Shoalgrass Halodule Wrightii Research Articles

Influence of the shoot density of Halodule wrightii Ascherson from rocky and sandy habitats on associated macroalgal communities

This study evaluated the influence of the shoot density of the shoal grass Halodule wrightii on the composition of the associated algal community, in rocky and sandy habitats on the coast of Ceará in northeastern Brazil. The phycological community included 18 species in 10 families, members of Rodophyta (72.2%), Chlorophyta (22.2%) and Phaeophyceae (5.5%). The largest proportion were epilithic (50%), followed by epiphytes on H. wrightii (38.4%), epipsammics (8%), and epiphytes on other algae (4%). Epiphytes on H. wrightii occurred mainly associated with rhizomes, but also tendrils of H. musciformis occurred attached to the leaves. The phycological community varied according to the density of H. wrightii, independently of particular characters of the meadows, although both habitat and other environmental variables seemed to influence the macroalgae composition and diversity. The rocky habitat was more diverse than the sandy habitat, but in the sandy habitat the shoal grass was important for algal settlement in areas where hard substrates were scarce or absent.

Amphipod control of epiphyte load and its concomitant effects on shoalgrass Halodule wrightii biomass

We investigated the interaction between epiphyte-grazer abundance and eutrophi- cation to assess the relative importance of top-down and bottom-up effects in subtropical seagrass meadows. In field experiments using a cageless technique to control amphipod abundance, we measured the effects of grazing and nutrient supply on the growth and productivity of shoalgrass Halodule wrightii and its epiphytes at both protected and wave exposed sites. Amphipod removal at the protected site resulted in 70% greater epiphyte loads on shoalgrass leaves and a 36% reduction in leaf biomass after 10 wk. At the wave-exposed site, where amphipod abundance was consistently low, we found no significant effects of grazer presence or nutrients on epiphyte load or leaf biomass. Average leaf length, however, was significantly reduced in nitrogen-enriched plots. Our results indicate that natural densities of amphipods can reduce seagrass loss by control- ling epiphytes.

Growth Patterns of Shoal Grass Halodule wrightii and Manatee Grass Syringodium filiforme in the Western Gulf of Mexico

IntroductIon Seagrass beds are a valuable resource because of the numerous roles they play in coastal systems. Seagrass blades provide habitat for abundant micro– and macro–algal communities, which in conjunction with the seagrass, contribute substantially to primary productivity in estuarine systems (Heck and Valentine 2006). The blades and roots stabilize sediments, which improves water clarity and increases light penetration, further increasing primary production (Zieman 1982, Gacia and Duarte 2001). Seagrass beds may also provide increased growth rates, critical habitat, and predation refuges for a variety of organisms (e.g., Irlandi and Peterson 1991, Hemminga and Duarte 2000) and have an increased abundance and diversity of juvenile and adult fish and other epibenthic organisms (Summerson and Peterson 1984, Heck et al. 1995). In Texas, like most of the western Gulf of Mexico (GOM), shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme), and turtle grass (Thalassia testudinum) are the most common seagrasses (Zieman 1982, Quammen and Onuf 1993, Withers 2002). Eighty percent of seagrass beds in Texas currently occur in the Laguna Madre (hereafter LM), a hypersaline lagoon that separates a coastal barrier island (Padre Island) from the Texas mainland (Pulich 1998, Tunnell and Judd 2002). Seagrasses were once common in many Texas bays, but have disappeared or declined in coverage in many areas due to anthropogenic causes (Pulich and Onuf 2007). Fortunately, the loss of seagrasses in Texas bays was offset by the increase in seagrass abundance in the LM due to the moderation of salinity after dredging of the Gulf Intracoastal Waterway (Quammen and Onuf 1993). Shoal grass can survive and grow in salinities from 5–80 (McMillan and Moseley 1967) and this seagrass dominated the LM for decades because of its ability to withstand these extreme salinities (Withers 2002). Shoal grass harbors a diverse resident fauna (Tolan et al. 1997) and the migratory redhead duck (Aythya americana) population depends on shoal grass in the LM for food during the winter season (Cornelius 1977). Shoal and manatee grass are currently present in the upper LM, with shoal grass historically being dominant (Quammen and Onuf 1993) and extensively studied (e.g., Dunton 1994, 1996). However, manatee grass is increasing in coverage in much of the LM and is steadily replacing shoal grass in this system (Quammen and Onuf 1993, Pulich and Onuf 2007). In other areas such as Corpus Christi Bay, shoal, manatee, and turtle grass have coexisted for the past 20 yrs (Czerny and Dunton 1995, K. Dunton pers. comm.). Although manatee grass is becoming increasingly abundant in Texas bays, its growth characteristics have not been measured in the western GOM. Changes in seagrass species composition can have significant community effects (Micheli et al. 2008), but the effects of a transition from shoal to manatee grass in the LM have not been extensively studied (but see Tolan et al. 1997). The goals of our study were to measure growth patterns of these two seagrass species in two locations in the western GOM that vary in salinity, epiphyte loads, and nutrient inputs.

Effects of the American oyster Crassostrea virginica on growth rates of the seagrass Halodule wrightii

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 389:117-126 (2009) - DOI: https://doi.org/10.3354/meps08163 Effects of the American oyster Crassostrea virginica on growth rates of the seagrass Halodule wrightii Dale M. Booth1,2,3,*, Kenneth L. Heck Jr.1,2 1Dauphin Island Sea Lab, 101 Bienville Blvd., Dauphin Island, Alabama 36528, USA 2Department of Marine Science, University of South Alabama, Mobile, Alabama 36688, USA 3Present address: Center for Environmental Studies, Horn Point Laboratory, 2020 Horns Point Road, University of Maryland, Cambridge, Maryland 21613, USA *Email: dbooth@hpl.umces.edu ABSTRACT: American oysters Crassostrea virginica have significant effects on ecosystem structure and function and may potentially determine whether coastal systems are dominated by either benthic or pelagic primary production. Field experiments were used to determine the effect of oyster density on the abundance and growth rate of shoalgrass Halodule wrightii in Mississippi Sound, Alabama, USA. Oysters were deployed at densities of 0, 15, 75, and 150 ind. m–2 in 1 m2 plots of H. wrightii in September 2004. Changes in shoalgrass shoot density, growth rate, total suspended solids (TSS), porewater nutrients, light transmittance, and epiphyte loads were all measured from October–November 2004, and June–July 2005. Results showed no significant relationship between oyster density, light transmittance, TSS, or epiphyte loading. However, there was a significant positive relationship between oyster density and porewater NH4 and PO4. Seagrass growth rates were highest at intermediate oyster densities, while high oyster densities negatively affected growth rates, shoot density, and plant biomass. KEY WORDS: Halodule wrightii · Submerged aquatic vegetation · SAV–oyster interactions · Growth rate · Habitat restoration Full text in pdf format PreviousNextCite this article as: Booth DM, Heck KL Jr (2009) Effects of the American oyster Crassostrea virginica on growth rates of the seagrass Halodule wrightii. Mar Ecol Prog Ser 389:117-126. https://doi.org/10.3354/meps08163 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 389. Online publication date: September 04, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

EFFECTS OF NUTRIENT ENRICHMENT ON PRIMARY PRODUCTION AND BIOMASS OF SEDIMENT MICROALGAE IN A SUBTROPICAL SEAGRASS BED(1).

Eutrophication of coastal waters often leads to excessive growth of microalgal epiphytes attached to seagrass leaves; however, the effect of increased nutrient levels on sediment microalgae has not been studied within seagrass communities. A slow-release NPK Osmocote fertilizer was added to sediments within and outside beds of the shoal grass Halodule wrightii, in Big Lagoon, Perdido Key, Florida. Gross primary production (GPP) and biomass (HPLC photopigments) of sediment microalgae within and adjacent to fertilized and control H.wrightii beds were measured following two 4-week enrichment periods during June and July 2004. There was no effect of position on sediment microalgal GPP or biomass in control and enriched plots. However, nutrient enrichment significantly increased GPP in both June and July. These results suggest that sediment microalgae could fill some of the void in primary production where seagrass beds disappear due to excessive nutrient enrichment. Sedimentary chl a (proxy of total microalgal biomass) significantly increased only during the June enrichment period, whereas fucoxanthin (proxy of total diatom biomass) was not increased by nutrient enrichment even though its concentration doubled in the enriched plots in June.

Benthic metabolism across a gradient of anthropogenic impact in three shallow coastal lagoons in NW Florida

Seagrasses are being lost at alarming rates worldwide, most often due to anthropogenic effects, but few reports have examined how seagrass loss affects the metabolism of coastal ecosys- tems. Here, we address this question by comparing both areal and system-integrated daytime ben- thic metabolic rates across 3 lagoons in the North Central Gulf of Mexico that display varying levels of abundance of the shoalgrass Halodule wrightii (i.e. from 64% of the bottom covered with shoal- grass down to between 4 and 0%), partially due to contrasting anthropogenic pressures. When com- paring the 2 shoalgrass-containing lagoons, shoalgrass patches featured higher areal rates of gross primary production (GPP) and respiration (R), and, to a lesser extent, higher rates of net production (NP), than did bare sediment. These results were robust despite across-lagoon differences in percent- age cover and areal biomass of shoalgrass and benthic microalgae (i.e. the lagoon with less cover also had lower areal shoalgrass biomass in the shoalgrass patches and higher areal microalgal chlorophyll a concentrations in the sediment). We did not find any consistent differences in the metabolic rates of bare sediment across the 3 lagoons, despite the fact that areal microalgal chlorophyll a concentra- tions in bare sediment increased as shoalgrass abundance decreased across the 3 lagoons. System- integrated rates of benthic GPP and R were higher in the lagoon with the highest shoalgrass cover when compared with the lagoons with little or no shoalgrass; but, surprisingly, system-integrated rates of benthic NP did not differ significantly across lagoons. This result suggests that the large decrease in shoalgrass abundance across the lagoons examined does not greatly affect the lagoon's potential capacity for accumulation and/or export of organic carbon. It also underlines the importance of deriving system-integrated estimates to properly understand how decreasing seagrass abundance can alter the daytime metabolism of coastal systems.

Effects of nutrient enrichment and grazing on shoalgrass Halodule wrightii and its epiphytes: results of a field experiment

We assessed the individual and combined effects of removing large predators and enriching water column nutrients on shoalgrass Halodule wrightii meadows in Big Lagoon, Florida, USA. To simulate the first-order effects of large predator reductions, we stocked 2.0 m 2 enclosures with elevated (~3 to 4× ambient) densities of the omnivorous pinfish Lagodon rhomboides, the domi- nant fish in local seagrass habitats, and we supplemented N and P in the water column to nearly 3× ambient levels. Monthly determinations of water column nutrients and chlorophyll a (chl a), coupled with bimonthly measurements of leaf epiphyte biomass, seagrass growth and biomass, and begin- ning and ending comparisons of mesograzer abundance, were used to evaluate the effects of increas- ing nutrient supply and changing food web structure. Results showed significant predator and nutri- ent effects, although there were fewer consumer effects and more negative nutrient effects on seagrasses than in our previous experiments, which had shown that mesograzers ameliorated the harmful effects of elevated nutrients on seagrasses. Epiphyte proliferation in enrichment treatments did not occur; thus, algal overgrowth could not explain the negative effects of nutrient loading on sea- grass biomass. Instead, nutrient loading resulted in nitrogen-rich shoalgrass, and it appears that this high-quality food stimulated pinfish herbivory. Elevated pinfish consumption of the enriched shoal- grass then resulted in the decline of seagrass biomass in enrichment enclosures. These results add additional complexity to understanding and predicting the effects of eutrophication in coastal waters.

First Evidence of a Nursery Habitat for Juvenile Gag in the Southern Gulf of Mexico

AbstractInformation on a nursery area for juvenile gag Mycteroperca microlepis in the southern Gulf of Mexico is offered for the first time in this study. A total of 817 gags that ranged in size from 1.5 to 34.5 cm standard length (SL) were captured with a small trawl net from January to December 2001 in a shallow sea grass bed (mainly turtle grass Thalassia testudinum, manatee grass Syringodium filiforme, and shoal grass Halodule wrightii). The gag biomass in the swept area was 217 ± 74 kg/km2 (mean ± SD) with a density of 48,393 ± 125 fish/km2. Peak ingress of gag was observed from March to May, when newly settled organisms (range, 1.5–2.8 cm SL; n = 28) and the smallest young of the year (age 0; 3.2–4.9 cm SL; n = 72) were caught. After settlement, juveniles reached maximum abundance by July (n = 186) with a size range of 8.0–18.2 cm SL and then began to leave the nursery area in September. The monthly occurrence of newly settled gag, length‐frequency analysis, and otolith aging indicate that age‐0 gag (range, 1.5–19.5 cm SL; n = 756) were present in this nursery area from March to December 2001 as a single discrete cohort. These results suggest that settlement is discontinuous for gag in this region. Back‐calculated hatching dates, estimated from otoliths of early juveniles captured in April (3.5–5.2 cm SL; n = 13), ranged between January 28 and February 10, 2001.

Nekton of new seagrass habitats colonizing a subsided salt marsh in Galveston Bay, Texas

Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core−1 thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change.

Relative importance of habitat selection and predation for the distribution of blue crab megalopae and young juveniles

We assessed the relative role of 3 potentially important processes affecting the distribu- tion of young juvenile blue crabs Callinectes sapidus Rathbun: (1) habitat selection at settlement, (2) selection of habitats by dispersing juveniles, and (3) habitat-specific predation rates, using cage experiments. The results suggest that active habitat selection by postlarvae and young juvenile crabs determines the habitat-specific distribution of juvenile blue crabs. Densities of blue crab settlers (megalopae and first instar crabs) in caged habitat patches were high and similar in artificial sea- grass, live shoal grass Halodule wrightii, and live oyster habitats (on average 69 to 82 settlers m -2 ), but significantly lower in mud (26 settlers m -2 ), indicating active selection for any structurally com- plex habitat at settlement. Second and third instar juvenile blue crabs (J2 and J3 crabs) also colonized the structurally complex habitats in higher numbers (34 to 115 crabs m -2 ), compared to mud (14 crabs m -2 ), demonstrating that young juvenile blue crabs are highly mobile and redistribute soon after metamorphosis. Densities of J2 and J3 blue crabs in uncaged treatments were significantly lower in all habitats than in caged treatments (on average 44% loss), suggesting high predation mortality. However, the loss of settlers and juvenile crabs was similar in all habitats, and had no significant effect on the juvenile crab distribution. Densities of potential predators were on average 5 times higher in the structurally complex habitats than in mud. An aggregation of predators in the refuge habitats, coupled with a refuge at low prey densities in unstructured habitats, appears to decrease the proximate effect of predation on the distribution of juvenile crabs. Concurrent high settlement of a small xanthid crab (cf. Eurypanopeus sp.) occurred almost exclusively in the oyster habitat, sug- gesting that species with a limited ability to redistribute after metamorphosis may display a more specific selection of the adult habitat at settlement.

Genetic structure of natural and restored shoalgrass Halodule wrightii populations in the NW Gulf of Mexico

The decline of seagrass communities worldwide has sparked an urgent need for effec- tive restoration strategies, which require a working knowledge of population genetic structure. Halodule wrighti is a common seagrass of the Caribbean region that is being restored to areas of the Gulf of Mexico, yet little is known of its population genetics. This study provides an assessment of individual, clonal and population effects on the genetic structure of 4 natural H. wrightii populations occupying 170 km of coastline in and around Galveston Bay, Texas, for comparison with 7 restored populations ranging in age from 2 to 7 yr. By using molecular markers, in the form of amplified frag- ment length polymorphisms (AFLPs), we found considerable variation in clonal richness at the popu- lation scale (from 0.54 to 0.82), with the restored populations occupying an intermediate to high posi- tion within this range. Replicate sampling within individual seagrass beds of 3 to 5 m diameter generally revealed higher levels of clonal richness, elevated by 4 to 22% over that at the population scale, suggesting that seed recruitment is more important at the local scale than at distances of >10 m. Genetic diversity was 2 to 3 times less than that expected for a widespread, outcrossing species like H. wrightii, although a 170% increase in the frequency of variable markers relative to the mean for all other populations was noted for a volunteer population that had recruited from a mixture of donor materials planted at a nearby restoration site. Within the spatial extent of this study, natural popula- tions adhered to a model of isolation-by-distance, whereas donor materials from these same natural populations were undergoing a rapid genetic convergence within a restored site where they had been planted together.

Vulnerability of newly settled red drum ( Sciaenops ocellatus ) to predatory fish: is early-life survival enhanced by seagrass meadows?

We conducted predation experiments to evaluate the vulnerability of red drum Sciaenops ocellatus (Linnaeus) larvae and early juveniles to pinfish Lagodon rhomboides (Linnaeus) predators. Experiments were designed to analyze the effects of habitat complexity, prey size, and rearing condition on prey vulnerability. Three structurally different habitats [unvegetated (substrate only), shoal grass Halodule wrightii (Aschers) and turtle grass Thalassia testudinum (Bank ex Konig)] were simulated in experimental mesocosms. Instantaneous hourly mortality rates (Z h−1 predator−1) for hatchery-reared red drum were significantly higher in the unvegetated habitat (0.189) than in either shoal grass (0.069) or turtle grass (0.046). A similar trend in predation mortality was observed for wild-caught red drum; instantaneous hourly mortality rates were 0.166, 0.047, and 0.021 in unvegetated, shoal grass, and turtle grass habitats, respectively. Mortality rates (adjusted means) for hatchery red drum were higher than for wild individuals in all three habitats; however, the differences were not significant. Predation mortality decreased with increasing prey size (3 to 9% decrease in Z per mm increase in length), suggesting that small red drum (i.e. new settlers) were most vulnerable to predators. We conclude that habitat complexity is critical to the survival of newly settled red drum, and changes in the complexity or areal coverage of natural seagrass meadows may affect early-life survival and possibly recruitment levels.

Condition of larval and juvenile red drum ( Sciaenops ocellatus ) from estuarine nursery habitats

RNA:DNA ratios of larval and juvenile red drum (Sciaenops ocellatus) collected from nursery habitats in the Aransas Estuary, Texas, in 1994 were quantified using a highly sensitive ethidium-bromide fluorometric technique. RNA:DNA ratios of wild red drum were evaluated by comparing individual values to a linear regression model derived for starved laboratory-reared red drum. Wild red drum were in relatively good condition with <5% of the RNA:DNA ratios within or below the 95% prediction interval of 4 to 5 d starved red drum. A multiple-regression model explained 54% of the variability in the RNA:DNA ratio of wild red drum, and identified length and water temperature (midday) as significant factors. RNA:DNA ratios increased with fish length [≃1.1 mm−1, over the size range investigated (5␣to 20 mm)]. The effect of temperature on the RNA: DNA ratio was assessed on different sampling trips, and ratios increased with increasing temperature. Abundance of larval and juvenile red drum in the Aransas Estuary varied as a function of both habitat (shoal grass Halodule wrightii, turtle grass Thalassia testudinum) and site (Aransas Bay, Redfish Bay); however, no differences in RNA:DNA ratios were detected between habitats or between sites. It is postulated that the nutritional condition of newly settled red drum from the Aransas Estuary in 1994 was relatively high, and that starvation was of minor importance.

Photosynthetic Production and Biomass of the Subtropical Seagrass Halodule wrightii along an Estuarine Gradient

Seasonal patterns of aboveground and belowground biomass, leaf chlorophyll (chl) content, and in situ differences in photosynthetic parameters were examined in the shoal grass Halodule wrightii along an estuarine gradient in the western Gulf of Mexico. Continuous measurements of biomass were collected over a 5-yr period (1989–1994) with respect to several abiotic factors in three estuarine systems that were characterized by significant differences in salinity and ambient dissolved inorganic nitrogen (DIN; NO2−+NO3−) regimes that ranged from 5–25‰ (0–80 μM DIN) in the Guadalupe estuary to 35–55‰ (0–9 μM DIN) in the upper Laguna Madre, Photosynthesis versus irradiance (P vs. I) parameters, measured from December 1989 to April 1991, showed no significant differences among the three sites, and there were no significant differences in leaf chlorophyll content and chl a:b ratios among sites over the entire 5-yr period. Saturation irradiance in Halodule wrightii is estimated at 319 μmoles photons m−2 s−1 based on measurements collected at the three sites over a 2-yr period. No strong seasonal variations were observed in total plant biomass, but root:shoot ratios (RSR) showed a clear pattern of maximum RSR values in winter and minimum values in summer. There were no significant differences in RSR among sites, and no consistent correlations could be established between plant parameters and sediment porewater NH4+, salinity, or temperature. Sediment porewater NH4+ values generally ranged from 50 μM to 400 μM (average 130–150 μM) but could not be correlated with significant differences in sediment composition between the sites. The high productivity of Halodule wrightii under a variety of light, nutrient, and salinity conditions explains its ubiquitous distribution and opportunistic strategy as a colonizing species. However, the persistence of a dense algal bloom in Laguna Madre coincident with low DIN levels (<5 μM) contradicts previously accepted relationships on nutrient stimulation of algal growth, and provides strong evidence that water quality parameters for estuarine seagrasses are decidedly estuarine-specific. Consequently, a knowledge of the long-term history of estuarine systems is critical to habitat managers, who are required to establish minimum water quality criteria for the protection of submerged aquatic vegetation in estuarine systems. *** DIRECT SUPPORT *** A01BY074 00028

Sources of predatory mortality in the bay scallop Argopecten irradians (Lamarck): interactions with seagrass and epibiotic coverage

Field observations and experiments were conducted to identify the sources of predatory mortality in adult bay scallops Argopecten irradians concentricus Say and to determine the physical factors that affected the rate of predation. Ring-billed gulls Larus delawarensis Ord, herring gulls L. argentatus Coues, and knobbed whelks Busycon carica Gmelin were responsible for the mortality of scallops placed in the field. Predation intensity by these predators varied significantly with depth of water coverage and habitat type. Predation by gulls was more intense on intertidal scallops than on scallops that were barely subtidal (covered by <5 cm of water), and less intense on scallops within shoalgrass Halodule wrightii Acherson beds than on scallops on bare sand bottom. Predation by whelks was less intense on scallops within subtidal eelgrass Zostera marina L. beds than on unvegetated subtidal sand in April, but a similar pattern in August was not significant. Epibiotic coverage by macroalgae and invertebrates did not alter the rate of predation in any of the experiments. The results from these experiments suggest that seagrass acts as a partial refuge from predation for bay scallops, and in part explains the marked habitat preference of this species. While other costs and benefits may be associated with the symbiosis of scallops and their epibionts, epibiotic coverage does not confer any protection from predation, and so in the absence of further information the relationship should be classified as commensalistic.

Seasonal abundance, distribution, and habitat selection of juvenile Callinectes sapidus (Rathbun) in the northern Gulf of Mexico

Estimates of juvenile blue crab Callinectes sapidus (Rathbun) abundance within high and low density Cuban shoal grass Halodule wrightii (Ascherson) grassbed microhabitats and adjacent unvegetated substratum were carried out by suction sampling at Ono Island, Alabama, during 1987–1988. Juvenile blue crabs were significantly more abundant in vegetated than in unvegetated substratum for all samples except July and August 1987 and May and July 1988. Newly recruited crabs (⩽5mm CW) exhibited some association with high density grass. Medium-sized individuals (5–10 mm CW) showed a tendency toward association with low density grass. Larger juveniles (>10 mm CW) did not exhibit an association with any particular substratum based on field sampling. Complementary laboratory studies of habitat selection by juvenile blue crabs were carried out at the Dauphin Island Sea Laboratory during July and August 1988. In these laboratory experiments, a preference for high density grass over unvegetated substratum and low density grass was observed for small crabs (11–37 mm CW). In the absence of potential predators (>45 mm CW), female crabs selected low density vegetation more often than other substrata, while male crabs more often selected high density vegetation. In the presence of these potential predators, females always chose vegetated substrata over unvegetated substratum while males increased their selection of high density grass. The larger potential predators selected low density vegetation more often than any other substratum. Field and laboratory studies indicated that habitat partitioning through active selection of substratum by size class of juvenile blue crabs occurred. Laboratory studies indicated habitat partitioning by sex. Substratum selection by crabs was influenced by the presence of larger blue crabs which are potential predators. Although predation may act as the ultimate selection agent for such partitioning, it was not the proximate cause in laboratory studies.