Cheryl Ann Butman

Hydrodynamic Facilitation of Gregarious Settlement of a Reef-Building Tube Worm

Experiments testing the effects of hydrodynamic processes and chemical cues on substrate selection were conducted with larvae of the marine tube worm Phragmatopoma lapidosa californica. In flume experiments, larvae were presented an array of sand treatments, including two substrates previously shown to induce metamorphosis in this species, under fast and slow flow regimes. Larvae preferentially metamorphosed on the inductive substrates in both flows. Delivery to the array was higher in fast flow because larvae tumbled along the bottom, whereas in slow flow, larvae were observed swimming in the water column. Thus, in addition to chemical cues, behavioral responses to flow conditions may play an important role in larval recruitment to the benthos.

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.

Sediment-trap experiments on the importance of hydrodynamical processes in distributing settling invertebrate larvae in near-bottom waters

Abstract The hypothesis that planktonic larvae of benthic invertebrates sink through the water like passive particles in turbulent flows near the seabed was tested in the field by exploiting biased sampling characteristics of sediment traps. Traps of several designs were calibrated in a laboratory flume using passively sinking larval mimics having fall velocities similar to those measured on nonswimming polychaete larvae. A priori predictions regarding the rank order in which various trap designs would collect passively sinking larvae in the field were specified by the rank order in which the traps collected larval mimics in the flume. Field experiments were conducted at two sites, 10- and 14-m depth, in Buzzards Bay, Massachusetts, U.S.A., and traps were moored 0.4–1.6 m above the seabed. In experiments during four field seasons, with deployments lasting from several hours to 11 days, trap collections of Mediomastus ambiseta (Hartman) polychaete postlarvae, total bivalve larvae and postlarvae, spionid/sabellariid polychaete larvae (individuals too small to identify definitively to family), spionid polychaete larvae, enteropneust larvae, and gastropod larvae nearly always corresponded to a priori predictions for passive particle collections between sedimenttrap designs. Results were statistically more significant during some collection intervals than during others, but the rank order of larval collections within each group of traps (deployed simultaneously) corresponded to the rank order of passive particle collections by the traps in the flume, with a couple exceptions. Collections of a polychaete, Pectinaria gouldii (Verrill), were more similar between trap designs (i.e., not biased, as predicted for passive particle collections) than the organisms mentioned above. Competent Pectinaria larvae may sink more quickly because of their larger size and reduced surface area (due to construction of a parchment tube while still suspended). There may be no trapping bias for particles sinking this fast. Collections of metamorphosing seastar larvae also were not in the predicted passive rank order, which may be due, in part, to larvae adhering to solid trap surfaces during metamorphosis. The passive sinking hypothesis could not be falsified in most of the field experiments, indicating that hydrodynamical processes may determine distributions of larvae in very near-bottom waters. Passive sinking by larvae is not, however, an explicit result of this study. Other processes that may have produced observed collections, such as chemical, sedimentary or biological differences among trap environments, must be tested against the passive sinking alternative hypothesis. If larvae sink like passive particles to within 0.4-m of the seabed, as results of this study suggest, then it is possible that larvae initially reach the seafloor at sites where particulates, with fall velocities similar to larvae, initially settle. Passive deposition may thus determine the relatively large-scale distribution of larvae, with active or passive redistribution of larvae, post-settlement selection, or post-settlement mortality determining localized distributions.

Spatial and temporal variability in aggregated grain-size distributions, with implications for sediment dynamics

The grain-size distribution of bottom sediments has important implications for diverse aspects of sediment dynamics, including prediction of the critical boundary shear stress and calibration of suspended sediment sensors. Past sampling strategies to obtain estimates of the seabed grain-size distribution typically have not considered spatial and temporal variability, and have been insufficient to resolve potential millimeter-scale vertical variations in grain size. Moreover, laboratory analyses have been predicated on chemically and/or ultrasonically disaggregating the sediments before resolving particle diameter, therefore the more dynamically relevantin situ grain-size spectrum is not measured. To test for such effects, three sites on the northern California continental shelf comprising a cross-shelf transect from a sandy, inner-shelf (60 m) site, to a muddy, mid-shelf (90 m) site and a relict, outer-shelf (130 m) site were studied. Replicate box cores were collected over two winter field seasons, and multiple subcores from each box core were vertically sectioned at 2-mm intervals. Gentle wet sieving techniques were used to determine the mass fraction in the 300,um size classes. In addition, a lesser number of standard disaggregated grain-size analyses were performed using a Coulter Counter. Results from the sandy, inner-shelf site indicate the presence of an ephemeral fine-grained (<20 μm) surface layer (0–2 mm) that substantively alters the grain-size distribution “seen” by the flow. In addition, there is evidence for a progressive and substantial winnowing of fine-grained sediment from the surface layer over the course of a winter storm season. At the deeper sites, the upper 2 mm of the bed contained 5–20% more material <20 μm than deeper portions of the sediment column. At all three sites, the level of small- (10s of centimeters) and intermediate- (10s of meters) scale spatial variability is modest. In addition, the disaggregated grain-size distribution at the two muddy sites is, in all cases, markedly finer than the in situ grain-size distribution. Therefore, calibrations and predictions based on a knowledge of the size distribution of the primary (i.e. disaggregated) particles could be in serious error.

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 ...

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.

A simple technique for fine-scale, vertical sectioning of fresh sediment cores

Biogeochemical processes occurring at the sediment-water interface and within the very near-surface sediments are known to affect or control what occurs above or below the sediment surface (see reviews ofNowell 1983· Jumars and Nowell 1984; Nowell and Jumars 1984; Grant and Mad~en 1986; Butman 1987; Rumohr et a!. 1987). Studies of these phenomena often require vertical sampling of the variables of interest over very fine (e.g., millimeter) spatial scales. Existing techniques for such fine-scale vertical sectioning of fresh sediment cores were developed primarily for studies of porewater chemistry (Craven et a!. 1986; Jahnke et a!. 1986; Reimers and Smith 1986) and of the distributions of meiofauna or microbes (Boaden and Platt 1971; Joint et a!. 1982; Palmer and Molloy 1986), where only very small samples (cores ::53.0 em in diameter) are required. Furthermore, in most cases, the verticalsectioning technique requires subcoring a much larger sediment sample (e.g., a box core) after it is taken. This potentially introduces error (contamination between layers) when sectioning at millimeter intervals, because near-surface sediments within the sample may mix vertically

Bivalve Feeding and the Benthic Boundary Layer

Benthic suspension feeders particularly some bivalves have strong structuring effects on rocky shore communities which are commonly dominated by the Mytilidae in most oceans (see Paine 1984). This dominance may be related to at least two important aspects of their ecology the sestonic nature of their food and their sessile mode of life. The food of seston feeders is produced in a three-dimensional environment which flows over the animals. Under favourable conditions this food is continuously replenished by currents sinking and reproduction of planktonic organisms. Thus owing to their sedentary nature and feeding behaviour there is strong coupling between the pelagic and benthic environments through bivalves. Energy from the pelagos is channeled directly to the benthos via the filtering activity of the organisms and is eventually used for various metabolic and growth activities (see Dame and Patten 1981). Given the above it can be postulated that the biomass of other feeding guilds (e.g. carnivores which depend directly on primary consumers and perhaps even grazers through the elimination of algal growth by sessile animals Dayton 1973) as well as the overall structure of intertidal communities is dependent on the “success” or “failure” of the suspension feeders. Thus given the central position of suspension feeders as structuring agents of the intertidal communities energy flow from the pelagos to the benthos can be determined by measuring energy flow through this guild. Similar principles hold for subtidal macrofaunal assemblages. Wildish and Peer (1983) for example reported similar relationships between the pelagos and benthos in the Bay of Fundy where suspension feeders contributed over 88% of the total macrofaunal production.

Non-selective settlement of Mercenaria mercenaria (L.) larvae in short-term, still-water, laboratory experiments

Abstract Sediment selection by settling larvae of the hard clam Mercenaria mercenaria (L.) was determined in 4-h, still-water, laboratory experiments where larvae were given a choice between two highly contrasting sediment treatments: a natural, organic-rich mud, and an abiotic, glass-bead mixture with a grain-size distribution similar to the mud. The experiments were designed to evaluate the effects of intrinsic (larval age) and extrinsic (sea water temperature) factors on sediment selectivity. These specific effects were tested because results of initial experiments designed to replicate those of Butman et. al, where M. mercenaria larvae had selected beads over mud in still water but not in flow, showed no significant selection. In the present study, only five of the 23 experiments conducted showed significant selection and, in all cases, for mud over beads. Larval age and water temperature had no significant effect on the outcome. In the earliest of these experiments, we discovered a problem with larval preservation in mud samples (dissolution of the larval shell due to low pH) that may have resulted in underestimates of the number of larvae settled in mud in Butman et al., thus confounding interpretation of those results. The conclusions of Butman et al. are therefore modified based on results of the experiments presented here: settling M. mercenaria larvae do not select between two extreme sediment treatments in 4-h, still-water, laboratory experiments. In addition, competency tests used here failed to establish a consistent, predictable, precipitous rise in competency for a given batch of M. mercenaria larvae. This may be due to natural, large variation in physiological development within a given larval pool such that a small proportion of competent larvae are available each day over an extended period. Furthermore, the between-experiment variability in selectivity and some results of the competency tests suggest that the duration of these experiments may be too short to document definitive, initial larval settlement. Occasional selection would then reflect continual redistribution of larvae prior to final settlement and metamorphosis.