Judith P. Grassle

Hydrodynamic enhancement of larval settlement in the bivalve Mulinia lateralis (Say) and the polychaete Capitella sp. I in microdepositional environments

To test whether larval settlement patterns of the opportunistic bivalve Mulinia lateralis (Say) and the opportunistic polychaete Capitella sp. I are influenced by near-bottom flow, laboratory still-water and flume-flow experiments were conducted using a sediment-filled array consisting of depressions and compartments flush with the flume bottom. Compartments were filled with organic-rich mud or a low-organic, glass-bead mixture of a comparable grain size. Previous flume experiments have shown that larvae of both species settle in greater numbers in mud compared with glass beads. Depressions create a hydrodynamic environment that traps passive particles, permitting tests of the relative importance of active selection versus passive deposition of larvae in regions of microtopography. In both flow and still water, Capitella sp. I larvae consistently selected organic-rich mud over glass beads, regardless of whether treatments were flush or depressions. Settlement was higher, however, in depressions (3.8 cm in diameter and 2.8 cm deep) for a given sediment treatment, particularly in glass bead treatments in flow. In flow and still-water experiments, M. lateralis larvae also chose mud over glass beads but, in some instances, higher settlement occurred in glass bead depressions (a “poor” choice) compared to flush mud (a “good” choice). These results suggest that near-bottom flow influences settlement distributions of both species (i.e. settlement enhancement in depressions), but the effect may be greater for M. lateralis larvae. Higher settlement generally observed in mud depressions compared with glass bead depressions suggests that larvae of both species may have been able to “escape” from depressions if the substratum was unsuitable, although M. lateralis larvae were poorer swimmers than Capitella sp. I larvae and were more vulnerable to passive entrainment and retention in depressions. Similar experiments with smaller depressions (9 mm in diameter and 9 mm deep) showed no settlement enhancement in depressions for Capitella sp. I and enhancement in only one of two flow experiments with M. lateralis larvae, suggesting that the hydrodynamic, trapping effect may be scale dependent for both species.

Horizontal swimming and gravitational sinking of Capitella sp. I (Annelida: Polychaeta, larvae: implications for settlement

Abstract Horizontal swimming and gravitational sinking of the lecithotrophic larvae of Capitella sp. I were measured on laboratory-reared organisms during their planktonic life. Swimming was quantified using a video-computer system for motion analysis under both infrared and directed white light conditions. Gravitational fall velocities were measured on anesthetized larvae 10 a temperaturecontrolled particle settling chamber. Swimming and sinking measurements were made on larvae within approximately 12 hours of hatching and at about 24-hour intervals for the following two days. Mean swim speeds were between 2.5 and 3.3 mm/sec on the first day and decreased to 1.6 to 2.3 mm/sec on day 3. Mean fall velocities ranged between 0.8 and 1.0 mm/sec and did not change significantly over the pelagic period. The larvae displayed both photokinetic and phototactic behaviors, swimming slower and toward the white lights, especially early in their development. The capability of these organisms to select settlement sites ...

ULTRASTRUCTURAL DIFFERENCES IN THE EGGS AND OVARIAN FOLLICLE CELLS OF CAPITELLA (POLYCHAETA) SIBLING SPECIES

Ultrastructural studies of ovarian follicle cells and mature eggs in four sibling species in the polychaete genus Capitella have revealed distinct and consistent morphological differences that parallel in some respects the differences between the species in egg size, and embryonic and larval development. Capitella spp. I and II are extremely similar in all respects: the follicle cells lack lipid and contain a modest amount of glycogen; the mature eggs are rich in lipid and glycogen and contain very similar proteid yolk granules. In both species mature eggs have a characteristic electron-dense band and a zone of mitochondria in the cortical ooplasm. These sympatric species have eggs that are similar in size and lecithotrophic larvae that are planktonic for only a short time. Capitella sp. III (Capitella jonesi) has ovarian follicle cells containing a small amount of lipid and no glycogen, while the mature eggs have a small amount of lipid, abundant glycogen, and large proteid yolk granules. These small eggs show no evidence of an electron-dense band or any concentration of mitochondria in the cortical ooplasm. This species has planktotrophic larvae that remain in the plankton for many weeks. Capitella sp. IIIa has ovarian follicle cells rich in both lipid and glycogen. The large mature eggs are rich in lipid, have relatively little glycogen, and have abundant proteid yolk granules. The cortical ooplasm contains electron-dense material similar to that observed in the eggs of species I and II but it is distributed in a discontinuous band. This species has direct development, and juvenile worms emerge from the parental brood tube after metamorphosis. The egg envelopes and microvilli of the eggs of all four sibling species undergo substantial morphological changes following release from the ovary into the coelom. The significance of these morphological and biochemical differences between the species is not known, but the lack of intraspecific variation in these characters suggests that their presence or absence reflects specific differences in the processes of yolk formation and utilization.

Sediment choice by settling larvae of the bivalve, Spisula solidissima (Dillwyn), in flow and still water

The surfclam, Spisula solidissima (Dillwyn), is a common species in many sandy locales off the east coast of North America from Labrador to North Carolina, yet it rarely occurs in muddy areas. In order to determine if this distribution may result, at least in part, from larval habitat selection, 24-h laboratory still-water and flume-flow experiments were conducted using a sediment-filled array. Sediment treatments included low-organic sand typical of adult habitat and a contrasting organic-rich mud. In flow experiments, larvae consistently chose sand over mud. In still water, however, results were variable, with higher settlement sometimes observed in mud. Measurements of settled individuals from four pairs of still-water/flow experiments indicated that mean shell lengths of clams in sand treatments were generally similar to those in mud for flow experiments, but in three of the corresponding still-water experiments clams in mud treatments were significantly smaller than those in sand. This difference may be a result of a deleterious effect of the mud on the larvae, resulting in a disproportionate representation of small, precompetent individuals in the mud treatments. These flow and still-water experiments indicate that larval habitat selection may contribute to the sediment-specific adult field distributions of the surfclam.

Larval habitat choice in still water and flume flows by the opportunistic bivalve Mulinia lateralis

Abstract Competent pediveligers of the coot clam Mulinia lateralis (Say) clearly preferred an organically-rich mud over abiotic glass beads in 24-h flume experiments, and often demonstrated the same choice in still-water experiments. We hypothesize that peediveligers with characteristic helical swimming paths above the bottom can exercise habitat choice in both still water nad flow, but that the limited swimming ambits of physiologically older periveligers require near-bottom flows to move the larvae between sediment patches so that they can exercise habitat choice. Although M. lateralis larvae are planktotrophic, their ability to delay metamorphosis in the absence of a preferred sediment cue is limited to about five days, a shorter time than the lecithotrophi larvae of the opportunistic polychaete species, Capitella spp. I and II. Field distributions of all three opportunistic species may result, at least in part, from active habitat selection for high-organic sediments by settling larvae.

The role of colonization in establishing patterns of community composition and diversity in shallow-water sedimentary communities

To determine whether pattern and diversity in benthic sedimentary communities are set primarily at colonization or by post-settlement biological interactions, we collected faunal cores and conducted reciprocal sediment transplant experiments at a sandy and a muddy site at 12 m depth, ∼3 km apart off New Jersey. Multivariate analyses of cores collected at these sites in September 1994 indicated differences in the taxa determining local pattern, with the bivalve Spisula solidissima and the polychaete Polygordius sp. being dominant at the sandy site, and oligochaetes, several polychaete species, and the bivalve Nucula annulata dominant at the muddy site. Individual cores from the sandy site were significantly less diverse than those at the muddy site. Short-term experiments (3-5 d) were deployed by divers at three different times (August-September, 1994). Replicate trays (100 cm 2 ) filled with azoic sand or mud were placed flush with the ambient seafloor at both sites. Multivariate comparisons indicated that sediment treatment in trays played a greater role in determining colonization patterns in the first experiment, site played a greater role in the second, and both variables contributed in the third. This pattern suggests that larval settlement and habitat choice played an important role in the first and third experiments, and that local transport of recently settled juveniles from the surrounding sediments was important in the second and third experiments. Sandy-site trays had significantly lower diversity than muddy-site trays, but there was no effect of sediment type in trays on diversity of colonizers. These experiments focused on small spatial scales and three short time periods, but they demonstrate that species patterns in some environments may be set by habitat selection by larvae and by juvenile colonization from the surrounding community. Post-colonization processes such as predation and competition likely play a major role for some species, but patterns of initial colonization corresponded well with those in the local community.

Adult macrofauna effects on Capitella sp. I larval settlement : A laboratory flume study

The opportunistic, deposit-feeding polychaete Capitella sp. I is the overwhelming numerical dominant in disturbed and enriched sediments and rarely co-occurs in appreciable numbers with other abundant mud-dwelling macrofauna. Rapid colonization and population increase in organic-rich sediments is typically followed by subsequent sharp decline. The mechanistic basis for these characteristics was explored in flume-flow experiments that tested whether settling Capitella sp. I larvae avoid sediments inhabited by macrofaunal adults or sediments reworked by them. The first set of experiments consisted of four treatments: conspecific adults or no adults in reworked or non-reworked sediment. Capitella sp. I settlement was significantly altered (depressed) only by pelletized sediment of conspecific adults. The second set of experiments involved similar treatments, but with adults of the deposit-feeding bivalve Tellina agilis. Neither adult presence nor sediment reworking significantly affected settlement of Capitella sp. I larvae. A third set of experiments that compared settlement in sediments with and without the suspension-feeding bivalve Mulinia lateralis demonstrated no significant treatment effect. These results suggest that larval settlement behavior could contribute to population growth in a boom and bust species when a critical limiting resource is overexploited. That is, sediments completely pelletized by Capitella sp. I adults may signal settling larvae that organic matter is depleted. Larvae may therefore settle in smaller numbers and are more likely to be dispersed away from abundant populations of adults. Active avoidance of conspecific adults or adults of other taxa is unimportant for the taxa at the densities tested here.

INITIAL RECRUITMENT AND GROWTH OF SURFCLAMS (SPISULA SOLIDISSIMA DILLWYN) ON THE INNER CONTINENTAL SHELF OF NEW JERSEY

Abstract Surfclam (Spisula solidissima Dillwyn) larval settlement and the initial growth of recruits were studied on the inner shelf of New Jersey. Initial recruitment was measured by taking weekly benthic core samples during the summer settling season, and larval supply was characterized using meroplankton samples taken every four hours in July. The temporal variation in recruitment at two inshore stations (12-m depth) was linked to larval supply from the water column, and spatial differences (inshore vs. offshore) in recruitment also appeared to be related to larval supply. Spatial and temporal variation in larval concentrations was associated with wind-driven cross-shelf circulation. Contrasting recruitment patterns between the two inshore stations could not be explained by larval supply alone and were likely affected by near-bottom flows. Growth rates of initial surfclam recruits (with initial shell lengths <360 μm) were estimated to be 10–20 μm d−1, and the growth rates of individuals >360 μm shell length were 25–50 μm d−1. This study provides realistic field estimates of early growth rates of surfclams and further evidence of the relationship between upwelling/downwelling events and surfclam larval supply and initial recruitment on the inner continental shelf.

Red Blood Cell Oxygen Binding in Capitellid Polychaetes

The oxygen equilibrium properties of red blood cells that circulate in the coelomic cavities of 10 morphologically similar capitellid polychaetes are generally species specific. Appreciable differences in oxygen affinity distinguish most of the species and, in several instances, cooperativity differs as well. The range of oxygen affinities is far greater than in the other closely related polychaetes examined to date. We suggest that the differences may prove to be adaptations to thermal properties of the environment, body size, or both.

Scanning Electron Microscopic Aids for Identification of Larval and Post-Larval Bivalves

ABSTRACT The identification of bivalve larvae and early postlarvae in plankton and benthic samples has long been a challenge, hampering both basic and applied research efforts in marine, estuarine, and freshwater environments. The usefulness of published optical micrographs of the early life-history stages of bivalves is limited because of the great morphological similarity of the imaged articulated shells, particularly at the early (straight-hinge) developmental stages. While a number of techniques have been refined in recent years and show promise for use in routine identifications of larval and post-larval bivalves (e.g., single-step nested multiplex polymerase chain reaction; in situ hybridization protocols through color coding with taxon-specific, dye-labeled DNA probes; coupled fluorescence in situ hybridization and cell sorting; and image analysis techniques using species-specific shell birefringence patterns under polarized light), no adequate comprehensive reference source exists that accurately depicts the morphology and morphometry of the shells of larval and post-larval stages of target bivalve species in a consistent format to assist in identification of such stages. To this end, scanning electron micrograph (SEM) sequences are presented of the disarticulated shell valves of laboratory-reared larval and post-larval stages of 56 species of bivalve molluscs from a wide spectrum of marine, estuarine, and freshwater habitats. Emphasis is placed on the usefulness of the morphology and morphometrics of consistently-oriented, disarticulated shell valves and associated hinge structures in discriminating the early life-history stages of these various bivalve species. Although the scanning electron micrograph sequences presented accurately depict the gross morphologies/ morphometrics and hinge structures of the disarticulated shell valves of the larvae and/or postlarvae of the 56 species of bivalves, it is important to emphasize that a scanning electron microscope is not necessary to observe even fine hinge structures associated with the early ontogenetic stages of these species. Such structures are readily visible using a wide range of optical compound microscopes equipped with high-intensity reflected light sources, although the disarticulated shell valves must be viewed in several planes of focus to discern the often subtle details seen clearly in the scanning electron micrographs. These morphological characters provide researchers with invaluable aids for the routine identification of the early life-history stages of these species isolated from plankton and benthic samples.