Seagrass Blades Research Articles

The effects of mucous tubes on the distribution, behavior and recruitment of seagrass meiofauna

An investigation of the effects of mucous tubes produced by amphipods, polychaetes and harpacticoid copepods was conducted on meiofaunal organisms inhabiting Thalassia testudinum Banks ex König blades in Tampa Bay, Florida. Samples of seagrass blades were collected from the field in October 1991 and the numbers of meiofauna, percent cover of mucous tubes and distance from the base of the seagrass blade were determined. Results indicated that meiofauna densities were greater on blade areas with greater percent cover of mucous tubes and that meiofauna distribution was independent of physical position (height) on the blade. Second, observations of behavior of blade-dwelling meiofauna were conducted simultaneously in areas with and without mucous tubes. Total meiofauna, harpacticoids and turbellarians occurred in significantly greater abundances in mucous tube areas and along with ostracods spent significantly more time in mucous tube areas than in non-tube areas. In a field experiment using conditioned glass substrata significantly greater numbers of meiofauna immigrated onto substrata with mucous tubes, constructed by the harpacticoid, Dactylopodia tisboides (Claus), than substrata without tubes. Evidence supports the conclusion that relatively imperceptible, ephemeral structures such as mucous tubes can affect the distribution and behavior of epibenthic meiofauna on seagrass blades.

Hydrodynamically induced synchronous waving of seagrasses: ‘monami’ and its possible effects on larval mussel settlement

Underwater observations of eelgrass ( Zostera marina L.) beds at the mouth of the Jordan River, Maine, USA, indicated that the eelgrass blades gently undulated with low-amplitude movements under low current speeds. When the above-canopy speeds exceeded 10 cm s −1, dramatic large-amplitude waving of many blades in synchrony occurred. The eelgrass waving caused wide variability in horizontal water current speeds measured above the canopy. During three summers (1986, 1989, 1990), the blade tips (distal 30 cm) had an average of about 3 × more recently settled (plantigrade) blue mussels ( Mytilus edulis) compared to regions lower (30–90 cm) on the blade. Because waving of seagrass blades results in the blade tips moving through much more of the water column than lower regions of the blade and in enhanced turbulent mixing above the plant canopy, we hypothesize that such movements increase the likelihood of blade encounter with mussel larvae, and explain enhanced mussel abundances on blade tips. We further hypothesize that the enhanced mixing may direct larvae into seagrass beds generally. Large-amplitude, synchronous waving of terrestrial grasses has been termed ‘honami,’ (Japanese: ho = cereal; nami = wave) and has been shown to dramatically alter aerodynamical conditions within and above the grass canopy. We suggest that ‘monami’ (mo = aquatic plant) is important in coastal hydrodynamics and has major implications for larval settlement and recruitment.

Influences of Larval Settlement Location and Rate on Later Growth and Mortality in a Sessile Marine Invertebrate Population (Spirorbis spirillum)

Planktonic larvae of the tube-buildlng polycheate Spirorbis spirillum (Linnaeus) settle abundantly along seagrass blades of Thalassia testudinum Banks ex Konig in bays of the northern Gulf of Mexico. Detailed demographic parameters of settled Individuals were measured in situ by using biological stains that mark the leading margin of tubes containing living S. spirillum and this method yielded similar results when compared with independent estimates of settlement rate, growth, and mortality. Larvae tended to settle near the base of growing seagrass blades and subsequent survivorship and tube growth were highest for Individuals nearest the base. Survivorship that is positively affected by settlement location and not negatively affected by the density of conspecifics suggests that temporal changes In settlement can have a significant Influence on benthic population size over time. This was supported by a positive correlation between population size and settlement and suggests that local extinction is potentially less likely to occur because planktonic larvae may be transported distances greater than the scale of ecological processes acting on the benthic stage.

Trophic cues induce metamorphosis of queen conch larvae ( Strombus gigas Linnaeus)

The planktotrophic larvae of the gastropod Strombus gigas Linnaeus are metamorphically competent for 6 days, at which time they must make contact with a benthic cue or lose the ability to metamorphose. In this laboratory study, larvae were exposed to potential metamorphic inducers from juvenile conch habitats. Conspecifics and dilute solutions of macroalgae did not trigger metamorphosis. However, Batophora oerstedi J. Agardh, a food source of juvenile conch induced the highest percentage of metamorphosis in veligers (mean = 62%, sd = 16). Other cues which induced metamorphosis were sediment (mean = 51%, sd = 11), Thalassia testudinum Köenig seagrass blades (mean = 36%, sd = 13), and Thalassia testudinum detritus mean = 29%, sd = 16). Further analyses of epiphytic, organic, and inorganic components of macrophytes and sediment indicated that the associated epiphytes were the primary components for inducing metamorphosis. When veligers were exposed to Thalassia testudinum detritus collected from 6 seagrass habitats encompassing a conch nursery ground near Lee Stocking Island, Bahamas, metamorphosis was statistically higher for detritus from four sites within the nursery than two sites outside the nursery periphery. This indicated that Thalassia testudinum detritus varied in composition and quality from one habitat to another and that conch larvae probably metamor phose in response to trophic cues found in their juvenile habitats. This adaptive behavior may be a mechanism to ensure that larvae are recruited into conch nurseries which offer good growth and survival to juvenile and adult stages.

Hydrodynamics, diffusion-boundary layers and photosynthesis of the seagrasses Thalassia testudinum and Cymodocea nodosa

Photosynthetic rates of aquatic plants frequently increase with increasing current velocities. This is presumably due to a reduction in the thickness of the diffusion boundary-layer which allows for a higher carbon availability on the plant surface. Blades of the seagrasses Thalassia testudinum and Cymodocea nodosa exposed to different current velocities under controlled laboratory conditions, showed increased photosynthetic rates with increasing flow only at low current velocities (expressed as blade friction velocities, u*). Carbon saturation of photosynthetic processes occurred at a relatively low u* level (0.25 cm s-1) for T. testudinum collected from a calm environment compared to C. nodosa (0.64 cm s-1) collected from a surf zone. No further enhancement of photosynthetic rates was observed at higher u* levels, suggesting limitations in carbon diffusion through the boundary layer below critical u* levels and possible limitations in carbon fixation by the enzymatic system at higher u* levels. These results, as well as those of previous theoretical studies, assumed the flow on the immediate seagrass-blade surface to be hydrodynamically smooth. The presence of epiphytes and attached debris causes the surface of in situ seagrass blades to be exposed to flows ranging from smooth to rough-turbulent. As a consequence, the boundary-layer thickness on moderately epiphytized blades under medium to high flow-conditions is not continuous, but fluctuates in time and space, enhancing carbon transport. In situ u* levels measured directly on blades of seagrasses indicate that T. testudinum and C. nodosa can be exposed to conditions under which the boundary layer limits photosynthesis during short periods of time (milliseconds) during low-energy events. As waves cause the thickness of the diffusion boundary-layer to fluctuate constantly, carbon-limiting conditions do not persist for prolonged periods.

Significance of copepod emergence to benthic, pelagic, and phytal linkages in a subtidal seagrass bed

The role of meiobenthic copepod emergence in linkages among benthic, pelagic, and phytal habitats was examined in a subtidal seagrass (Tl~alassia testudjnurn) bed at the mouth of Tampa Bay, Florida. USA. On 4 dates the ability of emerging copepods to affect the density and composition of benthic and phytal assemblages was tested during 2 h periods of minimum and maximum emergence. Copepod exchanges between benthic and phytal habitats and the duration of pelagic excursions also were examined throughout a night by measuring settlement into 2 types of sediment traps and onto natural and defaunated seagrass blades. The ability of emerging fauna to disperse through the water column was determined from horizontal collectors. Changes In copepod densities and composition during periods of active emergence did not indicate an unequivocal exchange between sediment and seagrass blade assemblages. Copepod resettlement onto the sediments was greater during periods of increased emergence and suggested that emerging copepods typically did not remain in the water column for a prolonged (> 2 h) time. The dispersal of copepods through the water column was haphazard and unaffected by prevailing currents. Species and habitat affiliation influenced both the dispersal and settlement behaviors of emerging copepods. Although appreciable numbers of sediment-associated copepods can enter and disperse through the water column, our results suggest that the effects of emergence on linkages between benthic, pelagic, and phytal habitats are minimal or limited in duration.

POPULATION ECOLOGY OF PAGURUS MACLAUGHLINAE GARCIA-GOMEZ (DECAPODA: ANOMURA: PAGURIDAE) IN THE INDIAN RIVER LAGOON, FLORIDA

ABSTRACT Pagurus maclaughlinae is an abundant component of sea-grass macrobenthos in Florida waters. As a grazer on sea-grass blades, it may play an important role in sea-grass ecology by reducing epiphyte cover on the blades. Population dynamics, habitat selection and utilization patterns, and interactions with a decapod predator were examined in field collections and laboratory experiments. At a site north of Link Port in the Indian River Lagoon, Florida, P. maclaughlinae was more abundant in beds of Thalassia as compared to those of Syringodium or Halodule. In the laboratory, Syringodium was selected in preference to Thalassia and Halodule, and choice was not influenced by presence of a predator. Sea grass with epiphytes was significantly selected over unfouled substrates. Recruitment occurred in October-February and in May-July, and the overall male/female ratio was close to 1, although month to month variation occurred. High percentages of ovigerous females were found in the fall and in the spring, and female maturation occurred at a CL of approximately 1.25-1.50 mm. Cerithium muscarum, Modulus modulus, and Nassarius vibex constituted -94% of the 13 species of gastropod shells used. Shell utilization differed between sexes, ovigerous and nonovigerous females, and among size classes. Predation experiments with the blue crab Callinectes sapidus suggested that predation rate was independent of shell species, that the presence of Thalassia offered significant protection against predation compared with bare sand, and that predation rates were not different between sea grasses.

Perception and selection of macrophyte detrital falls by the bathyal echinoid Stylocidaris lineata

Many deep-sea animals are known to exploit patchy food resources such as animal carcasses and sunken plant remains, but the mechanisms by which such foods are located remain generally unknown. The bathyal echinoid Stylocidaris lineata is an omnivorous deposit feeder that ingests sediment, dead animal remains, seagrass blades, and macroalgae such as Sargassum spp. Using a submersible, we investigated the ability of urchins to locate and exploit large falls of detritus. Individuals quickly arrived at packets of Thalassia testudinum and Sargassum spp. placed on the bottom, and they preferred these food items significantly over inert controls. However, the echinoids demonstrated no significant tendency to move toward the scent of upstream T. testudinum, either in situ or in laboratory flume experiments. Individuals moved at net speeds up to 30 m day −1. The existing evidence suggests that S. lineata locate food by chance encounter, not distant chemoreception.

Meiofauna on the seagrass Thalassia testudinum: population characteristics of harpacticoid copepods and associations with algal epiphytes

The composition and abundance of bladedwelling meiofauna was determined over a 15 mo period (1983–1984) from a Thalassia testudinum Banks ex Konig meadow near Egmont Key, Florida, USA. Harpacticoid copepods, copepod nauplii, and nematodes were the most abundant meiofaunal taxa on T. testudinum blades. Temporal patterns in species composition and population life-history stages were determined for harpacticoid copepods, the numerically predominant taxon. Sixteen species or species complexes of harpacticoid copepods were identified. Harpacticus sp., the most abundant harpacticoid, comprised 47.8% of the total copepods collected, and was present throughout the study. Copepodites dominated the population structures of the blade-dwelling harpacticoid species on most collection dates. Ovigerous females and/or copepodites were always present, indicating continuous reproductive activity. Results suggest that epiphytic algae influence meiofaunal abundance on seagrass blades, as densities of most meiofaunal taxa at Egmont Key were positively associated with percent cover of epiphytic algae throughout the study. The majority of significant correlations between meiofaunal density and cover of epiphytic algae involved filamentous algae, although encrusting algae dominated the epiphytic community. It appears that resources provided by epiphytic algae to seagrass meiofauna (additional food, habitat, and/or shelter from predation) may be associated with algal morphology.

Emergence and dispersal of phytal-dwelling meiobenthic copepods

The relationship between the emergence (= active upward swimming) of phytal-dwelling meiobenthic copepods from blades within subtropical seagrass beds and dispersal of copepods was examined. Samples of copepods emerging from blades were collected using emergence traps. The distances to which copepods dispersed were measured using horizontal traps positioned at 0, 5, 10, 15, and 20 m outside the source of colonists. On each of two dates, experimental (defaunated) blades were sampled from patches at various distances from the source bed to assess copepod colonization; additionally, patches were arranged to determine if they served as “trapping stones” to accelerate copepod invasion of nearby patches. Twelve species of copepods vertically migrated from seagrass blades; these same species moved horizontally through the water column at all distances tested (up to 20 m). Species richness, abundances of total adults and copepodites, and total nauplii, did not decrease as patch distance from the source bed increased. Diosaccid sp. A and Scottolana canadensis ([Wiley] Coull) were the only copepod species displaying differences in abundance with distance (patch location). Colonization was extremely rapid, with ambient densities being attained within 2 days. The frequency of adults and copepodites was dependent upon patch location for 9 and 12 species on sampling dates 2 and 4, respectively. On day 2 copepodites dominated patches for most species, while adults dominated on day 4, although no consistent trend in location of patch containing the highest frequencies of copepodites or adults was detected. The frequency of males and females was dependent upon patch location for only 1 species on each date, with numbers of females exceeding males. Although no “stepping stone” effect was detected, active emergence of phytal-dwelling meiobenthic copepods from blade surfaces contributed to their dispersal to newly created patches of vegetation.

Primary production dynamics in seagrass beds of Mississippi Sound: the contributions of seagrass epiphytic algae, sand microflora, and phytoplankton

Primary production rates of 4 autotrophic components in Halodule wrightii Aschers. beds off Horn Island in Mississippi Sound (USA) were measured over an annual cycle. Hourly production rates varied from as little as 0.9 mg C m-' for H, wrightii leaves in winter to as high as 1143 mg C m-' for eplphytic algae during summer. Stepwise multiple regression showed that only 15 % of the variation in hourly epiphytic algal production could be related to a single environmental variable (i.e. light energy). Variations in hourly production rates for the other productivity components were best explained by light energy, water temperature, tidal range and blade density; R2 for these regressions was 0.66 and 0.80 for phytoplankton and seagrass blades respectively. The epiphytic algal assemblage on H. wrightii leaves was dominated by the red alga Acrochaetium flexuosum Vickers and 12 taxa of araphid, monoraphid and biraphid diatoms. Phytoplankton over the beds were virtually all centric diatoms, whereas the microflora associated with the sandy sediments in which H. wrightii was rooted was dominated by small pennate diatoms. Annual production rates (g C were estimated as follows: epiphytic algae (905), phytoplankton (468), sand microflora (339) and Halodule wriyhtij (256). System production was dominated by the microalgae, with the contribution of H. rvrightji blades being only 13 % of the total. Epiphytic algae were the single most important productivity con~ponent, being responsible for 46 and 60 % of total system and benthic production respectively. The high standing crops and production rates of the epiphytic and benthic microalgae are of interest in that recent studies have indicated these algae can be the principal source of organic matter in seagrass food webs.

Benthic determinants of settlement for planktonic larvae: availability of settlement sites for the tube-building polychaete Spirorbis spirillum (Linnaeus) settling onto seagrass blades

In northwestern Gulf of Mexico bays planktonic larvae of Spirorbis spirillum (Linnaeus), a sessile tube-building polychaete, settle disproportionately on the bases of growing blades of the seagrass Thalassia testudinum Banks et Konig. Experimental relocation of blades and removal of epiphytic organisms demonstrated that searching larvae avoid epiphytic algae associated with distal regions of blades. Epiphyte-free space is readily accessible to S. spirillum larvae because larvae also tend to settle deeper in the water column where space is produced by plant growth. Observed settlement rates and the distance that S. spirillum settles from conspecifics and algae ( > 0.7 mm) were used to calculate free space “required” by settling larvae. Space required by settling larvae over time very rarely exceeded free space produced by growing seagrass blades (only 2 of 37 dates examined). Increased free space (through additions of blades cleared of epiphytes) within enclosures did not increase numbers of S. spirillum settling within experimental areas indicating high space availability for settlement. Larvae appear to search efficiently for space produced rapidly by blade growth. As a result, the benthic population size is susceptible to changes in the rate at which larvae arrive from the plankton, rather than to local changes in the opening of space for settlement. If, in other systems, larvae are capable of searching significantly large areas, as found in this study, then the contribution of changes in space availability to settlement variability must be carefully considered.

Does epibenthic structure negatively affect meiofauna?

The epibenthic harpacticoid copepod Parastenhelia megarostrum attains very high population densities in troughs of persistent sediment ripples on open intertidal sand banks in Pauatahanui Inlet, New Zealand. In closely adjacent eelgrass Zoslera capricorni Aschers beds, individuals of this species are only infrequently recorded from subcanopy sediments. Epibenthic structure in the form of flexible artificial seagrass plants was emplaced in the ripple field at two different times (June, September). Field experiments were undertaken to test the premise that presence of such structure would, by the creation of turbulent water flows around plants, lead to a progressive alteration in the microtopographic profile of the sediment leading to reductions in abundance of P. megarostrum. Moreover, these experiments were designed to test the effects of imposed overhead structure not only on this surface-dwelling copepod but also other components of the primarily infaunal meiobenthos. The experimental design consisted of two location treatments (inside and outside mimic grass plots) and two density levels of mimic plants within plots (30, 60 plants · 0.5 m −2), sampled at eight intervals representing time in days (D) since implantation of mimic plants (D1,2,4,6,8,12,16,31). Core samples were taken from within experimental plots, with replicates taken outside acting as undisturbed controls. The imposition of artificial epibenthic structure resulted in the rapid breakdown of ripple integrity, marked changes in profile and a net accumulation of sediment over the experimental period in both plant density treatments. In both 30- and 60-plant density plots, there were highly significant interactions between experimental period and location for P. megarostrum, with decreases in abundance inside plots. No equivalent interactions or main effects on abundance of the interstitial harpacticoid Paraleptastacus sp. or nematodes were revealed. Variance analyses indicated that abundance of P. megarostrum is less dependent on gross changes in the sediment surface than to the presence of overhead cover and that high densities in sediment depressions are a function of active aggregation rather than passive accumulation. Modification of sediment surface profile and mechanical disturbance by the sweeping action of seagrass blades clearly has a significant impact on epibenthic meiofauna but not on infaunal species.

Response of harpacticoid copepods to detrital accumulation on seagrass blades: a field experiment with Metis holothuriae (Edwards)

A field experiment was performed in Tampa Bay, Florida, to determine whether varying levels of detritus affects abundance of the harpacticoid copepod Metis holothuriae (Edwards). Three levels of seagrass detritus obtained from Thalassia testudinum Banks ex Koenig were affixed to Petri dishes containing a mucus-like medium. 14 dishes of each treatment were placed at random in a Thalassia bed and recovered after 24 h and 4 days. Numbers of copepods recovered from the experimental treatments varied among detritus levels; lowest numbers were found in dishes without detritus and the greatest numbers in the high-detritus dishes. ANOVA indicated that detritus level had a highly significant effect on the abundance of Metis while day of recovery and the level x day interaction were not significant. Behavioral observations in the laboratory indicated that Metis spent significantly more time feeding while on detritus-laden blades than on detritus-free blades. Conversely, copepods spent more time swimming when presented with detritus-free blades than when detritus was present.

Experimental investigations of benthic reentry by migrating meiobenthic copepods

The resettlement behavior of meiobenthic copepods, which actively migrated from sediments in a seagrass bed, was investigated in a shallow subtidal area in Tampa Bay, Florida, U.S.A. Experimental studies were conducted to determine whether meiobenthic copepods after emerging from sediments at sunset reenter the sedimentary substratum or select other subhabitats, water and seagrass blades. Migrating copepods were collected with emergence traps and transferred to experimental aquaria in the field which contained sediment, seagrass-blade and water treatments. Settlement into each type of treatment was measured in separate 2-h and 9-h experiments. Differences in densities of copepod taxa retrieved from emergence traps and introduced into experimental aquaria were recorded as were differing relative proportions of each copepod species returning to the substratum treatments. Settlement patterns of total copepods and three dominant copepod species, Zausodes arenicolus, Halicyclops sp. and Robertsonia hamata, departed from those expected by chance. The populations of R. hamata and Halicyclops sp. which settled were generally skewed towards males and a close matching of males and copepodites within treatment dishes was evident. Similar to nighttime-emergence patterns, timing and magnitude of postmigration reentry differs among copepod taxa and such reentry may be linked to reproductive events. Complex behavioral processes previously noted for fish and macrofaunal organisms in seagrass beds may also be important in recruitment and reassortment of meiobenthic copepods.

Density-dependent growth and grazing effects of juvenile queen conch Strombus gigas L. in a tropical seagrass meadow

A field experiment with 1-yr-old queen conch Strombus gigas L. in a seagrass meadow of the Exuma Cays, Bahamas, indicated that growth rates and mortality were density-dependent and related to food limitation. Juvenile queen conch were important consumers of macrodetritus including particles identified as dead seagrass blades. Herbivory on living macrophytes was minimal. Enclosure/exclosure treatments showed that the natural field density of juvenile conch (2 · m −2) reduced significantly the standing crop of senescent seagrass blades and macrodetritus but not that of living seagrass. Grain size, organic content and chlorophyll of sediments were not influenced by the presence of juvenile conch. Removal of seagrass detritus by conch, however, may have a major influence on the abundance and species composition of other benthic invertebrates that use detritus as a source of food or shelter.

Diel Activity Patterns and Foraging Dynamics of the Sea Urchin Tripneustes ventricosus in a Tropical Seagrass Community and a Reef Environment (Virgin Islands)

Abstract. Feeding and movement activity patterns and the foraging behavior of the sea urchin Tripneustes ventricosus were investigated in a Thalassia testudinum seagrass bed and on a patch reef at St. Croix, U.S. Virgin Islands.Most of T. ventricosus in the seagrass bed fed during both the day and night. Consumption of seagrass blades amounted to 1.4 g dw · individual‐1· d‐1 (mean sea urchin size 12.1 cm ambitus diameter). The population consumed approximately 3.6% of the daily seagrass production.The movement of T. ventricosus was diel with high nocturnal activity levels and rates of locomotion, and little activity during daylight hours. Foraging activity was reduced under turbulent conditions. Movement rates were not governed by food availability. T. ventricosus travelled 3.7 m · d‐1 on the patch reef. Individuals in the seagrass bed moved 8.8 m · d‐1, most of which was accomplished during the night. The area traversed contained more than one hundred times the amount of food consumed daily. It is assumed that this behavior requires high energy expenditures for locomotion, thus yielding a low net energy profit from feeding.The high rate of movement in seagrass beds may represent an innate behavioral adaptation to predation by night‐active helmet conchs (Cassis spp.). Fitness optimization by T. ventricosus may have been achieved by minimizing the predation risk during night hours. In an evolutionary context, survival by means of an increased energy expenditure for locomotion may compensate for the lower net energy gain from foraging.

Winter mass migration of juvenile queen conch Strombus gigas and their influence on the benthic environment

From February through March 1988, mass migration of juvenile queen conch Strombus gigas was observed in the Exuma Cays. Bahamas. This represents the second documentation of mass migration in the species but unlike the first aggregation of l-yr old conch, this migration was comprised primarily of individuals 2 yr old. L k e the earlier mass migration, advance of the aggregation was in the direction of the ebb tide. Progression of the aggregation started and stopped 3 times during the 1 mo observation period. Non-moving clusters of randomly oriented individuals were formed as a response to increasing wave action associated with the passage of cold fronts. Grazing by conchs in the aggregation had a significant influence on the biomass of seagrass macrodetritus and epiphytes, but not on living seagrass blades. A tagging experiment showed that the aggregation dispersed at 4.1 % d-' Rapid increase in conch density with the passage of the migration provides further support for the hypothesis that such migrations serve as a dispersal mechanism for luvenile conchs from centers of recruitment.

The role of prey detection in the selection of prey by pinfish Lagodon rhomboides (Linnaeus)

Selective feeding by fishes may be due in part to the conspicuousness of particular prey types; certain prey may be disproportionately detected by fish and thus could be subject to a greater risk of predation. In laboratory experiments designed to test the relative importance of chemoreception, vision, and prey activity in the process of prey detection, I allowed pinfish Lagodon rhomboides (Linnaeus) to choose among various seagrass-meadow-associated infaunal and epifaunal prey [ Melita, Cerapus, Caprella (amphipods); Hippolyte (shrimp); Americonuphis, Nereis (polychaetes); and Clytia, Sertularia (hydroids)]. In laboratory trials, pinfish approached individual prey enclosed in clear glass tubes with an open end (covered with 50-μm mesh Nitex screen to allow water exchange) that were provided with sand and seagrass blade substrata. Within each trial, one prey species was offered in an array consisting of an unaltered tube and tubes that had been manipulated to prevent detection of the enclosed prey using vision (opaque sides), olfaction (sealed tops), or both senses (opaque sides and sealed top). Pinfish most frequently approached prey in the clear-sided, screen-topped tubes and least frequently approached tubes with opaque sides and sealed tops; the tubes with sealed tops and clear sides or screened tops and opaque sides were approached with intermediate frequency. Approach frequencies were similar for all prey types tested. It appears that vision and chemoreception were used jointly by pinfish to locate the enclosed prey. In a second series of prey choice experiments, pinfish were offered four prey types (amphipods, shrimp, polychaetes, and hydroids), either live or freshly killed, in clear glass tubes with screen tops to determine if inactive, but otherwise similar prey, were approached as frequently as active prey. All prey types tested were approached with similar frequency; however, live prey were approached more frequently than dead prey (although not significantly more frequently). Small, mobile epifaunal prey (i.e., amphipods and other microcrustaceans) should be disproportionately detected on the seagrass blades by pinfish in the field and thus may be subject to a greater risk of predation than sessile prey.

Response of small motile epifauna to complexity of epiphytic algae on seagrass blades

Field collections and experiments were performed to examine the relationship between the biomass of epiphytic algae (= habitat complexity) on Thalassia testudinum blades and the density of associated motile epifauna. Samples collected at Egmont Key, Florida indicated a significant positive association between the density of harpacticoid copepods, the most numerous taxon on seagrass blades, and biomass of the dominant epiphyte, Giffordia mitchelliae. Similar results were noted for nematodes, amphipods, and crustacean nauplii. Seagrass blades with large amounts of epiphytic algae are often older than blades with lesser amounts of algae, and have had a longer time to be colonized by epifauna. A recolonization experiment was conducted in aT. testudinum bed in the Indian River lagoon, Florida examining associations between the density of epifauna on blades ofthe same age (blades defaunated and replanted in the field) with various amounts of epiphytes. After 2 days a significant positive relationship between epiphyte biomass and density of dominant epifauna (copepods, crustacean nauplii, and nematodes) was recorded, suggesting that results were attributable to differential amounts of epiphytic algae since length of colonization time did not vary among blades. Recolonization experiments in a T. testudinum bed at Ft. Desoto, Florida using artificial blades (polypropylene ribbon) and 3 densities (no, intermediate, and high) of artificial epiphytes (clumps of cotton-polyester thread) produced results comparable to those above within 3 days. Highest densities of dominant epifauna (copepods, nematodes, and polychaetes) were found on blades in the high epiphyte treatment. Mean numbers of copepods and also polychaetes on high and intermediate epiphyte treatments were not significantly different from each other, but were significantly higher than on blades with no epiphytes. Nematode densities declined significantly with decreasing amounts of artificial epiphytes. These results suggest that much of the relationship observed between motile epifauna and epiphytic algae on seagrass blades may be attributed to the physical structure of the algae, given the similar responses of most fauna to both living and artificial plant material. The pattern observed for harpacticoid copepods and epiphytic algae on seagrass blades in field collections and experiments varied among species. Laophontid sp., Harpacticus sp., and Diosaccus sp. were strongly positively associated with the amount of epiphytic algae, however, Amphiascus sp., Ectinosomatid spp., and Dactylopodia tisboides showed little association. The results of this study demonstrate that relationships which have been shown for larger plant structure and fauna also hold for small motile epifauna and fine scale vegetative architecture. I. Department of Biology, University of South Florida, Tampa, Florida, 33620, U.S.A.