Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

Desiccation,predation, and mussel-barnacle interactions in the northern Gulf of California

Summary Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

Effects of Adult Dreissena polymorpha on Settling Juveniles and Associated Macroinvertebrates

The impact of Dreissena polymorpha settlement on recruitment of juvenile mussels and density of other macroinvertebrates was studied in field experiments using blank concrete blocks and tiles (control), blocks and tiles with attached empty zebra mussel shells, and blocks and tiles with attached living mussels. On blocks, dominant invertebrate taxa showed colonization patterns coinciding with increased habitat complexity owing to zebra mussel settlement or the biodeposition of faeces and pseudofaeces. Adult and especially juvenile zebra mussels preferred blocks with empty shells to blank blocks and blocks with living mussels; this might possibly be caused by a chemical cue that induces gregarious settlement. Lower recruitment on blocks with attached living mussels compared to blocks with only shells could be the consequence of ingestion of larvae by adult mussels and of competition for food. On tiles, the sediments deposited and the organic content of the sediment were investigated. Sedimentation was significantly higher on shell‐only and live‐mussel tiles compared to blank tiles. Organic matter differed significantly between blank and live‐mussel tiles.     

Gracilaria-Mytilus interaction on a commercial algal farm in Chile

Mytilus chilensis is an invertebrate that competes for space with the alga Gracilaria chilensis in farmed areas in Chile and, for this reason, is considered a contaminant organism. Mussel beds are considered to play a role in the regeneration of nitrogen and, as a consequence, they could be an important source of ammonium for the algae. In this study, we manipulated the mussel cover in experimental plots, creating replicated areas with 0%, 30% and 60% coverage. In half of the plots the mussels were killed with a gas torch so their effect would be mechanical, without nutrient regeneration. After 15 days, each plot was planted with 12 G. chilensis bundles (100 g each) per square meter. Ammonium concentrations increased significantly in the waters around the mussel bed in contrast to areas without mussels or dead mussels. Mussel cover had a significant negative effect on the length of the G. chilensis bundles planted in the experimental plots. However, no significant differences were detected between experimental quadrats with live mussels and those with dead mussels during a 5 month period. These results indicate that the mechanical effect of the mussel can, to some extent, be responsible for the decline in G. chilensis abundance in farms where mussel beds have been established.     

Fouling community of the red king crab, <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius 1815), in a subarctic fjord of the Barents sea

We examined the species composition of red king crab (Paralithodes camtschaticus) fouling communities in Dolgaya Bay, a small fjord of the Barents Sea, in August 2005 and 2006. In total, there were 13 species observed on 301 crabs collected from water depths of 5–90 m. Barnacles (Balanus crenatus; prevalence 42.9%) and blue mussels (Mytilus edulis; 11.6%) were the most common epibionts, while amphipods (Ischyrocerus commensalis) were the most common symbionts (28.6%). Infestation rates in Dolgaya Bay were different from those in an “open” area of the Barents Sea (Dalnezelenetskaya Bay), probably due to differences in hydrodynamic conditions. Differences in infestation prevalence and intensity were detected neither between male and female crabs nor between crabs collected at 5–35 m versus 90 m depths. Prevalence of common fouling species increased with host size. Amphipods I. commensalis colonized the carapace and limbs in Dolgaya Bay less frequently than in Dalnezelenetskaya Bay, probably due to interspecific competition with barnacles occupying the dorsal parts of the host. Juvenile barnacles and mussels dominated the fouling communities on the crabs. The age of barnacles did not exceed 2–4 months. However, the presence of 4-year-old mussels suggests that these older mollusks have been directly transferred from mussel beds to the hosts. Our results indicate that colonization by epibionts and symbionts is generally not disadvantageous for the crab hosts, except for some possible negative impacts of amphipods occupying the gills.     

Context-dependent effects of freshwater mussels on stream benthic communities

1. We asked whether unionid mussels influence the distribution and abundance of co‐occurring benthic algae and invertebrates. In a yearlong field enclosure experiment in a south‐central U.S. river, we examined the effects of living mussels versus sham mussels (shells filled with sand) on periphyton and invertebrates in both the surrounding sediment and on mussel shells. We also examined differences between two common unionid species, Actinonaias ligamentina (Lamarck 1819) and Amblema plicata (Say 1817). 2. Organic matter concentrations and invertebrate densities in the sediment surrounding mussels were significantly higher in treatments with live mussels than treatments with sham mussels or sediment alone. Organic matter was significantly higher in the sediment surrounding Actinonaias than that surrounding Amblema. Actinonaias was more active than Amblema and may have increased benthic organic matter through bioturbation. 3. Living mussels increased the abundance of periphyton on shells and the abundance and richness of invertebrates on shells, whereas effects of sham mussels were similar to sediment alone. Differences in the amount of periphyton growing on the shells of the two mussel species reflected differences in mussel activity and shell morphology. 4. Differences between living and sham mussel treatments indicate that biological activities of mussels provide ecosystem services to the benthic community beyond the physical habitat provided by shells alone. In treatments containing live mussels we found significant correlations between organic matter and chlorophyll a concentrations in the sediment, organic matter concentrations and invertebrate abundance in the sediment and the amount of chlorophyll a on the sediment and invertebrate abundance. There were no significant correlations among these response variables in control treatments. Thus, in addition to providing biogenic structure as habitat, mussels likely facilitate benthic invertebrates by altering the availability of resources (algae and organic matter) through nutrient excretion and biodeposition. 5. Effects of mussels on sediment and shell periphyton concentrations, organic matter concentrations and invertebrate abundance, varied seasonally, and were strongest in late summer during periods of low water volume, low flow, and high water temperature. 6. Our study demonstrates that freshwater mussels can strongly influence the co‐occurring benthic community, but that effects of mussels are context‐dependent and may vary among species.     

Stylochus tauricus,a predator of the barnacle Balanus improvisus in the Black Sea

The flatworm Stylochus tauricus Jacubova has been found associated with the barnacle Balanus improvisus Darwin, on which it feeds. The predation rate (the number of barnacles eaten by one polyclad in a month) ranges between 5–10. Inside the empty shells of B. improvisus some egg-plates of S. tauricus were observed. Pelagic Götte's larvae aged 2–3 days possess 4 lobes while those aged 7–8 days have 5 lobes. Flatworms can prey on the young of another species Balanus eburneus Gould, whereas predation on the mussels Mytilus galloprovincialis Lam. is rare. There is a direct correlation between predator abundance and prey ingested.     

Morphological Adaptations of the Mussel Crenomitilus grayanus (Bivalvia) to Attached Life

The morphology of the mussel Crenomytilus grayanus from different biotopes manifesting the adaptation of this mollusk to attached life on the bottom surface was studied. It was established that colonization of rocky coastal areas with active hydrodynamics by mussels is related to retardation of linear growth rates, the formation of convex shells, and vigorous development of retractor muscles of the foot and byssus. An environment protected from the effect of surfs on silted bottoms facilitated the fast growth of mussels, which resulted in the formation of an elegant flat shell. In that biotope mussels are specified by a rather poor development of the muscles responsible for its attachment to the substrate.     

Susceptibility of larval dragonflies to zebra mussel colonization and its effect on larval movement and survivorship

Colonization by the zebra mussel, Dreissena polymorpha, was quantified for five dragonfly species that differed in size and larval habits in a Michigan lake. Both larger size and a non-burrowing habit independently increased susceptibility to colonization. In 2005, over 50% of the final instars of the sprawlers Didymops transversa and Hagenius brevistylus were colonized, as well as younger instars. Rarely colonized were Progomphus obscurus and Dromogomphus spinosus, whose larvae burrow under sand, and the sprawler Epitheca princeps, whose final instars were lightly covered with sand. Hagenius larvae that had been preyed upon carried more mussels than those dying of other causes. More generally, mussel attachment decreased the probability that sprawlers left the water to emerge, the distance that some species traveled before emerging, and the ability of an overturned sprawler to right itself. On average, final instars of Didymops and Hagenius remaining in the water carried three times as many mussels as individuals known to emerge. Compared to uncolonized individuals, Epitheca and Progomphus with mussels emerged closer to the water line. Among colonized Didymops, the distance traveled on land before emerging decreased with increasing mussel load. Of the colonized Didymops that could right themselves, righting time increased with mussel load. Because the two common species of sprawlers were disproportionately colonized, and mussel attachment decreased their chances of emerging, our results suggest that D. polymorpha has the potential to affect the community structure of this guild of aquatic and terrestrial predators. Handling editor: R. Bailey     

Commensals of Mytilus galloprovincialis in the Black Sea: Urastoma cyprinae (Turbellaria) and Polydora ciliata (Polychaeta)

The intensity of infection of the mussel Mytilus galloprovincialis Lamarck in the Black Sea by the turbellarian Urastoma cyprinae (Graff), which lives on its gills, was found to be higher in larger hosts, reaching a maximum in mussels of 50–70 mm length. Greater numbers occured in mussels inhabiting a silty bottom than in cultivated mussels suspended above the bottom. Over the period 1982–1987, U. cyprinae was most numerous in winter and especially so in years that were colder. The spionid polychaete Polydora ciliata (Johnston) also infects M. galloprovincialis, burrowing into the shell. Young spionids of up to 1 mm length occured in mussels with a shell length of 35 mm. Numbers of this commensal reached a maximum in mussels of intermediate size.     

Epiphytic Refugium: Are Two Species of Invading Freshwater Bivalves Partitioning Spatial Resources?

Enumeration of benthic (bottom dwelling) and epiphytic (attached to plants) zebra and quagga mussels (Dreissena polymorpha and D. bugensis, respectively) at Lake Erie near-shore sites in fall of 2000 revealed an unexpected prevalence of the zebra mussel on submerged plants. Even at Buffalo, New York, USA, where benthic dreissenids have been 92–100% quagga mussel since 1996, zebra mussels constituted 30–61% of epiphytes numerically. This may reflect a partitioning of settling space consistent with interspecific competition. A seasonal epiphytic refugium might allow the zebra mussel to persist even where the benthos is almost exclusively quagga mussel. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of environmental stress on intertidal mussels and their sea star predators

Consumer stress models of ecological theory predict that predators are more susceptible to stress than their prey. Intertidal mussels, Mytilus californianus, span a vertical stress gradient from the low zone (lower stress) to the high zone (higher thermal and desiccation stress), while their sea star predators, Pisaster ochraceus, range from the low zone only into the lower edge of the mussel zone. In summer 2003, we tested the responses of sea stars and mussels to environmental stress in an experiment conducted on the Oregon coast. Mussels were transplanted from the middle of the mussel bed to cages in the low and high edges of the mussel bed. Sea star predators were added to half of the mussel cages. Mussels and sea stars were sampled between June and August for indicators of sublethal stress. Mussel growth was measured, and tissues were collected for heat shock protein (Hsp70) analyses and histological analyses of reproduction. Sea stars were weighed, and tissues were sampled for Hsp70 analyses. Mussels in high-edge cages had higher levels of total Hsp70 and exhibited spawning activity earlier in the summer than mussels in the low-edge cages. Sea stars suffered high mortality in the high edge, and low-edge sea stars lost weight but showed no differences in Hsp70 production. These results suggest that stress in the intertidal zone affected the mobile predator more than its sessile prey, which is consistent with predictions of consumer stress models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An indirect effect of biological invasions: the effect of zebra mussel fouling on parasitisation of unionid mussels by bitterling fish

Invasive species represent a major threat with both direct and indirect effects on natural ecosystems, including effects on established and coevolved relationships. In a series of experiments, we examined how the interaction between two native species, a unionid mussel (Unio pictorum) and the European bitterling (Rhodeus amarus), a fish that parasitises unionids, was affected by the non-native zebra mussel (Dreissena polymorpha). The zebra mussel fouls hard substrates, including shells of living unionids, and its presence is often associated with a decrease in population density of native unionid mussels. Bitterling lay their eggs into live unionids and the embryos develop inside their gills. Using a range of zebra mussel densities, we demonstrated that zebra mussel fouling had a negative effect on the number of bitterling eggs inside the mussel host, with abundances of 5–10 zebra mussels (shell size 15–25 mm) per unionid critical for bitterling ability to utilise the host. In a further experiment, we found that bitterling did not discriminate between unfouled unionids and those fouled with five zebra mussels. Most ovipositions into fouled hosts, however, were unsuccessful as eggs failed to reach the unionid gills. We discuss implications of such unsuccessful ovipositions for bitterling recruitment and population dynamics.     

Indigenous microflora and opportunistic pathogens of the freshwater zebra mussel, Dreissena polymorpha

Freshwater fouling invertebrate zebra mussels (Dreissena polymorpha) harbor a diverse population of microorganisms in the Great Lakes of North America. Among the indigenous microorganisms, selective species are opportunistic pathogens to zebra mussels. Pathogenicity to zebra mussels by opportunistic bacteria isolated from the mussels was investigated in this study. Among the more than 30 bacteria isolated from temperature-stressed mussels, Aeromonas media, A. veronii, A. salmonicida subsp. salmonicida, and Shewanella putrefaciens are virulent pathogens to juvenile zebra mussels. Inoculation of a bacterial concentration of A. media, A. salmonicida subsp. salmonicida and S. putrefaciens at 10⁷ cells per zebra mussel resulted in 100% mortality within 5 days, and only 64.9% for A. veronii. In contrast, mortality was less than 12.3% following inoculation of a sterile phosphate buffer solution as a control. In addition, mortality was dependent on the size of the pathogen population used in inoculation and the incubation temperature, indicating the close relationship between the bacterial population and subsequent death. On the mussel tissue, a dense microbial population was evident from the moribund mussels viewed with Scanning Electron Microscope (SEM). Opportunistic bacteria invaded and destroyed the D. polymorpha tissue after 7 days of incubation when the bacterial inoculation was larger than 10⁵ per zebra mussel. Our results suggest that mussels are reservoirs of opportunistic pathogenic microorganisms to aquatic organisms and humans and a better understanding of the microbial ecology of the mussels will provide insights to the possible health hazards from these microorganisms.     

Variation in scope for growth: a test of food limitation among intertidal mussels

Wellington Harbour supports large populations of the mussels Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus that are almost entirely absent from nearby coastal locations in Cook Strait. We calculated scope for growth (SFG) using ambient Cook Strait water over a broad temporal scale and a broad range of seston conditions to determine if negative SFG explains this phenomenon. Although all three mussel species had positive mean SFG values, variation in SFG was high and negative values often occurred: A. maoriana 19.1 J g⁻¹ h⁻¹, 43% of mussels showed negative SFG; M. galloprovincialis 1.26, 52% negative SFG; P. canaliculus 45.6, 27% negative SFG. Negative SFG was most often due to negative absorption efficiency caused by metabolic faecal loss that is characteristic of mussels feeding in environments with low seston quality. From our ecophysiology data we constructed a model to estimate SFG based on physiological responses to the narrow range of seston conditions typical of Cook Strait (Model One), and a model to estimate SFG based on physiological responses of mussels to the broad range of seston conditions typical of Wellington Harbour and Cook Strait (Model Two). We used seston data collected over an 18-month period from sites in Wellington Harbour and Cook Strait to derive 159 estimates of species-specific mussel SFG from both models. Both models produced higher estimates of SFG for mussels in the Harbour compared with those at Cook Strait sites. This was consistent with elevated particulate concentrations in the Harbour than at Cook Strait sites, and in agreement with previous studies. For Cook Strait mussels, both models produced negative estimates of net energy balance for long periods of time (several months), whereas for Harbour mussels negative SFG estimates were generally short in duration. We conclude that our short-term laboratory-based determinations of SFG and our long-term bioenergetics modelling estimates do not conclusively support the hypothesis of food limitation for three coexisting taxa of mussels in the intertidal region of Cook Strait, New Zealand. Handling editor: P. Viaroli     

Guild structure in water mites (Unionicola spp.) inhabiting freshwater mussels: choice,competitive exclusion and sex

Summary Unionicolid water mites inhabit freshwater unionid mussels during the nymphal and adult stages of their life-cycle. Regular sampling of mussels from two sites in St. Mark's River, Fl. established that each of four species of water mite (Unionicola abnormipes, U. fossulata, U. serrata and U. formosa) occurred mainly in one or two of the mussel species available at each site.The role of preference for particular mussel species during host location was assessed for the first three mite species by choice experiments, in which mites were offered different mussel species simultaneously. In five out of six experiments, mites entered normally unused mussels as often as they did normally used ones. Additionally, a sexual difference in choice was found for U. fossulata, with males preferring one mussel species and females showing no preference. One mussel species, (Anodonta imbecilis), normally unused but chosen by mite species during the lab. experiments, is inhabited exclusively by the fourth mite species, U. formosa, in the field. An experiment showed that U. formosa excludes other mite species aggressively from Anodonta imbecilis.The results illustrate the sometimes misleading nature of simple sampling data as an indication of host specificity or host preference in parasites. They suggest also that the population dynamics of some parasites might be more fruitfully compared to unrelated, free-living species than to other parasites.     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

Group living and the effects of spatial position in aggregations of Mytilus edulis

Summary The mussel Mytilus edulis typically occurs in aggregations and several consequences of living in groups were studied. Isolated individuals and individuals associated in relatively small groups (6–9 mussles/group) grew more and therefore had greater reproductive output than mussels associated with relatively large groups of 21–28 individuals. Mussels located in the centers of groups exhibited reduced growth and thus lower reproduction relative to mussels located on the edges of groups whose growth and reproduction was similar to that of isolated individuals. Sampling from natural populations indicated that most mussels grow within the matrix of very large groups and hence will experience reduced growth and reproduction. Patterns of growth exhibited by mussels in association with living and model mussels showed that the adverse effects on growth exhibited by mussels in relatively large groups are not a function of the mere physical relief of a mussel clump, but are caused by some property of living neighbors.Laboratory experiments on mussel predation by the crab Pachygrapsus crassipes indicated that crabs prey disproportionately on mussels growing on the edges of groups.The consequences of group living in mobile and nonmobile organisms are considered, and it is suggested that a greater number of negative effects will arise in groups as mobility decreases. In addition, the noted ecological similarity between groups of sessile organisms and spreading clones and its evolutionary implications are discussed.This is contribution no 171 from the Smithsonian Marine Station at Link Port