Alan L. Shanks

PROPAGULE DISPERSAL DISTANCE AND THE SIZE AND SPACING OF MARINE RESERVES

This study compiled available information on the dispersal distance of the propagules of benthic marine organisms and used this information in the development of criteria for the design of marine reserves. Many benthic marine organisms release propagules that spend time in the water column before settlement. During this period, ocean currents transport or disperse the propagules. When considering the size of a marine reserve and the spacing between reserves, one must consider the distance which propagules disperse. We could find estimates of dispersal distance for 32 taxa; for 25 of these, we were also able to find data on the time the propagules spent dispersing. Dispersal distance ranged from meters to thousands of kilometers, and time in the plankton ranged from minutes to months. A significant positive correlation was found between the log-transformed duration in the plankton and the log-transformed dispersal distance ( r 5 0.7776, r 2 5 0.60, df 5 1, 25, P 5 0.000); the more time propagules spend in the water column the further they tend to be dispersed. The frequency distribution of the log-transformed dispersal distance is bimodal (kurtosis 52 1.29, t 52 4.062, P , 0.001) with a gap between 1 and 20 km. Propagules that dispersed ,1 km spent less time in the plankton (,100 h), or if they remained in the plankton for a longer period, they tended to remain in the waters near the bottom. Propagules that dispersed .20 km spent more than 300 h in the plankton. The bimodal nature of the distribution suggests that evolutionary constraints may reduce the likelihood of evolving mid-range dispersal strategies (i.e., dispersal between 1 and 20 km) resulting in two evolutionarily stable dispersal strategies: dispersal , 1k m or.;20 km. We suggest that reserves be designed large enough to contain the short-distance dispersing propagules and be spaced far enough apart that long-distance dispersing propagules released from one reserve can settle in adjacent reserves. A reserve 4-6 km in diameter should be large enough to contain the larvae of short-distance dispersers, and reserves spaced 10- 20 km apart should be close enough to capture propagules released from adjacent reserves.

Pelagic Larval Duration and Dispersal Distance Revisited

I present dispersal distances for 44 species with data on propagule duration (PD) for 40 of these. Data were combined with those in Shanks et al. (2003; Ecol. Appl. 13: S159–S169), providing information on 67 species. PD and dispersal distance are correlated, but with many exceptions. The distribution of dispersal distances was bimodal. Many species with PDs longer than 1 day dispersed less than 1 km, while others dispersed tens to hundreds of kilometers. Organisms with short dispersal distances were pelagic briefly or remained close to the bottom while pelagic. Null models of passively dispersing propagules adequately predict dispersal distance for organisms with short PDs (<1 day), but overestimate dispersal distances for those with longer PDs. These models predict that propagules are transported tens of kilometers offshore; however, many types remain within the coastal boundary layer where currents are slower and more variable, leading to lower than predicted dispersal. At short PDs, dispersal distances estimated from genetic data are similar to observed. At long PDs, genetic data generally overestimate dispersal distance. This discrepancy is probably due to the effect of rare individuals that disperse long distances, thus smoothing genetic differences between populations. Larval behavior and species’ life-history traits can play a critical role in determining dispersal distance.

Marine snow: sinking rates and potential role in vertical flux

The sinking rate of intact marine snow (macroscopic aggregates) was determined in settling chambers on seven occasions in Monterey Bay, California and in the northeastern Atlantic. Sinking rates ranged from 43 to 95 m day−1 (average 68 m day −1). The average calculated flux from the upper 20 m of the water column was 30.7 × 104 aggregates m−2 day−1. These calculations suggest that from 3 to 5% of the POC (particulate organic carbon) and 4% to 22% of the PON (particulate organic nitrogen) standing stocks were removed from the surface water each day by the sinking of marine snow.

Laboratory-made artificial marine snow: a biological model of the real thing

Cylindrical tanks of unfiltered seawater were rotated on a roller table until the particles in the seawater formed aggregates resembling marine snow. During the summer of 1987 comparisons were made between marine snow in field samples from two coastal sites on seven separate dates, and aggregates formed in the laboratory in seawater samples taken on the same dates. Aggregates in field and laboratory samples were photographed and their dimensions were determined. Particulate composition of the aggregates was characterized by the abundance of diatoms, benthic diatoms, diatom frustules, mineral grains, fecal pellets, and fungal spores. Laboratory-prepared aggregates had a significantly greater short axis, and significantly larger calculated volume than field aggregates. Particulate compositions of field aggregates were paralleled by similar changes in the laboratory product. Dry weights of known numbers of aggregates collected on three dates indicated no significant differences in calculated densities or porosities of marine snow formed in the field and in the laboratory. We suggest that this method of forming marine snow in the laboratory may provide researchers with a useful experimental tool.

POPULATION PERSISTENCE OF CALIFORNIA CURRENT FISHES AND BENTHIC CRUSTACEANS: A MARINE DRIFT PARADOX

Because flow in the California Current is unidirectional for months, larvae of coastal species may drift and settle downstream of their parents, potentially causing the parental population to go extinct from upstream to downstream; this is a marine equivalent of the drift paradox in streams. Unidirectional larval drift might, however, be minimized by when, where, and how organisms reproduce. We compiled data on California Current fishes (89 nearshore, 65 shelf/slope) and benthic crustaceans (35 nearshore, 15 shelf/slope) and found three unique sets of life history traits displayed by (1) shelf/slope species, (2) nearshore pelagic-spawning fishes (all from the Southern California Bight [SCB] and Baja California), and (3) all other nearshore species. Pelagic larval durations (PD) of shelf/slope species are long (∼136 d); offspring are pelagic winter through summer and are found at depth below the mixed layer offshore. Offspring experience northward flow in winter and southward flow in spring/summer, perhaps minimizing net alongshore drift. Adults are both long-lived and highly fecund. Nearshore species have short PDs (∼45 d), and offspring are pelagic during spring/summer upwelling and are found nearshore and near the bottom. Nearshore larvae may experience southward flow during upwelling and northward flow during relaxation events, minimizing net alongshore transport. Nearshore species have shorter lives and are less fecund than shelf/slope species. Nearshore pelagic-spawning fishes (all from the SCB and Baja) have short PDs (∼48 d), but in contrast to other nearshore species, their larvae are pelagic from spring into fall and are found far from shore. Adults are long-lived, highly fecund, and have numerous broods per year. These life history traits may have evolved to exploit eddies and countercurrents present in the SCB and off Baja for larval retention. Each set of life history traits appears to improve chances of offspring recruiting to parental populations. The pelagic phase, rather than being dispersive, may be selected to achieve a migration between larval pelagic and adult benthic habitats.

Internal-wave-mediated shoreward transport of cyprids, megalopae, and gammarids and correlated longshore differences in the settling rate of intertidal barnacles

The hypothesis that cyprids are transported shoreward in the convergence zone (slick) over internal waves, was tested by comparing cyprid abundances in the near-surface water (depth <20 cm) of the convergence and divergence zones over internal waves. If the hypothesis is correct, then abundances of cyprids in slicks should be significantly higher than abundances in the water between slicks. On three of four sampling dates in two different locations in the San Juan Archipelago, the cyprids of Balanus glandula and Semibalanus cariosus were significantly and as much as 13-fold more abundant in the convergence zones. Other invertebrates, the megalopae of Lophopanopeus bellus belhts and Pugettia spp. and gammarid amphipods, were also at times significantly more abundant in the convergence zones. These data agree with prediction, suggesting that cyprids and other larvae can be transported shoreward by internal waves. Internal waves have the potential not only to carry larvae shoreward, but they also may deposit larvae in different amounts along a shore. We made a preliminary test of this hypothesis in Park Bay. We followed surface drifters transported by internal waves and found that most drifters were carried into the bays southern end. In areas where surface drifters were frequently carried by internal waves, barnacle settling rate in the intertidal was significantly and ≈ 10-fold greater than in areas where surface drifters were seldom transported by internal waves. These data suggest that settling rate of intertidal barnacles along a shoreline was related to the pattern of transport of the cyprids by internal waves.

Movement and feeding activity of red sea urchins (Strongylocentrotus franciscanus) adjacent to a kelp forest

Movement and feeding were studied in a population of red sea urchins, Strongylocentrotus franciscanus (Agassiz, 1863), found within and immediately seaward of a kelp forest offshore from Santa Cruz, California, USA. Mean sea urchin movements varied from 7.5 cm/day inside the kelp forest to over 50 cm/day at 15 and 100 m outside the kelp forest. The percentage of sea urchins feeding decreased from 66% inside the kelp forest to 16 and 15% at 15 and 100 m outside the kelp forest. These data indicate that movement by these sea urchins is a response to a low food supply.

TIDAL PERIODICITY IN THE DAILY SETTLEMENT OF INTERTIDAL BARNACLE LARVAE AND AN HYPOTHESIZED MECHANISM FOR THE CROSS-SHELF TRANSPORT OF CYPRIDS

At an intertidal study site in southern California the daily settlement of barnacle cyprids (probably Chthama/us spp.) was followed during the summer of 1983. Daily settlement was not significantly cross correlated with wind speed or direction but was significantly cross correlated with the maximum daily tidal range at lags of +1 to +4 days; peak settling occurred several days before the spring tide. This pattern of settle ment is nearly identical to that of the megalopa of an intertidal crab, Pachygrapsus crassipes, and this suggests that, like these megalopae, cyprids may be transported onshore in slicks over tidally forced internal waves.

Adding teeth to wave action: the destructive effects of wave-borne rocks on intertidal organisms

SummaryObservations in rocky intertidal areas demonstrate that breaking waves ‘throw’ rocks and cobbles and that these missiles can damage and kill organisms. Targets in the intertidal were dented by impacts from wave-borne rocks. New dents/day in these targets was positively correlated with the daily maximum significant wave height and with new patches/day in aggregations of the barnacle Chthamalus fissus. Impact frequency was highest in the upper intertidal and varied dramatically between microhabitats on individual boulders (edges, tops and faces). These patterns were reflected in the microhabitat abundances of ‘old’ and ‘young’ barnacles. Comparisons were made of the survivorship and the frequency of shell damage in two populations of the limpet Lottia gigantea living in habitats which differed primarily in the number of moveable rocks (i.e. potential projectiles). The mortality rate and frequency of shell damage were significantly higher in the projectilerich habitat. In addition only in this habitat did the frequency of shell damage covary significantly with seasonal periods of high surf. Investigation of the response of limpet shells to impacts suggests that shell strength varies between species and increases with shell size. Species-specific patterns of non-fatal shell breakage may have evolved to absorb the energy of impacts. In two of the intertidal habitats studied, wave-borne rock damage was chronic and, at least in part, may have governed the faunal makeup of the community by contributing to the physical ‘boundaries” of the environment within which the inhabitants must survive.

Orientated swimming by megalopae of several eastern North Pacific crab species and its potential role in their onshore migration

The megalopal stage of most near shore and intertidal crabs must return to the coast to complete their development. Crab megalopae are strong swimmers and if they swam consistently shoreward they could conceivably swim back to shore. This hypothesis was tested for megalopae of Pachygrapsus crassipes (Randall), Lophopanopeus bellus bellus (Stimpson, 1860), Cancer oregonensis (Dana), and C. gracilis (Dana) by investigating their swimming orientation when housed in a transparent container with a view of the underwater illumination or swimming freely in the sea. In the transparent container, megalopae tended to swim in the direction of the suns bearing. Free swimming megalopae swam straight courses and displayed significant preferred swimming directions. Pachygrapsus crassipes and L. bellus bellus megalopae swam at the sea surface and parallel to the current direction. Free swimming C. oregonensis and C. gracilis swam at about 3 to 5 m depth and in the direction of the suns bearing. Megalopae of Pachygrapsus crassipes and C. oregonensis were observed on 4 and 3 days respectively and they did not preferentially swim in a shoreward direction. These results suggest that they do not migrate back to shore by swimming. The orientated swimming may, however, assist the megalopae in returning to shore or locating a settlement site. Swimming with the surface current might help megalopae to migrate shoreward in the convergence zone over internal waves or Langmuir circulation cells. Swimming in the direction of the suns bearing might represent a search behavior for a benthic settlement site.