Ladd E. Johnson

SURVIVAL OF FUCOID EMBRYOS IN THE INTERTIDAL ZONE DEPENDS UPON DEVELOPMENTAL STAGE AND MICROHABITAT1

Embryos of the fucoid alga Pelvetia fastigiata (J. Ag.) DeToni were outplanted into the intertidal zone to assess survival during the physical stress brought about by emersion during a single low tide. Survival varied among microhabitats. Under the adult Pelvetia canopy, survival of 6‐h‐, 24‐h‐, 48‐h‐, and 1‐wk‐old embryos was nearly 100%. Almost all embryos of all ages died in exposed habitats on bare rock or within habitats where the Pelvetia canopy was removed experimentally. However, within red algal turfs, where most juvenile Pelvetia occur, survival was unusually age specific: 24‐ to 48‐h‐old embryos survived poorly compared to younger (6 h old) or older embryos (1 wk old). Survival patterns reflected microhabitat temperatures during the experiments. The fate of young post‐settlement stages must be studied at these fine temporal and spatial scales to understand the organization of intertidal communities.

Gametogenesis, gametes and zygotes : an ecological perspective on sexual reproduction in the algae

Ecological aspects of sexual reproduction in freshwater and marine algae are reviewed in the context of reproductive seasonality, gamete release, pheromones, fertilization success, polyspermy, parthenogenesis, and the dispersal of zygotes. Fertilization success in freshwater and marine macroalgae is higher than previously assumed, and the biological and physical variables that contribute to this are reviewed and analysed. These variables include synchronous release of gametes, plant architecture, immobilization of female gametes, cytological specializations (e.g., in the spermatia of some red algae), agglutinins, and a number of important aspects of water motion. Little is known about fertilization success in unicellular algae, but various aspects (gametogenesis, resting zygotes) of reproduction in such algae are considered. The review concludes with questions that should be addressed in future studies; a number of these require more attention to hydrodynamic variables.

Dispersal and recruitment of a canopy-forming intertidal alga: the relative roles of propagule availability and post-settlement processes

Abstract. The daily settlement of eggs and zygotes of the monoecious brown alga Pelvetia compressa (J. Agardh) De Toni was measured on artificial substrata in areas inside and outside patches of adults in the high intertidal zone of central California. Settlement was generally 1–2 orders of magnitude higher under the adult canopy. This pattern seems to be due to the synchronous release of gametes during the daytime low tide. The release of gametes also appears periodic over longer time scales (e.g., 3- and 14-day cycles). In spite of the high availability of propagules under the adult canopy, juveniles were most abundant outside patches, where propagule availability was lower. In both areas, juveniles were disproportionately associated with patches of a red algal turf [primarily Endocladia muricata (Postels & Ruprecht) J. Agardh and Masticarpus papillata (C. Agardh) Kützing]. The turf, which is less common under the P. compressa canopy, may offer protection from dislodgment, grazing, and/or desiccation and thus facilitate recruitment at this site. Overall, post-settlement processes appear more important in determining population structure than does the availability of propagules in areas in and around patches of adults. However, the apparent small range of dispersal of P. compressa may make propagule availability an important limitation to the establishment of new populations and may restrict gene flow between populations.

Historical invasions of the intertidal zone of Atlantic North America associated with distinctive patterns of trade and emigration

Early invasions of the North American shore occurred mainly via deposition of ballast rock, which effectively transported pieces of the intertidal zone across the Atlantic. From 1773–1861, >880 European ships entered Pictou Harbor, Nova Scotia, as a result of emigration and trade from Europe. The rockweed Fucus serratus (1868) and the snail Littorina littorea (≈1840) were found in Pictou during this same period. With shipping records (a proxy for propagule pressure) to guide sampling, we used F. serratus as a model to examine the introductions because of its relatively low genetic diversity and dispersal capability. Microsatellite markers and assignment tests revealed 2 introductions of the rockweed into Nova Scotia: 1 from Galway (Ireland) to Pictou and the other from Greenock (Scotland) to western Cape Breton Island. To examine whether a high-diversity, high-dispersing species might have similar pathways of introduction, we analyzed L. littorea, using cytochrome b haplotypes. Eight of the 9 Pictou haplotypes were found in snails collected from Ireland and Scotland. Our results contribute to a broader understanding of marine communities, because these 2 conspicuous species are likely to be the tip of an “invasion iceberg” to the NW Atlantic from Great Britain and Ireland in the 19th Century.

Distribution and feeding ecology of the seastars Leptasterias polaris and Asterias vulgaris in the northern Gulf of St Lawrence, Canada

Extensive field observations were made in the Mingan Islands, northern Gulf of St Lawrence, to evaluate spatial and temporal variations in the use of habitat and prey resources by two major subtidal predators, the seastars Leptasterias polaris and Asterias vulgaris . Although both seastars have similar size structures and generally overlapped in their spatial and temporal distribution, the degree of overlap varied in different sites and appeared to be related to prey abundance, substratum type and slope. Three general patterns were observed: (1) both species aggregating in shallow water and decreasing in numbers with depth; (2) the two species showing inverse depth distributions; and (3) both seastars occurring in low numbers across the subtidal zone. Temporal changes in availability of the preferred prey of the two seastars, the mussel Mytilus edulis , appeared to be a major factor affecting their abundance and distribution. The two seastars occurred together in dense aggregations on mussel beds in shallow water. They consume similar-sized mussels until the number of mussels becomes reduced when A. vulgaris , but not L. polaris , begins to select larger mussels. Once a mussel bed is decimated, the seastars appear to move away, possibly in search of other beds. Intensive seastar foraging limits the distribution of mussels to a few metres in depth. Below the mussel zone, the two seastars are spatially segregated at a small spatial scale (1 m 2 quadrat) and select different alternative prey, L. polaris feeding mainly on the crevice-dwelling clam Hiatella arctica and A. vulgaris on the ophiuroid Ophiopholis aculeata . The size partitioning of the preferred prey in shallow water, and spatial segregation and selection of different alternative prey at greater depths may reflect mechanisms permitting the two seastars to coexist.

Landscape patterns of an aquatic invader: Assessing dispersal extent from spatial distributions

Assessing the spatial distribution of organisms across landscapes is a key step toward determining processes that produce observed patterns. The spatial distribution of an invasive aquatic mollusk, the zebra mussel (Dreissena polymorpha), was examined in two lake-rich areas (Belarus and midwestern United States) with contrasting invasion histories. Spatial distribution patterns of invaded lakes were determined using Ripleys K. Aggregation of invaded lakes was found at similar spatial extents ( 120 km in Belarus. The observed spatial extent of aggregation likely reflected the scale of secondary geographic spread, whereas the scale of long-distance dispersal events was reflected by the spatial extent of segregation. Isolated Belarus lakes were less likely to be invaded than those connected by waterways. Although one-dimensional aggregation of invaded lakes along connected Belarus waterways was not observed, nearest neighbor analysis indicated that zebra mussel dispersal occurred at distances <15 km within these waterways. Based on observed spatial pattern, we concluded that zebra mussels have not yet saturated European and North American lake landscapes, including many suitable lakes. Similar distribution patterns of invaded lakes in Belarus and North America suggest that similar processes have influenced zebra mussel spread in both landscapes.

Algal colonization in urchin barrens: defense by association during recruitment of the brown alga Agarum cribrosum

We investigated the process whereby juveniles of the kelp Agarum cribrosum escape grazing by the green sea urchin, Strongylocentrotus droebachiensis, on urchin barrens in the rocky subtidal zone in the Mingan Islands, northern Gulf of St. Lawrence. The highest recruitment of juvenile A. cribrosum occurred under the canopy of the large filamentous phaeophyte Desmarestia viridis, where urchin densities were markedly reduced, compared to the surrounding area. This pattern of distribution appeared to be related to the wave-induced sweeping motion of D. viridis, although currents may modify the back and forth motion of the alga by pushing the canopy towards a specific direction, thereby allowing urchins to invade the non-swept areas. The density of juveniles under D. viridis plants increased with plant size and increasing proximity to the holdfast. Living under D. viridis slightly reduced the growth rate of the A. cribrosum juveniles, but this loss in growth was clearly outweighed by the gain in protection from sea urchin grazing. The time scale over which D. viridis provides protection is in the order of months, as D. viridis is an annual alga that disappears in early autumn. This defensive association of juvenile A. cribrosum with D. viridis is possibly a successional step leading to the formation of mature stands of A. cribrosum.

A method for spatial and temporal assessment of gastropod grazing intensity in the field: the use of radula scrapes on wax surfaces

The feeding apparatus of many marine molluscan herbivores leaves distinctive marks on the surface of dental wax. This method can be used in the field to assess the spatial and temporal patterns of gastropod grazing on rocky shores. Among the common gastropod grazers of intertidal habitats on the Isle of Man, distinctive rasping marks were made by docoglossan (Patella vulgata.), rhipidoglossan (Calliostoma zizyphinum. and Gibbula spp.) and larger taenioglossan (Littorina obtusata.) grazers. Our technique for the field deployment of wax surfaces is simple, inexpensive, and permits a realistic placement of the wax surface in the environment. This placement is achieved by casting the wax into small discs (14 mm diameter) and setting them into pre-formed holes in the rock surface. By quantifying either the number of discs scraped or the area of the wax surface scraped, patterns of grazing intensity (defined as areal extent of the surface grazed in a given period) can be assessed over a variety of spatial and temporal scales. To illustrate this method and refine its use, we recorded the grazing patterns of the limpet Patella vulgata for periods from 12 h to 17 days. The optimal period of deployment depended on the specific habitat, but we often found periods of 1–14 days to be appropriate. Regular arrays of discs also demonstrated that grazing intensity was spatially variable at a scale of 0.25 m, and that grazing intensity increased throughout the late winter and spring. This method provides a cheap and direct measure of feeding intensity that is directly relevant to understanding the effect of grazing molluscs on algal communities. Moreover, it can be used over a variety of spatial and temporal scales.

Use of prey resources by the seastars Leptasterias polaris and Asterias vulgaris: a comparison between field observations and laboratory experiments

The subtidal predators Leptasterias polaris and Asterias vulgaris strongly overlap in their size, distribution and use of prey resources. We evaluate the factors determining their use of prey (feeding rate and prey selection) by comparing prey use in the laboratory and in the field. Both seastars have the same preferred prey, the mussel Mytilus edulis, select medium-sized mussels (1.5–3.0 cm), and show similar responses to increasing mussel density (functional response). Differences between the two seastars in their feeding abilities appear to affect both their size selection of mussels and use of alternative prey when mussels become rare or absent. A. vulgaris selects larger mussels than L. polaris when mussel abundance is low, and this may optimize its energetic intake. L. polaris is be better able to prey on the crevice-dwelling clam Hiatella arctica and A. vulgaris on the ophiuroid Ophiopholis aculeata. These differences in prey selection may reduce potential competitive interactions between the two seastars, as they cause prey partitioning when the preferred prey is rare or absent. The greater feeding rate of A. vulgaris on larger mussels likely provides it with a competitive advantage when the abundance of preferred prey is limited.

Patterns of Spread of the Zebra Mussel (Dreissena polymorpha (Pallas)): the Continuing Invasion of Belarussian Lakes

The invasion of the freshwaters of Belarus by the zebra mussel, Dreissena polymorpha (Pallas), began at least 200 years ago by the opening of shipping canals linking the Black Sea and Baltic Sea drainage basins. However, zebra mussels have invaded only 93 (16.8%) of 553 studied lakes; at least 20 of these lakes were invaded within in the past 30 years. Zebra mussels were found disproportionately in lakes that were mesotrophic, larger, and had some commercial fishing. Although larger lakes have more intensive fisheries with larger catches, the intensity of the fishery and average catch did not affect the probability of zebra mussel invasion. Zebra mussels were not found in dystrophic lakes (10% of the lakes studied), probably due to their low pH and calcium content. Zebra mussels became locally extinct in one lake due to anthropogenic eutrophication and pollution. Many ecologically suitable lakes have yet to be invaded, which suggests that natural vectors of overland dispersal, e.g., waterfowl, have been ineffective in Belarus. Thus, future spread of this species will continue to depend on human activities such as commercial fishing.