Environment‐dependent variation in selection on life history across small spatial scales

Variation in life‐history traits is ubiquitous, even though genetic variation is thought to be depleted by selection. One potential mechanism for the maintenance of trait variation is spatially variable selection. We explored spatial variation in selection in the field for a colonial marine invertebrate that shows phenotypic differences across a depth gradient of only 3 m. Our analysis included life‐history traits relating to module size, colony growth, and phenology. Directional selection on colony growth varied in strength across depths, while module size was under directional selection at one depth but not the other. Differences in selection may explain some of the observed phenotypic differentiation among depths for one trait but not another: instead, selection should actually erode the differences observed for this trait. Our results suggest selection is not acting alone to maintain trait variation within and across environments in this system.     

Grazing damage and encrustation by an invasive bryozoan reduce the ability of kelps to withstand breakage by waves

Increased breakage of macroalgal fronds during large wave events can significantly reduce canopy cover and biomass. We examined the effects of encrustation by the invasive bryozoan Membranipora membranacea and damage by the snail Lacuna vincta on the ability of kelp blades (Saccharina longicruris, Laminaria digitata, and Laminaria complanata) to withstand wave forces. Using standard materials testing procedures, we documented significant reductions in the maximum stress before breakage, toughness, and extensibility of blade material following bryozoan encrustation. Histological sections of blade tissue indicated a significant degradation of the outer layers of cells following prolonged encrustation by M. membranacea as a likely cause of weakening. Full-thickness perforations and partial-thickness grazing scars also reduced blade strength, suggesting that grazing damage can initiate cracks that lead to blade breakage. Our findings provide a mechanistic link between the damaging effects of mesograzers and encrusting bryozoa on their algal hosts and the export of detrital material from subtidal kelp beds.     

Scuba diving damage on coralligenous builders: Bryozoan species as an indicator of stress

The effects of scuba diving activities on coralligenous concretions builders in the Mediterranean Sea are reported for a non-marine protected area (non-MPA). Five erect bryozoan species, which are particularly vulnerable due to their fragile structure, were investigated: Adeonella calveti, Myriapora truncata, Pentapora fascialis, Reteporella grimaldii, and Smittina cervicornis. These species were sampled at frequently and rarely visited diving sites to the maximum depths reachable with the most common recreational diving certifications (i.e., 40 m). To do this, a non-destructive photographic sampling technique was used to minimize any impact on the benthic communities. For each sample, the number of colonies, their width, coverage, type of injury and degree of exposure were quantified using image analysis software. The number of species differed between the frequently and rarely visited diving sites, and frequently visited sites had significantly lower number of colonies and reduced colony width. Species-specific response to stress was associated with colony morphology and skeleton rigidity. Both the type of injury and colony position within the coralligenous habitat were affected by the presence of scuba divers. A linear model was proposed to analyse the response of bryozoans to scuba diving pressure. The results reported here suggest that P. fascialis and R. grimaldii could serve as robust species indicators of diving stress.     

Invasive species as a new food source: does a nudibranch predator prefer eating an invasive bryozoan?

Membranipora membranacea is an invasive bryozoan that was first found in the Gulf of Maine in 1987 and within two years became the dominant organism living on kelps. Membranipora may have become dominant so quickly because it had little competition in a relatively unoccupied niche; however, lack of predation has also probably played a major role. Where Membranipora is native, there is usually a specialist nudibranch predator that keeps the population in check. For example, in European populations, the nudibranch Polycera quadrilineata prefers Membranipora while Onchidoris muricata is known to prefer another bryozoan, Electra pilosa. Electra, Membranipora, and Onchidoris are all now found in the Gulf of Maine while Polycera is not. We tested whether Onchidoris would (1) eat Membranipora at all, (2) eat Membranipora and Electra at different rates, and (3) show a preference for eating Membranipora or Electra when given a choice. We found that Onchidoris does eat Membranipora, and it generally eats Membranipora faster than Electra. However, when given a choice, Onchidoris prefers Electra. Onchidoris typically reproduces in the spring and grows over the fall and winter, but has recently been found reproducing in the winter in New Hampshire. Although it does not survive the winter as well as Electra, Membranipora is the dominant organism living on many macroalgae in the late summer and fall. Thus, the large Membranipora food source now available in the summer and fall may allow Onchidoris to reproduce earlier.     

GENETIC VARIATION AND POLYMORPHISM IN THE INDUCIBLE SPINES OF A MARINE BRYOZOAN

Of particular value in understanding the evolution of genotypes with broad phenotypic ranges (phenotypic plasticity) are the few examples of organisms with adaptive plasticity, such as those that develop a defensive morphology in response to cues from predators. We know little about the heritability of inducible defensive characters or the range of phenotypes available for selection to act on in the field. Membranipora membranacea is a colonial marine bryozoan that produces spines within two days of exposure to waterborne predator extracts. Surveys done in 1993 and 1995 showed that the population at Friday Harbor Labs, Washington, was polymorphic for inducible spine type and was composed of a constitutively spined type that produced spines in the absence of a predator stimulus, an unspined phenotype that did not produce spines irrespective of a stimulus, and an inducibly spined phenotype that would produce spines if exposed to the appropriate cue. In 1995, the frequencies of these types were determined through a laboratory common-garden experiment; 178 colonies were cultured from metamorphosis through approximately 30 days and then exposed to the cue. The inducible type was the dominant, comprising 80.3% of the population. The constitutive type made up 6.2% of the population, and the remaining 13.4 % was the unspined type. The frequency of the three types was similar to a preliminary trial of the experiment run in 1993. Experiments also showed that the lengths of the spines of the inducible type varied continuously among genotypes. To assess causes of variation in the inducible spine response and its clonal heritability, 16 clones were subdivided and grown in a common environment and exposed to a single dosage of spine inducing substance (SIS). Spine length showed high clonal heritability. The range of colony responses from a single environment varied from relatively unresponsive to highly responsive colonies with a very low threshold of response. Norms of reaction were quantified for spine lengths of inducible genotypes originating from two field environments by testing them in a concentration series of SIS. Both spine length and spine type varied with concentration of inducer. Within a clone, colonies were more likely to produce membranous spines than corner spines at higher concentrations. At low concentrations, only straight spines were produced. This study showed that populations of M. membranacea at Friday Harbor are a mix of inducible, nonspined and constitutively spined individuals. Even the inducible individuals showed high heritable variation in the length of spine activated, suggesting that there is considerable scope for the evolution of this character. A norm-of-reaction experiment further showed that the type of spine produced, membranous or corner, varied with the concentration of the cue. Factors maintaining the polymorphism and the broad range of genotypes could include high costs of defending the spined types coupled with a shifting biotic regime.     

Phylogeny reconstruction in the neogene bryozoan metrarabdotos: A paleontologic evaluation of methodology

An adequate stratigraphic record can not only aid in both cladistic and stratophenetic reconstruction of phytogenies, but can also serve in estimating the temporal consistency of the resulting phylogenetic trees. For hypothetical data sets, cladistically constructed trees can be as consistent with the temporal distribution of sampled populations or species as those constructed stratophenetically. Empirical testing in taxonomic groups with sufficiently dense fossil records is needed to show whether, and under what conditions, this potential can be realized. A stratophenetic tree and cladistic trees based on several approaches to character weighting were constructed for Caribbean Neogene species of the bryozoan Metrarabdotos with multiple‐character data from closely spaced sequential populations. The modular morphology and highly punctuated evolutionary pattern of these species blur the distinction between continuous and discrete characters, so that all available characters are potentially of equal significance in establishing phytogenies, rather than just those with discrete states conventionally used in cladistic analysis. However, only the cladistic trees generated with all characters weighted to emphasize contribution to species discrimination have temporal consistencies that are clearly significant statistically and approach that of the stratophenetic tree in magnitude. These results provide a start toward establishing general guidelines for cladistic analysis of taxa with stratigraphie records too sparse for stratophenetic reconstruction.     

Evolutionary palaeoecology of symbioses between bryozoans and hermit crabs

Modern hermit crabs form associations with many organisms which encrust, bore into, or cohabit the living chambers of gastropod shells occupied by the crabs. Among these hermit crab symbionts are bryozoan species which develop massive, commonly multilayered, colonies encrusting hermit crab shells. These colonies extend the living chamber of the crab through a characteristic process of helicospiral tubular growth originating from the shell aperture. The scant information available on the ecology of Recent bryozoan‐hermit crab symbioses is reviewed. Symbioses have been recorded from intertidal to upper slope environments, and from tropical to cold temperate zones. None of the hermit crab species are obligatory symbionts of bryozoans, and the majority of the modern bryozoan species involved are also not obligatory symbionts. Fossil examples always lack the hermit crabs, which have a poor fossilization potential; however, the distinctive tubular growth pattern and other features of the bryozoans enable recognition of ancient examples of the symbiosis. The earliest inferred associations between bryozoans and hermit crabs date from the Mid Jurassic, but associations remained uncommon until the Neogene. A remarkably wide taxonomic diversity of Recent and fossil bryozoans are known or inferred symbionts of hermit crabs. The broad evolutionary pattern of the association demonstrates multiple originations of the symbiosis by bryozoans belonging to at least 5 cyclostome and 12 cheilostome families. Only the Miocene‐Recent cheilostome family Hippoporidridae has an evolutionary history closely tied to symbiosis with hermit crabs. There is no evidence for coevolution.     

Bryozoaires et Brachiopodes des “Calcaires bajociens a Bryozoaires” des Causses (France) et leur paléoécologie

Two Lower Bajocian new localities, with many silicified bryozoans and brachiopods have been discovered in Causses (France): Saint-Rome-de-Cernon (Aveyron) and Mende (Lozère). The study of the faunas allows to increase the anterior knowledges supplied by the study of localities in the South of Causses (neighbourhood of Alzon and Sumène = North of the Seuil sud-caussenard).The interest is, at first, the distance between the localities: Mende in the North, Saint-Rome-de-Cernon in the West. Moreover, in the second, well-preserved faunas were fossilized without transport. Bryozoans and brachiopods are very numerous. Large and complete bryozoan colonies have been get out by action of HCl. Fine internal structures like rhynchonellid Nannirhynchia or like thecidean transversarium and hemispondylium have been studied. At last, observations and paleoecological hypothesis are drawn, likewise comparison between three localities or locality-groups known in the Causses basin.The fauna from Saint-Rome-de-Cernon is biocoenosis, the substratum was soft, the hydrodynamism low and the depth, according to lamellibranchs, about 200 meters. The beds with bryozoans and small brachiopods contain a fauna with two predominant bryozoan species: Ceriocava straminea and Mesenteripora wrighti. These two species could built large colonies, in spite of fine granulometry of the sedimentary bottom. These colonies provided supports to less adapted bryozoans, to thecidids and to lamellibranch Lopha. The beds with terebratulids and bryozoans contain lesser bryozoans and thecidids. Terebratulids are the main group and are fixed with their peduncle to organic support above the irregular deposits. Beside these benthic animals, the rhynchonellid Nannirhynchia campestriensis is epibiontic on gorgonids or epiphytic on floating sea-weeds.On the contrary, all fossils from Mende have been more or less carried, excepted large sponges perhaps. The most numerous bryozoans (Ceriocava straminea, Mesenteripora wrighti) and numerous Lenticulines (Foraminifera) lived on hard bottom with moving water in community with crinoids. It is possible that a smaller part of bryozoans, requiring lower hydrodynamism, lived, either in sheltered parts of the crinoid community or closer to the sedimentation zone. Thecidids were fixed either on bryozoans in crinoid communities or on shelly remains and on bryozoans nearer to the sedimentation zone. Like in Saint-Rome-de-Cernon, Nannirhynchia campestriensis was epibiontic, or epiphytic, or both.In comparison with the whole localities from the Seuil sud-caussenard, we notice the simplicity of Saint-Rome-de-Cernon and Mende. The localities from the Seuil sud-caussenard were assemblages proceeding from 2 or 3 biotops. On the contrary, at Saint-Rome-de-Cernon we have a biocoenosis and at Mende a thanatocoenosis supplied by one biotop almost.     

Facilitation and inhibition of larval attachment of the bryozoan Bugula neritina in association with mono-species and multi-species biofilms

In this study, we investigated the effect of mono-species and multi-species biofilms on larval attachment of the bryozoan Bugula neritina. The effect of biofilms was examined through a double-dish choice bioassay in which larvae were given the choice of attaching either to a clean surface of a container or to surfaces covered with biofilms. Larvae attached in response to mono-species biofilms of 5 out of 7 bacterial isolates from a subtidal region, but they avoided surfaces covered by biofilms of 7 out of 8 isolates obtained from an intertidal region. In the follow-up choice experiments with multi-species biofilms developed for 2 days, 7 days, 14 days, 28 days and 30 days, larvae preferentially attached to filmed surfaces over the unfilmed surfaces. When biofilms from 2 different tidal regions (intertidal and subtidal) were offered as choices in the double-dish bioassay, larvae in all cases attached on the subtidal biofilms. Two-day-old subtidal biofilms with low densities of bacteria induced significantly higher (p < 0.05) attachment than did 30- day-old intertidal biofilms, which had high bacterial density. Terminal Restriction Fragment Polymorphism (T-RFLP) analysis revealed that the bacterial communities were substantially different in the subtidal and intertidal regions during all periods of the experiment. Attachment of B. neritina on subtidal biofilms did not depend on the bacterial density but rather was negatively correlated with diatom density, thickness of the exopolysaccharide layer and biofilm age. Our results suggest that the larvae of B. neritina can discriminate between biofilmed and clean surfaces and between biofilms developed under different tidal zones.     

Using clones and copper to resolve the genetic architecture of metal tolerance in a marine invader

The global spread of invasive species may be facilitated by adaptation to the practices that humans use to manage those species. For example, marine invertebrates that adapt to metal-based antifouling biocides on ship hulls may be more likely to be introduced to and establish in metal-polluted environments. We tested this idea by studying clonal variation in tolerance to, and ability to recover from, exposure to copper in a widespread invasive marine bryozoan, Watersipora subtorquata. We cloned colonies of this organism to independently test multiple environments in a genotype by environment design, and then created a genetic variance-covariance matrix. Genotypes were exposed to a gradient of copper concentrations and growth measured during exposure and after a recovery period. There was a significant genotype × environment interaction in growth during exposure and recovery. We found clonal variation in tolerance and ability to recover from exposure to copper, with growth during exposure apparently trading off against growth after exposure. A weak genetic correlation between growth during and after exposure further indicated that they are separate traits. Overall, the genetic variation within this population indicates that there is considerable potential for adaptation to copper, but this comes at a cost to growth in unpolluted environments.