Larry G. Harris

Changing Community States in the Gulf of Maine: Synergism Between Invaders, Overfishing and Climate Change

Human activities, including overfishing and species introductions, have had a dramatic impact on benthic communities in the Gulf of Maine within the past two decades. Prior to the 1970s, the climax community in the shallow subtidal was composed of Laminaria spp. kelp beds with an understory of arborescent red algae. In the 1980s, a population explosion of the green sea urchin, Strongylocentrotus droebachiensis, created an alternate community state, urchin barrens. Recently, a new community has been observed in former urchin barrens and kelp beds. This assemblage is principally composed of the introduced species: Codium fragile subsp. tomentosoides (green alga), Membranipora membranacea (bryozoan), Diplosoma listerianum (tunicate), Bonnemaisonia hamifera (red alga) and the opportunistic species Mytilus edulis (mussel) and Desmarestia aculeata (brown alga). In addition to changes in relative abundance, many of these species have greatly expanded their distribution and habitat selection. A model detailing mechanisms for the transition of the traditional kelp bed and urchin barren communities to others is presented and implications for this new community are discussed.

Community Recovery After Storm Damage: A Case of Facilitation in Primary Succession

Manipulations of early arriving, fast-growing algal stands, which appeared soon after a severe storm denuded a Southern California marine reef habiatat, indicated that the dense cover protected newly settled kelp plants from excessive damage by grazing fishes. This is an example of refuge facilitation in primary succession after a major natural disturbance, a mechanism that may contribute substantially to the regeneration of a kelp forest.

Temperature, Herbivory and Epibiont Acquisition as Factors Controlling the Distribution and Ecological Role of an Invasive Seaweed

The invasive canopy alga, Codium fragile ssp. tomentosoides, first observed at the Isles of Shoals in 1983, has become the dominant canopy species to 8 m throughout the islands. Codium populations are replacing themselves at most sites in what appears to be a new, climax, canopy species. However, Codium densities have declined in protected Gosport Harbor areas where it first became established. Codium has only slowly expanded its presence in adjacent nearshore subtidal habitats. Recent studies suggest a combination of factors that may be influencing the relative success of populations between habitats. The herbivorous sea slug, Placida dendritica, may be reducing populations in protected areas in spite of predators such as the green crab, Carcinus maenas, while surge may inhibit herbivore buildup in exposed habitats. Temperature instability due to localized, wind-driven upwelling may be slowing the buildup of subtidal Codium populations in nearshore sites. The combination of Codium dominance and the acquisition of increasing epibiont diversity are producing a new, potentially more complex community state than the previous kelp-dominated climax typical of the Gulf of Maine.

Predatory Impacts of Two Introduced Crab Species: Inferences from Microcosms

Abstract We utilized microcosms that mimicked the rocky intertidal community to examine the predatory impacts of two introduced crab species, Carcinus maenas and Hemigrapsus sanguineus, on naturally occurring assemblages of organisms on rocks in the laboratory and in the field. The two crab species had similar consumption patterns causing significant declines in Semibalanus balanoides (barnacles), juvenile Mytilus edulis (mussels), Spirorbis sp. (polychaetes), and ephemeral algae. In both two-day and fourteen-day field experiments, the decline in S. balanoides in the H. sanguineus treatments was significantly greater than in the C. maenas treatments suggesting that H. sanguineus may have a higher per capita impact on barnacles than C. maenas. As H. sanguineus increases in abundance in northern New England, it may have greater impacts on prey species than C. maenas did when it became established.

An infraciliary network in statocyst hair cells

SummaryUltrastructural analysis of the statocyst, a primitive vestibular organ, of the nudibranch molluscHermissenda crassicornis, indicates that in addition to the basal foot, there is an infraciliary rootlet system between basal bodies of adjacent sensory cilia. These rootlets project perpendicularly from the basal bodies and parallel to the cell surface in an astral array. A polarity within the network also appears to exist; the array is longest and most extensive on the side of the basal body directed away from the cell centre, but the overall arrangement of the basal bodies indicates a multidirectional sensitivity for each of the 13 sensory cells. This rootlet system, in conjunction with the attachment system of the basal bodies to the cell membrane (button anchors), may serve an integrative function for the mechanical stimuli experienced by sensory cells and/or be involved with their transductive processes by maximizing the stress to, and membrane distortion of, the transductive site caused by weighting of the cilia. Evidence was also obtained for the intracellular synthesis of statoconia by the nonsensory supporting cells.

Synxenic and Attempted Axenic Cultivation of Rotifers

Three species of rotifers have now been grown synxenically and, to a limited extent, axenically. Brachionus variabilis thrives in suitable media containing Chlorella pyrenoidosa and a bacterial species. Lecane inermis and Philodina acuticornis var. odiosa are bacteriophagous, the former doing best with two bacterial species (dixenically), the latter doing well with Escherichia coli alone (monoxenically).

Invasive seaweeds transform habitat structure and increase biodiversity of associated species

Summary The visual landscape of marine and terrestrial systems is changing as a result of anthropogenic factors. Often these shifts involve introduced species that are morphologically dissimilar to native species, creating a unique biogenic structure and habitat for associated species within the landscape. While community-level changes as a result of introduced species have been documented in both terrestrial and marine systems, it is still unclear how long-term shifts in species composition will affect habitat complexity or its potential to influence the biodiversity of species that occur at the base of the food web. We analysed quadrat photos collected at several subtidal sites in the Gulf of Maine over a 30+ year period, and collected individual seaweed species to determine their complexity and the biodiversity of meso-invertebrates associated with each species. By coupling the relationship of 30+ years of shifts in seaweed assemblages, morphological structure of the seaweed assemblage, and their meso-invertebrates, we determined introduced seaweeds have increased by up to 90%, corresponding to a rise in two-dimensional (2D) structure, and a decline in canopy height of subtidal rocky habitats. The highly complex two-dimensional habitat provided by introduced filamentous red seaweeds supports two to three times more meso-invertebrate individuals and species that form the base of the food web than simpler forms of morphological habitat. Synthesis. The present study demonstrates a long-term shift in foundation species towards a dominance of invasive seaweeds that directly reduce canopy height and increase the 2D biogenic structure of the habitat. These introduced seaweeds harbour greater biodiversity of species found at the base of the food web than seaweeds with simpler forms such as the native kelp species. Such shifts in habitat structure will propagate to food webs by influencing the structure of lower trophic-level meso-invertebrates and indirectly upper trophic-level species that feed on these invertebrates and use the seaweed structure as refuge.

Introduced species provide a novel temporal resource that facilitates native predator population growth

Non-native species are recognized as important components of change to food web structure. Non-native prey may increase native predator populations by providing an additional food source and simultaneously decrease native prey populations by outcompeting them for a limited resource. This pattern of apparent competition may be important for plants and sessile marine invertebrate suspension feeders as they often compete for space and their immobile state make them readily accessible to predators. Reported studies on apparent competition have rarely been examined in biological invasions and no study has linked seasonal patterns of native and non-native prey abundance to increasing native predator populations. Here, we evaluate the effects of non-native colonial ascidians (Diplosoma listerianum and Didemnum vexillum) on population growth of a native predator (bloodstar, Henricia sanguinolenta) and native sponges through long-term surveys of abundance, prey choice and growth experiments. We show non-native species facilitate native predator population growth by providing a novel temporal resource that prevents loss of predator biomass when its native prey species are rare. We expect that by incorporating native and non-native prey seasonal abundance patterns, ecologists will gain a more comprehensive understanding of the mechanisms underlying the effects of non-native prey species on native predator and prey population dynamics.

AN ANALYSIS OF THE DEFENSIVE MECHANISMS OBSERVED IN THE ANEMONE ANTHOPLEURA ELEGANTISSIMA IN RESPONSE TO ITS NUDIBRANCH PREDATOR AEOLIDIA PAPILLOSA

1. The defensive mechanisms shown by the west coast, intertidal sea anemone, Anthopleura elegantissima, in response to its nudibranch predator Aeolidia papillosa are identified and evaluated in the context of the environment where A. elegantissima occurs. The defensive mechanisms include intertidal distribution, clone formation, alarm response, bulging of the column, crawling and releasing from the substrate.2. A. papillosa are primarily located at the periphery of clones so that anemones in the interior of the clone have a refuge from predation. Assuming that cloning is an adaptation for space competition in A. elegantissima, then the additional advantage derived as a defensive mechanism should increase selection for clone formation.3. A. papillosa was less able than A. elegantissima to withstand desiccation from exposure at low tide. This suggests that the intertidal distribution of A. elegantissima is a defensive adaptation which reduces the threat of predation by A. papillosa at least during the warme...

The influence of substrate material on ascidian larval settlement

Submerged man-made structures present novel habitat for marine organisms and often host communities that differ from those on natural substrates. Although many factors are known to contribute to these differences, few studies have directly examined the influence of substrate material on organism settlement. We quantified larval substrate preferences of two species of ascidians, Ciona intestinalis (cryptogenic, formerly C. intestinalis type B) and Botrylloides violaceus (non-native), on commonly occurring natural (granite) and man-made (concrete, high-density polyethylene, PVC) marine materials in laboratory trials. Larvae exhibited species-specific settlement preferences, but generally settled more often than expected by chance on concrete and HDPE. Variation in settlement between materials may reflect preferences for rougher substrates, or may result from the influence of leached chemicals on ascidian settlement. These findings indicate that an experimental plate material can influence larval behavior and may help us understand how substrate features may contribute to differences in settlement in the field.