hypothesis that macrofaunal biomass and production of coral reef lagoons would decrease with distance from the reef flat and would change seasonally. In general, there were no simple relationships between infaunal standing stock or production and distance from the reef flat or season. Bioturbation by callianassid shrimps negatively affected the abundance of smaller infauna, suggesting a community limited by biogenic disturbance rather than by supply of organic material. Polychaetes and crustaceans were dominant amongst the smaller infauna (0.5 to 2mm) while larger animals (> 2 mm) were mostly polychaetes and molluscs. Mean biomass of infauna at both sites and all seasons was 3 181 mg C m^?2. The smaller animals (0.5 to 2 mm) contributed about 40% of total macrofaunal respiration and production although they represented only 15% of the total macrofaunal biomass. The biomass of macrofauna was about equal to that of the bacteria and meiofauna, while respiration represented 10 to 20% of total community respiration. Consumption by macrofauna accounts for only 3 to 11% of total organic inputs to sediment, with a further 14 to 17% being lost by macrofaunal respiration.     

Importance of predation by infaunal polychaetes in controlling the structure of a soft-bottom community in Maine,USA

Field manipulations in a Maine, USA intertidal mud flat showed that Nereis virens adults were an important factor in regulating the abundance of Corophium volutator, an amphipod which comprised 63% of the total number of individuals in the benthic community. Removal of N. virens adults resulted in an increase in C. volutator abundance, while addition of N. virens adults led to a decrease. There was a significant negative correlation between the abundance of N. virens adults and that of C. volutator. C. volutator was also negatively correlated with the density of all the other infaunal species combined. These results suggest that there is at least a 3-level interactive system (N. virens —C. volutator — other infaunal species) within the infauna in Maine; N. virens is a factor in controlling the abundance of C. volutator, and C. volutator may play a role in regulating the densities of other infaunal species. Models of generalized cropping in soft-bottom systems may be too simple if they ignore complex trophic relationships within the infauna.     

Fish feeding on mobile benthic invertebrates: influence of spatial variability in habitat associations

Patterns of habitat association and foraging were examined for a group of tropical goatfishes (family Mullidae) that feed on mobile benthic invertebrates at Lizard Island (Great Barrier Reef). All goatfish possess barbels that disturb the substratum during feeding. Foraging methods were examined for the six most common species and used in conjunction with data on habitat associations to estimate the distribution and potential impact on the benthic invertebrate assemblage of foraging-related disturbance. Particular species exhibited broad habitat associations which differed little over two surveys (January 1989, January 1990). All species showed different preferences for the substrata they foraged. Preferences for substrata exhibited by the most common reef-associated species, Parupeneus multifasciatus, differed among locations separated by 1 km, between sites 150 m apart, and between depths (shallow and deep). Habitat preferences changed with ontogeny. Based on their habitat associations and foraging preferences, species were divided into habitat generalists and specialists. Specialists associated primarily with soft sediments. Habitat generalists, such as P. multifasciatus and P. cyclostomus, are likely to have an impact on their mobile invertebrate prey that is localised, diffuse and transitory, making any experimental analysis difficult and expensive. Habitat specialists form a guild of fishes with complementary feeding modes that efficiently exploit soft sediments and are more amenable to experimental manipulation. Experiments designed to detect the impact of foraging by these fishes must be repeated at different locations and times and must account for depth differences in foraging pressure.     

Community responses to UV radiation. II. Effects of solar UVB on field-grown diatom assemblages of the Carribean

The effects of ambient solar UV on community productivity and structure were assessed during primary succession of benthic diatoms on artificial substrate in a coral reef of the Caribbean. Artificial substrates, partially enclosed by UV cutoff filters, were placed at a depth of 60 cm below surface. During the initial colonization stages, the assemblages exposed to the full solar spectrum had a mean productivity 43.4% lower than the assemblages exposed to PAR+UVA only. Some differences in species diversity of assemblages under the different UV treatments were also observed. Sensitive species to UVB exposure were Mastogloia angulata, M. ovata, M. paradoxa, Nitzschia longissima, Plagiogramma staurophorum, Rhopalodia musculus, and Surirella ovata. These UVB effects gradually diminished as succession proceeded; 5 to 6 weeks after the placement of the substrates in the water, no significant differences in productivity were observed between the different treatments, while after 6 weeks of growth, species diversity and evenness were higher, although not statistically significant, in the UVB-exposed assemblages. During the first 2 weeks of growth, the productivity under PAR+UVA was significantly lower than that under PAR only. Received: 12 July 1996 / Accepted: 20 November 1997     

Differences in the structures of fish assemblages inThalassia testudinum beds in Guadeloupe,French West Indies,and their ecological significance

The structures of fish assemblages in twoThalassia testudinum beds in Guadeloupe, French West Indies, one adjacent to mangroves and the other adjacent to coral reefs, were compared between January 1983 and May 1984. The aim of the study was to compare the influences of mangroves and coral reefs on the utilization of seagrass beds by fishes through examination of species composition, catch rate, size of fishes and temporal changes. The two fish assemblages were similar in terms of the number of species they had in common (nearly 44% of the total number of species collected) and the great abundance of juveniles. They both comprised species that usually inhabit other habitats, i.e., estuaries, open waters or coral reefs. Estuary-associated species (e.g. Gerreidae) were the most abundant species in the seagrass bed near the mangroves, while small pelagic species (e.g. Clupeidae) were the most abundant species in the seagrass bed near the coral reefs. The seagrass bed near the mangroves was preferentially utilized as a nursery area by small juveniles of various species (e.g. Clupeidae, Sparidae, Gerreidae, and at least one coral reef species,Ocyurus chrysurus). The abundance of these species varied frequently, suggesting successive arrivals and departures of juveniles over time. The seagrass bed near the coral reefs was characteristically utilized by fishes that are more able to avoid predation, i.e., fishes that forage over seagrass beds at night and shelter in or near the coral reefs during the day (large juveniles of coral reef species and adults of schooling pelagic species, respectively). The constant migrations of these fishes between the coral reefs and seagrass beds explained the relative stability of the structure of the fish assemblage in the seagrass bed over time. Thus, the two seagrass beds were not equivalent habitats for fishes. The distinct ecological influences of the mangroves (as a nursery for small juveniles) and coral reefs (as a shelter for larger fishes) on the nearby seagrass beds was clearly reflected by the distinct utilizations of these seagrass beds by fishes.     

The infaunal benthos of a natural oil seep in the Santa Barbara channel

We are studying a diverse infaunal benthic community that exists in the fine sand sediments of a shallow (16 m) natural oil seep near Santa Barbara, California, USA. The study area and sampling methods are described in this introductory paper. Data presented indicate the adequacy of sampling in revealing horizontal patchiness and vertical faunal distributions. The infauna of the seep and of a nearby comparison area, without seepage but of similar depth and sediment type, are compared. The data indicate a consistently larger but fluctuating density of organisms at the seep station. However, Shannon-Weaver diversity (H=1.6 to 1.7), Peilou's evenness (J=0.80 to 0.81), and measures of dominance-diversity with estimates of graphical skewness (0.66 to 0.68) and kurtosis (1.2 to 1.4) are all similar for the two stations. The rank correlation of common species at the stations is significant according to Spearman's rho. Species common to both stations account for 85 to 95% of the individuals, further indicating the high degree of similarity between stations. Denser populations of oligochaetes and the maldanid polychaete Praxillella affinis pacifica suggest some advantages for deposit feeders at the seep station. Mats of the bacterium Beggiatoa sp. are associated with localized intense oil seepage. Hypotheses suggesting trophic enrichment and biochemical adaptation at the seep are presented.     

Abundance and arm loss of the infaunal brittlestar Ophiophragmus filograneus (Echinodermata: Ophiuroidea), with an experimental determination of regeneration rates in natural and planted seagrass beds

The regeneration rate of arms of the infaunal brittlestar Ophiophragmus filograneus was determined in natural and planted beds of the seagrass (Halodule wrighttii) in Tampa Bay, Florida, USA in 1990. Annually, brittlestar densities did not differ on average between the two beds, but they did show marked differences on individual dates. Rates of arm loss could not be determined for the two sites separately, but were estimated for the entire area. Between 52 and 94% of the brittlestars collected showed evidence of arm regeneration. Approximately one-third of those collected were regenerating one arm, with smaller proportions regenerating two or more arms. Regeneration rate was greater overall for individuals that had lost more tissue; however, the rate of soft-tissue regeneration per arm regenerated was significantly greater for those that had lost one arm compared to those that had lost three arms. The rate of soft-tissue regeneration did not differe between individuals from the two seagrass beds, and regeneration rate was lower than reported for other infaunal brittlestars. Skeletal regeneration per arm was similar in both arm-removal treatments. This result supports the hypothesis that echinoderm skeleton is energetically inexpensive and may help to lengthen arms during early regeneration. The rate of skeletal production was three-fold greater in individuals from the planted seagrass than in those from the natural. bed. As increased skeletal regeneration requires some additional energy, this indicates a greater energy expenditure and differential allocation by brittlestars regenerating in the planted seagrass areas.     

Turbinaria ornata as an herbivory refuge for associate algae

Habitat associations are an integral part of coral reef community structure. Commonly, one organism lives in such close association within or near another that a spatial refuge occurs, whereby one of the organisms provides protection to the other. This is often the result of defenses of the host deterring an associate organism’s consumers. In Moorea, French Polynesia, the range and abundance of the brown macroalga, Turbinaria ornata, have increased drastically since 1980 such that dense aggregations of this macroalga are a dominant component of the backreef habitat. Turbinaria ornata is both mechanically and chemically defended from herbivores. Other species of macroalgae grow within aggregations of Turbinaria and may benefit from these defenses. This study investigates whether aggregations of Turbinaria create a refuge from herbivory for associate macroalgae. When Turbinaria aggregations were removed experimentally, there was a significant increase in the number of associate algal species. Moreover, an herbivory assay using the palatable local alga Acanthophora spicifera identified herbivory as the mechanism for lower diversity on bommies lacking Turbinaria aggregations. The local increase in algal richness due to the refuge from herbivory afforded by Turbinaria may be an important contribution to macroalgal and community dynamics on reefs in Moorea, French Polynesia.     

Variability in nursery function of tropical seagrass beds during fish ontogeny: timing of ontogenetic habitat shift

Seagrass beds are often considered to be important nurseries for coral reef fish, yet the effectiveness of these nursery functions (refuge and food availability) at different juvenile stages is poorly understood. To understand how the demands of juvenile fish on seagrass nursery functions determines the timing of ontogenetic habitat shifts from seagrass beds to coral reefs, we conducted visual transect survey and field tethering and caging experiments on three different sizes of the coral reef fish Pacific yellowtail emperor (Lethrinus atkinsoni) during its juvenile tenure in seagrass beds at Ishigaki Island, southern Japan. The study showed that although the number of individual L. atkinsoni juveniles decreased by >90 % during their stay in the seagrass nursery, the shelter and/or food availability functions of the nursery, at least for a juvenile size of approximately 5 cm total length (TL), provided the best survival and growth option. The timing of ontogenetic migration to coral reefs of larger fish (>8 cm TL) was attributed to foraging efficiency for larger food items in different habitats. Overall, the function of the seagrass bed nursery changed with juvenile body size, with marginally higher survival and significantly greater growth rates during early juvenile stages in seagrass beds compared to coral reefs. This would contribute to the enhancement in the number of individuals eventually recruited to adult populations.     

Ultraviolet radiation effects on the behavior and recruitment of larvae from the reef coral Porites astreoides

We tested the rarely considered hypothesis that the ultraviolet portion (UVR, 280–400 nm) of the light spectrum affects patterns of recruitment in reef-building corals. The premise for this hypothesis rests in the fact that biologically relevant intensities of UVR penetrate to considerable depths (>24 m) in the clear waters surrounding many coral reefs, and that reef organisms allocate substantial resources to prevent and repair UVR damage. The ability of larvae spawned by the brown morph of the Caribbean coral, Porites astreoides, to detect and avoid UVR was assessed in petri dishes where one-half of the dish was shielded from UVR and the other exposed. Observations made every 30 min between 10:30 and 13:30 h showed significantly higher densities of larvae swimming in regions shielded from UVR. To determine how this behavior affects settlement patterns, larvae collected from P. astreoides adults at 18 m depth were released into chambers deployed at 17 m depth where they were given a choice of three different light regions in which to settle: PAR (PAR=400–700 nm), PAR+UVAR (UVAR=320–400 nm), and PAR+UVAR+UVBR (UVBR=280–320 nm). At the end of the experiment, greater numbers of P. astreoides larvae had settled in the region of the tube where UVR was reduced than would be expected if dispersion were random. To our knowledge, this is the first demonstration in any reef-building coral species that planula larvae can detect UVR and that it affects their choice of a settlement site. These results indicate that the capacity to detect and avoid habitats with biologically damaging levels of UVR may be one factor contributing to the successful recruitment of coral larvae.     

Resource use by five sympatric parrotfishes in the San Blas Archipelago,Panama

Resource use by five sympatric species of parrotfish was quantified in the San Blas Archipelago of the Republic of Panama from March to August 1987. Detailed observations of parrotfishes on patch reefs and surrounding seagrass beds showed that they partition resources with respect to habitat, food and size, but not time. Although parrotfishes shared resources, the proportions of each resource used differed significantly among species. Scarus iserti (Bloch) scraped filamentous microalgae that grew from eroded coral pavement on lower slopes of patch reefs and in halos, the area of sparse vegetation surrounding reefs. Sparisoma viride (Bonnaterre) foraged on upper slopes of patch reefs where they mostly took bites from dead coral and associated algae. S. aurofrenatum (Cuvier and Valenciennes) had the broadest diet, which consisted mostly of seagrasses and macro- and microalgae that were attached to dead coral on lower reef slopes and in halos. Although S. chrysopterum (Bloch and Schneider) commonly occurred on patch reefs, it primarily foraged in seagrass beds that surround them. S. rubripinne (Cuvier and Valenciennes) was distributed most widely, ranging from seagrass beds to reef crests, where it took bites from seagrasses, dead coral and macroalgae. Juveniles of all species occurred on lower slopes or in halos where they scraped filamentous microalgae from coral pavement. As they matured, parrotfishes moved into other habitats changing access to different types of food. All of these parrotfishes fed throughout the daytime, and resource use did not differ between morning and afternoon.     

Experimental evidence for the effects of the thalassinidean sandprawn <Emphasis Type="Italic">Callianassa kraussi</Emphasis> on macrobenthic communities

Bioturbation by thalassinidean sandprawns is known to structure soft-bottom communities, and field observations have suggested that the sandprawn Callianassa kraussi is a significant force influencing macrofaunal communities. To investigate causal relationships, a field experiment was undertaken in Durban Bay, South Africa, in which experimental cages were used to exclude or include C. kraussi and the abundance of macrofauna in these treatments documented. Cage effects were assessed by comparing macrofauna in inclusion cages with that of unmanipulated areas containing high densities of C. kraussi equivalent to those in inclusion cages. Measurements were made in 3 months, in March, June and September 2005. Total abundance and species richness of macrofauna were significantly greater in exclusion cages than in inclusion treatments during all sampling seasons, while diversity differed between these treatments in June and September only. Ordinations indicated that macrofaunal assemblages in exclusion cages differed statistically from inclusion and control treatments in all three sampling seasons. In general, the surface-grazing gastropod Nassarius kraussianus and suspension and deposit-feeding species such as the polychaetes Prionospio sexoculata and Desdemona ornata, cumaceans, and the bivalves Dosinia hepatica and Eumarcia paupercula were significantly more abundant in prawn-exclusion plots, implying that they are negatively affected by bioturbation by C. kraussi, whereas burrowing infauna were not affected.     

Feeding on intertidal microbial mats by postlarval tiger shrimp, <Emphasis Type="Italic">Penaeus semisulcatus</Emphasis> De Haan

A series of experiments investigated the potential role of microbial mats in nutrition of the early settlement stages of Penaeus semisulcatus. From 3 days post-metamorphosis, the microbial mat supported high growth and survival rates in postlarvae, equivalent to that supported by a control diet of Artemia nauplii and mussel. Examination of gut contents indicated that benthic postlarvae feed indiscriminately on the microbial mat. However, when postlarvae were fed separated size-fractions of the microbial mat, only the fraction containing a high concentration of infauna (mainly nematodes) was able to support the same growth as intact microbial mat. This appears to be due to the low nitrogen content (0.4–0.9 mmol g⁻¹) of the various size-fractions, compared to that of infauna (4.0 mmol g⁻¹). The stable isotope composition of the dietary size-fractions and postlarval shrimp tissue supports the hypothesis that the shrimp assimilated C and N primarily from the associated infauna. This may be due to selective feeding that is not apparent from stomach contents, due to rapid digestion of fauna soft tissues, or to differential assimilation of infaunal prey relative to other microbial mat components. The results demonstrate that microbial mats may support survival and growth in early-stage penaeid shrimp postlarvae on intertidal mud flats.     

Stability of a Coal Waste Artificial Reef

The feasibility of using U.K. coal-fired power station waste materials for artificial reef production is being examined. in June, 1989, an experimental artificial reef was constructed in Poole Bay, off the central south coast of the U.K., using three different mixtures of pulverised fuel ash (PFA), flue gas desulphurisation (FGD) gypsum and slurry, stabilized with cement and formed into blocks. Fifty tonnes of 40 × 20 × 20 cm blocks were formed into eight conical reef units replicating three different PFA/gypsum mixtures and one concrete control. the reef structure is 10m below chart datum on a flat sandy sea-bed.

Combustion of coal concentrates the heavy metal content in the resultant ash. the purpose of stabilization of the ash as blocks is twofold: to immobilize heavy metals (or other components) and to provide hard substratum for the attachment of organisms. to examine the effectiveness of this stabilization and hence the environmental compatibility of the block materials, heavy metal (Cd, Cr, Cu, Pb, Mn, Ni, Zn) content of the blocks has been monitored routinely over two years, to determine leaching rates. Sectional profiles indicate partial replacement of calcium content by magnesium. Associated with this there has also been some redistribution of heavy metals. Only in the case of cadmium has there been a detectable loss from the surface of blocks. Chromium and manganese concentrations appear to have increased. the metal content of the reef epibiota (including ascidians, Ascidia mentula; hydroids, Halecium spp.; bryozoans, Bugula spp. and red algae) growing on the ash blocks has been compared to that of epibiota attached to the concrete controls and surrounding sea-bed. to date no evidence of excess bioaccumulation of metals has been detected.

The physical integrity of the ash reef blocks has been maintained. There is evidence that the blocks are increasing in compressive strength.

An indication of the fishery enhancement potential of the experimental structure is given by the presence of eight commercially fished species (crustaceans and molluscs) including lobsters (Homarus gammarus).     

Reef-basel herbivores and the distribution of two seagrasses (Syringodium filiforme and Thalassia testudinum) in the San Blas Islands (Western Caribbean)

Two seagrasses, manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum), predominated in the areas bordering Ukkup Tupo, San Blas Islands, Western Caribbean. These seagrasses occupied the following three concentric zones extending outward from the reef: a near-reef turtle grass zone, an intermediate manatee grass zone and an off-reef turtle grass zone. Feeding experiments between January and March 1980 indicate that the absence of manatee grass close to the reef resulted from grazing by reef-associated herbivores, mainly day-active fishes and night-active sea urchins (Diadema antillarum). Grazing on manatee grass by fishes was approximately six times greater than grazing by sea urchins; thus, it appears that herbivorous fishes restrict the near-reef distribution of manatee grass at the study area. Where grazing was heaviest, the inner boundary of the manatee grass zone was farthest from the reef. The volume of manatee grass grazed during experiments was five times the volume of turtle grass consumed, strongly suggesting that the former species is a preferred food item. This is the first evidence for selective grazing on seagrasses.     

Herbivory by the Caribbean king crab on coral patch reefs

Caribbean coral reefs are increasingly dominated by macroalgae instead of corals due to several factors, including the decline of herbivores. Yet, virtually unknown is the role of crustacean macrograzers on coral reef macroalgae. We examined the effect of grazing by the Caribbean king crab (Mithrax spinosissimus) on coral patch reef algal communities in the Florida Keys, Florida (USA), by: (1) measuring crab selectivity and consumption of macroalgae, (2) estimating crab density, and (3) comparing the effect of crab herbivory to that of fishes. Mithrax prefers fleshy macroalgae, but it also consumes relatively unpalatable calcareous algae. Per capita grazing rates by Mithrax exceed those of most herbivorous fish, but Mithrax often occurs at low densities on reefs and its foraging activities are reduced in predator-rich environments. Therefore, the effects of grazing by Mithrax tend to be localized and when at low density contribute primarily to spatial heterogeneity in coral reef macroalgal communities.