Size and structure of echinoderm populations associated with different coexisting coral species at Aldabra Atoll,Seychelles

The associated echinoderm fauna of 125 living heads of massive microatolls of Porites spp., branched heads of Porites nigrescens Dana and small, densely packed heads of the hydrocoral Millepora exacea Forskål of a shallow sublittoral lagoon flat at Aldabra Atoll, Seychelles was examined. The weight and number of echinoderms increased with coral size in all cases. The microatolls were the largest coral heads and sheltered the most diverse, underlying echinoderm cryptofauna comprising the largest bodied species, regardless of class or family. The infauna in heads of P. nigrescens was less diverse and smaller bodied than the echinoderms under microatolls. Only with P. nigrescens did species number increase with increasing coral size. Heads of M. exacea were the smallest and provided a habitat of compact interstitial spaces for the most depauperate, smallest bodied infauna which included juveniles of larger species. Ophiuroids dominated the coral-associated echinoderm faunas whereas larger bodied classes, such as echinoids and holothuroids, were less well represented. Large bodied ophiuroids such as certain ophiodermatids occurred only under microatolls, whereas the usually small bodied ophiotrichids dominated in the heads of M. exacea. The generally ubiquitous ophiocomids were well represented in, or under all corals. Heads of M. exacea could be nursery areas for recently metamorphosed juvenile ophiuroids and echinoids which, as they grow, seek shelter in corals with larger interstitial spaces such as P. nigrescens. Significant interspecific ophiuroid associations were related to space availability for larger species under microatolls and in heads of P. nigrescens. The possibility of microhabitat partitioning in corals among some ophiuroids is suggested.     

Use of artificial holes in studying community development in cryptic marine habitats in a tropical rocky intertidal region

Previous studies on the rocky intertidal shores of the Bay of Panama indicate that for many sessile and mobile organisms holes and crevices are important refuges from consumers. To study the dynamics of these hole-dwelling species assemblages, we devised a method allowing repetitive, non-destructive sampling and species manipulations in artificial holes. These are fiberglass sleeves with flanges at the outer edge that are inserted into receptacles made of quick-setting concrete. Predator and herbivore manipulations include: fences excluding slow-moving benthic consumers, bars across hole mouths restricting entry of large fishes and homogeneous (shelterless) surfaces created around a treatment decreasing abundance of small crabs. This design has withstood the rigors of two wet seasons in Panama. Results from two sites in Panama indicate that colonization occurs rapidly (6–7 months) in holes established in mid dry season. Encrusting fleshy algae (Ralfsia sp.) and ephemeral green algae (Cladophora spp.) colonize first and are succeeded by encrusting species of coralline algae, bryozoans and colonial tunicates and by erect fleshy red algae. Where consumers are present, encrusting corallines predominate; without consumers, bryozoans, tunicates and erect algae are more abundant. Zonation patterns develop within the holes, with the desiccation/heattolerant Ralfsia sp. dominating at the outer edges. With increasing deph, encrusting coralline algae, bryozoans and colonial tunicates reach their respective peak abundances. In contrast to the high variability observed among naturally occurring holes, replicates of each treatment tend to be similar. Between-treatment and between-site comparisons are less so. The naturally-occurring high level of small-scale patchiness is thus presumably due to variation in recruitment and in the local (microspatial) consumer regime. Substratum heterogeneity is therefore directly and indirectly important in maintaining a high local diversity in this community. As in other studies, consumers or disturbances are key factors in regulating patterns of community structure. More experiments of longer duration are necessary to ascertain the relative importances of consumers/disturbance and competition in controlling such assemblages.     

Settlement, recruitment and potential predators and competitors of juvenile echinoderms in the rocky subtidal zone

Recruitment patterns of marine invertebrates are affected both by settlement and early post-settlement events. This study examined the settlement and recruitment patterns of echinoderms at three sites in the rocky subtidal zone of Bocabec Cove, Bay of Fundy, Canada using artificial turf collectors and quadrats on the natural substrate. Potential predators were quantified at two of the sites along transects and in 1-m² quadrats. Both potential predators and competitors were quantified in 0.0625-m² quadrats. Settlement varied across sites (1.5–3 km apart) and two years of sampling (2004, 2005). The site of most potential settlement differed for the three groups of echinoderms: sea urchins (Strongylocentrotus droebachiensis), sea stars (Asterias spp.) and sea cucumber (Psolus fabricii). Settlement densities on the artificial turf collectors tended to be greater than the densities of settlers on the natural substrate. On the natural substrate, the only significant difference between densities of juveniles over time was that newly settled sea stars were found in July and were not found the following October. Large lobsters and carnivorous worms were potential predators with densities that varied between sites. Potential competitors that differed in abundance between sites were herbivorous gastropods and conspecifics for sea urchins; and carnivorous worms for sea stars. This study suggests that patterns of recruitment are either set up by patterns of settlement or by events during the first few weeks/months on the benthic substrate for these echinoderms.     

High intertidal community organization on a rocky headland in Maine,USA

A mosaic patchwork of the barnacle Balanus balanoides L., the mussel Mytilus edulis L., and the alga Fucus vesiculosus L. was found in the transitional region between the mid and high intertidal zones on a rocky headland on Mount Desert Island, Maine, USA. The development of the mosaic was observed by following recruitment and survival of B. balanoides in denuded patches located at the same tidal level along a 60 m stretch of shore. Barnacle recruitment was least under canopies of F. vesiculosus and greatest in open areas kept moist at low tide by surf. Barnacle survival after settlement was least under the F. vesiculosus canopy due to the whiplash effect of the algal fronds in the surf and greatest in open areas free from competition from mussels. In open areas, early mortality was correlated with settlement density. In areas of dense settlement (60 spat cm^-2) up to 90% mortality resulted within 5 months from crowding associated with growth. In older individuals crowding produced hummocks of elongated, weakly attached barnacles which were more prone to removal by surf than uncrowded barnacles. Mussels exerted competitive dominance over barnacles for space and the presence of mussel beds prevented further barnacle recruitment. Mussels suffered extensive mortality during winter storms when surf removed dense mats of weakly attached mussels. The patchy distribution of mussels and barnacles results from irregular rock substrata producing numerous environmental patches with respect to wave exposure and drainage at low tide, and from densitydependent mortality of both mussels and barnacles which creates patches of new colonizable space within each environmental patch.     

Impact of sewage on the distribution,abundance and community structure of rocky intertidal macro-organisms

The biological effects of a low-volume domestic sewage discharge were studied near Wilson Cove, San Clemente Island, California (USA), from February to June, 1972. There were fewer species and less cover near the outfall (7 macro-invertebrates, 17.6%; 13 macrophytes, 91.7%) than in nearby “unpolluted” control areas (9 macroinvertebrates, 9.2%; 30 macrophytes, 103.4%). The outfall biota was less diverse than that of the controls, as shown by 5 different diversity indices. A great reduction in community stratification (spatial heterogeneity) and, hence, community complexity occurred near the outfall; this reduction in stratification was primarilly due to the absence of Egregia laevigata, Halidrys dioica, Sargassum agardhianum and Phyllospadix torreyi. These were replaced in the mid-intertidal near the outfall by a low turf of blue-green algae, Ulva californica, Gelidium pusillum and small Pterocladia capillacea, and in the lower intertidal by Serpulorbis squamigerus covered with Corallina officinalis Var. chilensis. A statistically-based determination of assemblages or groups of organisms (i.e., cluster analysis) revealed 3 discrete outfall and 3 discrete control area groups; 3 assemblages contained samples from both areas. The distributional patterns of these groups indicate that near the outfall the degree of dilution of discharged sewage is more important in regulating zonation than is tidal height. The enhancement of the suspension feeder Serpulorbis squamigerus and the omnivores Ligia occidentalis, Pachygrapsus crassipes and Anthopleura elegantissima in the outer fringe of the outfall plume hypothetically is due to their ability to utilize sewage as a food source. A critical effect of the outfall may be to decrease environmental stability thereby favoring rapid-colonizers and more sewage-tolerant organisms. The outfall macrophytes were characterized by relatively higher net primary productivities, smaller growth forms, simpler and shorter life histories, and most were components of early successional stages.     

Amphipods and isopods in the rocky intertidal: dispersal and movements during high tide

Animals in the intertidal, both mobile and sessile, generally exhibit some zonation pattern, in which each species shows a preference for, or is confined to, some height levels. The study of zonation patterns is, however, almost exclusively based on surveys made during low tide, when many animals are relatively inactive. We studied zonation patterns of amphipods and isopods on rocky shores in southwestern Iceland, both by traditional sampling at low tide as well as by sampling during high tide. The distributional patterns seen at high tide differed significantly from that at low tide. One amphipod, Anonyx sarsi, was common around baits at all levels at high tide but absent from the intertidal at low tide. Several other species were either relatively more common or tended to be recorded higher, or in one instance, lower on the shore when the tide was in than at low tide. There was also evidence of some species changing habitats within the intertidal with the tidal cycle. Many species, however, moved little away from their respective zones occupied at low tide, and for some species, including some capable of rapid swimming, very limited mobility was indicated. We conclude that low-tide surveys of the intertidal give an incomplete picture of the community structure, and even key species may be missed in such surveys.Communicated by L. Hagermann, Helsingør     

Spatiotemporal variations in rocky intertidal malacofauna throughout Japan in the 1970s and 1980s

Spatial and temporal variations of benthic fauna over a wide region (>1,000 km) that includes complex ocean currents have seldom been quantitatively studied. The present study revealed malacofaunal variations on Japanese rocky shores affected by the Soya, Tsushima, Oyashio, and Kuroshio currents, and in the Inland Sea. On 29 shores in seven regions in these sea areas, previous researchers have recorded the number of molluscs in quadrats in spring and summer of 1978 and 1984–1986. Reanalyses of these records revealed the following patterns. (1) Shore-to-shore variation in malacofauna was correlated with shore-to-shore geographical distance less clearly for more southern regions. This might show that in southern regions many taxa with low relative abundance are haphazardly recorded on some shores and blur the effect of shore-to-shore distance on malacofaunal dissimilarity. (2) Region-to-region variation in malacofauna was significantly correlated with region-to-region geographical distance. This suggests the influences of environments and paleontological events specific to each current surrounding each region. (3) The pattern of dissimilarity between the Inland Sea and the other sea areas greatly changed from the 1970s to 1980s. This was due in part to the following density fluctuations on some shores in the Inland Sea: increases in the non-indigenous mussel Mytilus galloprovincialis, cultured oyster Crassostrea gigas, and littorinid Littorina brevicula reportedly abundant in sheltered areas; and decreases in the littorinid Nodilittorina radiata reportedly abundant in exposed areas and muricid gastropods Thais spp. showing imposex. This suggests an increase of anthropogenic perturbations during the study period in the Inland Sea which is surrounded by large cities and ports. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage

Despite the great interest in characterizing the functional structure and resilience of functional groups in natural communities, few studies have examined in which way the roles and relationships of coexisting species change during community succession, a fundamental and natural process that follows the release of new resources in terrestrial and aquatic ecosystems. Variation in algal traits that characterize different phases and stages of community succession on rocky shores are likely to influence the magnitude, direction of effects, and the level of redundancy and complementarity in the diverse assemblage of herbivores. Two separate field experiments were conducted to quantify per capita and population effects and the functional relationship (i.e., redundancy or complementarity) of four herbivore species found in central Chile during early and late algal succession. The first experiment examined grazer effects on the colonization and establishment of early-succession algal species. The second experiment examined effects on the late-successional, dominant corticated alga Mazzaella laminarioides. Complementary laboratory experiments with all species and under natural environmental conditions allowed us to further characterize the collective effects of these species. We found that, during early community succession, all herbivore species had similar effects on the ephemeral algae, ulvoids, but only during the phase of colonization. Once these algae were established, only a subset of the species was able to control their abundance. During late succession, only the keyhole limpet Fissurella crassa could control corticated Mazzaella. The functional relationships among these species changed dramatically from redundant effects on ephemeral algae during early colonization, to a more complementary role on established early-successional algae, to a dominant (i.e., keystone) effect on late succession. This study highlights that functional relationship within consumer assemblages can vary at different phases and times of community succession. Differentiation in herbivore roles emphasizes the need to evaluate consumer's impacts through different times of community succession, and through experimental manipulations to make even broad predictions about the resilience or vulnerability of diverse intertidal assemblages to human disturbances.     

Variation and persistence of the middle rocky intertidal community of central Chile,with and without human harvesting

Humans are an important intertidal predator in central Chile. Following a five-year study we report on the effects of the exclusion of humans from the rocky intertidal at Las Cruces, central Chile. The middle intertidal of harvested and non-harvested areas diverged in species diversity and composition during the experiment. In harvested areas the middle intertidal rocky shore was dominated throughout the study by a monoculture of mussels, Perumytilus purpuratus. When humans were excluded, the middle intertidal community switched to one dominated by barnacles (predominantly Jehlius cirratus and Chthamalus scabrosus); this community has persisted for at least three years, despite the presence of forces e.g. mussel larvae, that have the potential to alter the community structure. Such changes were mediated by the muricid gastropod Concholepas concholepas, a key-stone predator. As a consequence of the above changes, the species diversity, H', (primary space occupiers) in the non-harvested area increased from H'=0 at the beginning of the study in 1983 (when the middle intertidal community was dominated by mussels), to values ca H'=2 toward the middle of the study in 1984 (which coincided with the maximum predatory impact of C. concholepas) and subsequently decreasing to ca H'=0.5 at the end of the study in 1987 (when the middle intertida community was dominated by barnacles).Please address all correspondence to J. C. Castilla, Department of Zoology, University of Washington, Seattle, Washington 98195, USA, until March 1990     

Unprecedented bleaching-induced mortality in Porites spp. at Rangiroa Atoll, French Polynesia

In April-May 1998, mass coral bleaching was observed in the lagoon of Rangiroa Atoll, Tuamotu Archipelago, French Polynesia. Six months later, the extent of bleaching-induced coral mortality was assessed at three sites. Corals in the fast-growing genus Pocillopora had experienced >99% mortality. Many large colonies of the slow-growing genus Porites (mean horizontal cross-sectional area 5.8 m²) had also died - a phenomenon not previously observed in French Polynesia and virtually unprecedented world-wide. At one site, 25% of colonies, or 44% of the pre-bleaching cover of living Porites, experienced whole-colony mortality. At the two other sites, recently dead Porites accounted for 41% and 82% of the pre-bleaching live cover. Mortality in Porites was negatively correlated with depth between 1.5 and 5 m. Using a 50-year dataset of mean monthly sea surface temperature (SST), derived from ship- and satellite-borne instruments, we show that bleaching occurred during a period of exceptionally high summer SST. 1998 was the first year in which mean monthly SSTs exceeded the 1961-1990 upper 95% confidence limit (29.4°C) for a period of three consecutive months. We suggest that the sustained 3-month anomaly in local summer SST was a major cause of coral mortality, but do not discount the synergistic effect of solar radiation. Recovery of the size-frequency distribution of Porites colonies to pre-bleaching levels may take at least 100 years.     

Community succession following massive ice-scour on a rocky intertidal shore: recruitment,competition and predation during early,primary succession

The importance of species interactions and recruitment variability was examined during the first year and a half of primary succession (1988–1989) on an exposed rocky seashore near Halifax, Nova Scotia. Previous work suggested that emergent rock on these shores is normally dominated by fucoid rockweeds because predatory whelks control the sessile animal competitors, mussels and barnacles, and because herbivorous littorinids control ephemeral algal competitors. Abundances of all species except seasonal ephemeral algae were very small throughout this experiment and we found no significant effects of carnivory, herbivory, plant-animal competition or plant-plant competition. A slight facilitation of Fucus recruitment is attributed to a thin mat of ephemeral, blue-green algae. Very few other studies have directly manipulated intertidal ephemeral algae. As primary succession may be very rare in this assemblage, these results may be specific to these circumstances, but they highlight the varying importance of species interactions with variable recruitment. In particular, it appears that variations in recruitment success may be important to community structure, even when recruitment is not limited by propagule supply. The scale of the study also provides insight into successional processes occurring after the recent, extensive ice-scour of exposed seashores in this region.     

Population and species diversity fluctuations in a rocky intertidal community relative to severe aerial exposure and sediment burial

From a series of 10 quarterly assessment between October 1975 and May 1978 (inclusive), fluctuations in abundance were determined for macroinvertebrates and macrophytes of a rocky intertidal community on Santa Cruz Island, California, USA. During afternoon low tides in late fall and winter of the first 2 yr of study, organisms of the upper and middle intertidal zones were subjected to prolonged aerial exposure. Many species there tolerated this exposure, but die-backs occurred for a barnacle (Chthamalus fissus/C. dalli) and several algae (Endocladia muricata, Pelvetia fastigiata f. gracilis, Corallina officinalis var. chilensis, Corallina vancouveriensis, Cylindrocarpus rugosus and Codium fragile). These die-backs were succeeded by blooms of Ulva californica and Porphyra perforata. In the upper and middle intertidal zones, the major cover organisms that could tolerate prolonged aerial exposure were disproportionately prevalent and appeared to be maintained by the periodic repression of species intolerant to such exposure. In February 1978, heavy rainfall caused sediment inundation of the middle and lower intertidal zones. This event was followed by declines in abundance of a barnacle (Tetraclita rubescens) and several algae (Pelvetia fastigiata f. gracilis and corallina spp.). Shannon-Wiener H species diversity fluctuated from a high in October to a low the following May during both 1975–1976 and 1976–1977 in conjunction with the period of increased daytime aerial exposure in late fall and winter. A further decline in diversity following sediment inundation in February 1978 contributed to a long-term trend of declining H species diversity (3.06 in October 1975 to 1.87 in May 1978). We hypothesize that predictable late fall to winter aerial exposure stresses, in combination with a random physical disturbance (sediment burial), exceeded an optimal intermediate level of disturbance predicted for maximal species diversity.     

Food resource utilization among five species of embiotocids at King Harbor,California, with preliminary estimates of caloric intake

The diets of 5 species of embiotocids (Rhacochilus vacca, Embiotoca jacksoni, Hypsurus caryi ryi, Phanerodon furcatus and Micrometrus minimus) from King Harbor, Redondo Beach, California, USA, are examined by means of a dietary survey, field feeding observations and bomb calorimetry of prey items. Each species is shown to have statistically different diets from all others, with the exception of H. caryi which apparently is a dietary intermediate between E. jacksoni and P. furcatus. Dietary overlap between E. jacksoni and H. caryi appears to have resulted in some competitive exclusion along other niche parameters. Estimated caloric intakes suggest that R. vacca has the highest caloric yield per bite and the lowest feeding rate of the species studied. Possible energetic advantages are found for diet specialization in R. vacca.     

Role of competition and predation in determining habitat occupancy of Cerithiidae (Gastropoda: Prosobranchia) on the rocky,intertidal, Red Sea coasts of Sinai

Six cerithiid snail species occur on rocky intertidal flats along the Sinai coasts of the Red Sea: Clypeomorus moniliferum, Cerithium caeruleum, C. scabridum, C. columna, Clypeomorus tuberculatum and Cerithium echinatum. The present study, conducted at 22 stations covering almost the whole length of Sinai, covered the 4 yr period from October 1972 to August 1976, and describes the habitat occupancy of 5 of these species (C. echinatum is excluded for lack of data). Several of these species (sometimes all) often occur together, and in such cases are distincly segregated by habitat. However all species considerably overlap in their distribution along the axes of four major interrelated abiotic gradients, thus excluding the possibility that habitat segregation is determined by larval settlement preferences. Other distributional patterns observed at some sites, such as lack of overlap or contact between belts of the various species and the relative abundance of food available to all species, make postlarval competitive interactions unlikely. The existence and the degree of proximity of a coral reef with its associated predatory fishes, influence the cerithiids' distributional patterns. Differences between the cerithiid species in their vulnerability to fish predation, associated with differences between sites in the abundance and the accessibility of predatory fishes, and in the availability of refuges for each cerithiid species, can satisfactorily explain the observed distributional patterns including co-occurrence with habitat segregation. It is proposed that habitat segregation is caused by predation on young stages by generalist fishes which may totally eliminate a certain species at a given site; the same site may provide refuges for recruits of another species, allowing these to survive to an advanced age. In general, the flat's structural complexity is associated with its diversity of refuges from predation, and hence with the number of co-existing species. This mechanism for co-existence and habitat segregation in tropical Cerithiidae may also be instrumental in maintaining the high species diversity of other tropical benthic communities.Paper No. 12 in the series Colonization of the Eastern Mediterrancan by Red Sea species immigrating through the Suez Canal     

Biomass, diversity and production of rocky shore macroalgae at two nutrient enrichment and wave action levels

The littoral zone of temperate rocky shores is normally dominated by perennial macroalgae (e.g. Fucus, Ascophyllum, Laminaria), but nutrient enrichment and/or permanently decreased wave action may lead to structural community changes from dominance of perennials to increased amounts of annual opportunistic species (mainly green algae). Macroalgal biomass, diversity and production as well as relationships between the two latter were studied using Solbergstrand’s rocky shore mesocosms in SE Norway in connection with a long-term experimental manipulation of nutrient addition and wave action (high and low levels of both factors applied in a crossed way to eight outdoor basins). After more than 2 years of experimental treatment, the total standing stock of macroalgae was larger in low nutrient than in high nutrient treatments as well as in high wave compared to low wave treatments (in autumn only). For macroalgal functional groups, bushy and filamentous brown and filamentous red algae were generally favoured by low nutrient concentrations, while annual filamentous and sheet-like green algae were stimulated by the nutrient enrichment. There was only one significant interaction between nutrient enrichment and wave action (for brown filamentous algae in autumn) and also only one significant main effect of the wave treatment (for bushy brown algae in autumn). Surprisingly, the high nutrient treatments supported a higher diversity of macroalgae, whereas the low nutrient treatments generally showed higher production rates. Moreover, significantly negative correlations were found between macroalgal diversity and primary productivity in both summer and autumn. This study shows that it is the biological components of the communities subjected to external forcing (nutrient addition or decreased wave action) that regulate production and this contradicts the common misperception that resource production in natural systems simply can be fast-forwarded by fertilization. The negative relationships between diversity and productivity, although a consequence of unexpected results for diversity and production, are also novel and hint towards species identities having more important functional consequences than general species dominance patterns and the amount of species per se. These results also emphasise the context dependency of findings within the field of biodiversity and ecosystem functioning.     

Porites and the Phoenix effect: unprecedented recovery after a mass coral bleaching event at Rangiroa Atoll,French Polynesia

The 1997/1998 El Niño Southern Oscillation (ENSO) was the most severe coral bleaching event in recent history, resulting in the loss of 16 % of the world’s coral reefs. Mortality was particularly severe in French Polynesia, where unprecedented mortality of massive Porites was observed in lagoonal sites of Rangiroa Atoll. To assess the recovery of massive Porites 15 years later, we resurveyed the size structure and extent of partial mortality of massive Porites at Tivaru (Rangiroa). Surveys revealed an abundance of massive Porites colonies rising from the shallow lagoonal floor. Colony width averaged 2.65 m, reaching a maximum of 7.1 m (estimated age of ~391 ± 107 years old). The relative cover of recently dead skeleton within quadrats declined from 42.8 % in 1998 to zero in 2013, yet the relative cover of old dead skeleton increased only marginally from 22.1 % in 1998 to 26.1 % in 2013. At a colony level, the proportion of Porites dominated by living tissue increased from 34.9 % in 1998 to 73.9 % in 2013, indicating rapid recovery of recent dead skeleton to living tissue rather than transitioning to old dead skeleton. Such rapid post-bleaching recovery is unprecedented in massive Porites and resulted from remarkable self-regeneration termed the ‘Phoenix effect’, whereby remnant cryptic patches of tissue that survived the 1997/1998 ENSO event regenerated and rapidly overgrew adjacent dead skeleton. Contrary to our earlier predictions, not only are large massive Porites relatively resistant to stress, they appear to have a remarkable capacity for recovery even after severe partial mortality.     

The effects of sand movement on intertidal seaweeds and selected invertebrates at Bound Rock,New Hampshire,USA

A seasonal study of sand movement and the benthic intertidal organisms at Bound Rock, New Hampshire, USA was conducted between November, 1973 and February, 1975. The site is subjected to irregular sand fluctuations, as well as diurnal, neapspring and major summer sand intrusions. The abundance and distribution of intertidal species was interrelated with the historical sand fluctuations at the area. For example, the lower limits of Mytilus edulis, Balanus balanoides and Porphyra umbilicalis approximated the highest summer sand elevations. Highly abraded rock surfaces in the lower intertidal zone were dominated by opportunistic annuals (e.g. Enteromorpha spp.) and perennial psammophytic or sand-loving seaweeds (e.g. Ahnfeltia plicata and Sphacelaria radicans). Overall, the intertidal seaweed populations at Bound Rock showed a lower number of perennials and fewer species than adjacent rocky shores. The low species diversity of seaweeds at the study site is attributable to unstable environmental conditions and a limited number of habitats. Even so, several psammophytic microhabitats are evident at Bound Rock depending upon the extent of sand burial and abrasion. The morphological and reproductive adaptations of several psammophytic species are discussed.Published with the approval of the Director of the New Hampshire Agricultural Experiment Station as Scientific Contribution No. 812.Scientific Contribution No. 53 of the Jackson Estuarine Laboratory.     

Abundance,diversity and temporal variability in a California intertidal nudibranch assemblage

A 40-month study of a nudibranch assemblage within a 250 m² intertidal area at Asilomar State Beach, California, USA, indicated that dominance was shared among 9 species which comprised 87% of the total number of individuals enumerated. These 9 species also showed high frequency of occurrence in the area. The number of nudibranch species counted per month was shown to be independent of weather and tide conditions, but the numbers of individuals and diversity values were correlated with wave action, lower values and numbers being tabulated during times of storms. Cumulative plots of diversity and species number based upon 15 min enumeration times indicated adequate assessment of diversity and species number after 60 min of sampling. The number of species and individuals and the diversity varied from month to month over the 40 months, but these variations were shown to be statistically insignificant or to be correlated with weather. Hence, it was concluded that the assemblage was a stable one, structured primarily by the 9 dominant species. No statistically significant seasonal or annual changes were observed in the assemblage. Diversity was found to correlate most highly with species number and less with number of individuals. The average number of species found per month was 13, the average number of individuals 104, and the average monthly diversity (HH}) was 1.82. Significant positive and negative correlations between abundances of certain species were found. These were attributed to occurrence of prey items or to synchrony of reproduction. There was no evidence of migration, and resident status for one dominant species was established. Comparison of the present study with earlier work suggested that a stable nudibranch assemblage was not merely a local phenomenon.     

Defense by exploitation in the Florida carpenter ant, Camponotus floridanus, at an extrafloral nectar resource

When resources in a territory have a patchy distribution, intruders may successfully exploit unguarded patches. In such cases, territory owners may use exploitative strategies to reduce the gains of the intruders. The territorial ant Camponotus floridanus attends the leaf nectaries of Urena lobata, which are also visited by the ant Pseudomyrmex mexicanus and other intruders. Residents visited the nectaries at a high rate and in a systematic way, and thereby depressed the mean standing crop per nectary. This reduces the gains of randomly visiting intruders which obtain the mean standing crop per nectary visit. Two or three residents were present on large plants and kept the mean standing crop at the same low level as at small plants with a single ant. This is an ideal free distribution of the ants. The resident ants visited the nectaries at a rate which increased in proportion to the nectar production per nectary. This is the expected systematic visitation when nectar production varies between nectaries. It is suggested that systematic visitation and maximization of the visitation rate are evolutionarily stable strategies in both residents and intruders. However, the intruders are constrained by the residents, so that they visit the nectaries less frequently and in a random manner, and thus have a lower gain rate. When the resident was temporarily absent, the intruders visited the nectaries at a high rate and systematically. Received: 31 May 1999 / Received in revised form: 15 November 1999 / Accepted: 6 December 1999     

Effects of nutrients, herbivory, and depth on the macroalgal community in the rocky sublittoral

We studied the interacting roles of nutrient availability and herbivory in determining the macroalgal community in a rocky littoral environment. We conducted a factorial field experiment where we manipulated nutrient levels and herbivory at two sublittoral depths and measured macroalgal colonization and the following young assemblage during the growing season. At the community level, grazing reduced algal colonization, though the effect varied with depth and its interaction with nutrient availability varied in time. In shallow water, the total density of macroalgae increased in response to nutrient enrichment, but the ability of grazers to reduce macroalgal density also increased with the nutrient enrichment, and thus, the community could not escape from the top-down control. In deep water, the algal density was lower, except in July when nutrient enrichment caused a very dense algal growth. Grazing at the greater depth, though effective, was generally of smaller magnitude, and in July it could not limit algal recruitment and growth. Species richness peaked at the intermediate nutrient level in deep but not in shallow water during most of the growing season. Grazing had no effect on diversity of the algal community at either depth and only a minor effect on species richness at the greater depth. Opportunistic and ephemeral algae benefited from the nutrient enrichment but were also grazed to very low densities. Slowly growing and/or perennial species colonized poorly in the nutrient enriched treatments, and depending on the species, either suffered or indirectly benefited from herbivory. For all species, effects of nutrients on colonization depended on depth; usually both nutrient and herbivory effects were more pronounced at the shallow depth. We conclude that grazers are able to reduce macroalgae over a large range of nutrient availabilities, up to 12-fold nutrient enrichment in the current experiment, and that the sublittoral depth gradient generates variation in the algal community control exerted by both herbivory and nutrient availability. Thus temporal and spatial variability in both top-down and bottom-up control and in their interaction, especially along the depth gradient, may be crucially important for producer diversity and for the successional dynamic in a rocky sublittoral environment.