Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.     

Sneaking up on ‘enemies’: alleviating inherent disadvantages in competitive outcomes in a nearly 3-million-year-old palaeocommunity from Florida,USA

Bryozoans offer one of the few systems in which competitive interactions for living space can be studied in the fossil record. Here, we describe the outcome of competitive overgrowths in a 3-million-year-old bryozoan palaeocommunity encrusting shells of the bivalve Anomia simplex from the lower Tamiami Formation in Florida (upper Pliocene, Piacenzian). We found that win–lose overgrowths are more common than stand-offs in interspecific encounters, while stand-offs are more common than win–lose overgrowths in intraspecific encounters. We observed more intraspecific encounters and fewer interspecific interactions than expected under a null hypothesis, suggesting that bryozoans of the same species are likely to be clustered. For some species, intraspecific encounters are more likely to result in the apparent fusion of the two colonies, with the development of rows of kenozooids along the contact edge, probably reflecting relatively low dispersal. We also identified some clear winners and some clear losers. A negative correlation was found between the number of colonies observed and the probability of winning for a species, resulting in a dominance of loser species in the assemblage, a pattern previously described as typical for early colonizers of hard substrates. Our results also confirm the finding of earlier studies that having large zooids and, subordinately, multilayered growth are key traits for success in overgrowth competition, with angle of encounter also having an effect for both poor and good competitors that take advantage of ‘attacking’ colonies of other species from the rear and the flank.     

Epifaunal associates of Fucus serratus at Dale, south-west Wales

The macrofauna associated with Fucus serratus at Dale in south-west Wales contained 30 taxa of which five were common: these comprised four encrusting bryozoans and a hydroid. The bryozoans Flustrellidra hispida and Electra pilosa showed significant variations in abundance according to the size and structure of plants colonised. Electra was most abundant on plants where other bryozoans were least numerous. Colony growth in Electra was proportional to the amount of space provided by individual plant segments whilst growth in competitively superior species appeared to be independent of the size of the plant segments colonised. Dynamena pumila was larger and more abundant on larger segment faces where competition was presumably weaker. Each species showed a specific pattern of zonation along the fronds. The dominant species were generally more abundant on the concave surfaces of the plants. There was some evidence for the tendency for certain taxa to co-occur but the degree of association between species was weak and probably of limited biological significance. Species diversity varied with plant structure. More species occurred on concave surfaces at all plant levels but the overall pattern of diversity on the two segment faces was similar; diversity was greatest in the shrubbier mid-frond regions. Differential distribution of the dominant encrusting taxa within this Fucus community results in a considerable degree of niche segregation and ecological isolation.     

Ecology of cryptic coral reef communities. IV. Community development and life histories of encrusting cheilostome bryozoa

Life histories of encrusting cheilostome species from the cryptic reef community at Rio Bueno, Jamaica, were studied on fouling panels over 3 yr. Recruitment and growth were generally slow compared with those reported for temperate cheilostomes. Most species that became abundant and persisted throughout the study did so through relatively rapid growth to a large size by a few successful colonies, rather than by accumulating great numbers of small colonies. This pattern, which reflects a striking increase in maximum survival with increasing colony size, is the basis for the extremely patchy distributions of bryozoans under corals. Reproduction in these species is delayed, and only a few long-lived, large colonies ever reproduce. Only one species, Drepanophora tuberculatum (Osburn), approached the characteristic opportunistic pattern of high recruitment, small colony size, and early reproduction.Grazing and nesting activities of one yellowtail damselfish greatly affected distributions of major taxa and cheilostome species on one set of panels. Species more abundant on grazed panels are more heavily calcified and showed other protective features, compared with species more abundant elsewhere. Despite intensive grazing by the fish, overgrowth interactions occurred frequently on both sets of panels. The fish affected what organisms were present, but did not obviously reduce the amount of overgrowth.     

Overgrowth competition between clades: implications for interpretation of the fossil record and overgrowth indices

Overgrowth interactions (2693 in total) were observed among three major groups (arguably clades) of bryozoans--cheilostomatids (57 species), ctenostomatids (3 species), and cyclostomatids (14 species). The bryozoans studied here occur in shallow water at high-temperate polar latitudes where they encrust hard substrata such as rock piles. The main study site was the intertidal and infralittoral zones of Kodiak Island, Alaska, but observations were also made in similar zones of South Georgia Island and the Falkland Islands in the South Atlantic Ocean. Cheilostomatids dominated the number of species, individuals, and interactions at all depths. Intraclade interactions formed 73.7% of the encounters for cheilostomatids, 1.6% for ctenostomatids, and 5.7% for cyclostomatids. The competitive ranking of the three clades was broadly ctenostomatids > cyclostomatids > cheilostomatids. Significantly, these results contradict all previous quantitative studies of bryozoan overgrowth, in which cheilostomatids are reported to overgrow cyclostomatids at a higher rate. From these studies and the literature, we calculated win indices to vary from 0 to 0.42 for living cyclostomatids, from 0.08 to 0.9 for living cheilostomatids, and from 0.25 to 0.75 for living ctenostomatids. The win indices of cyclostomatid and cheilostomatid clades show significantly more variation in living assemblages than in fossil assemblages. This disparity may be due to differential preservation (polar and subpolar assemblages last less than 4 years). The diversity was very high in terms of both species richness and interaction types (outcomes between competitor pairs). Comparison with the literature suggests the possibility that nearshore diversity of bryozoans may be bimodal (have two peaks) between high arctic and antarctic latitudes. Indices of success in overgrowth competition have been constructed in various ways. For cheilostomatids, the method of calculation had little influence on the ranking of representatives. In contrast, the apparent success of ctenostomatids and cyclostomatids varied hugely with how the index was calculated. This inconsistency is due to the use of very different strategies in overgrowth competition; among the two latter groups, many interactions involve tied outcomes.     

Spatial competition among clonal organisms in extant and selected paleozoic reef communities

Summary Occurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are common in reef deposits due to biostratinomic feedback, i.e. dense skeletal accumulations provide hard substrates for clonal recruitment, thus facilitating greater frequency of live-dead encounters than in lower biomass level-bottom communities dominated by solitary organisms. Differentiating between these two types in ancient reefs is difficult, often impossible. Most live-live interactions among clones in extant reef communities involve competition for space. Clonal spatial competition is divisible into four types: 1) direct-aggressive: encrusting overgrowth; 2) indirect-passive: depriving neighbors of resources, chiefly sunlight, by growth above them; 3) stand-off: avoidance of competition by organisms adopting positions that avoid or minimize direct polyp/zooid contact; and 4) overwhelming: one clone/ species volumetrically or numerically overwhelms the other, meeting minimal resistance. Despite class-order level differences in taxa, our results indicate that extant analogs, based on the arrangement and distortion of skeletons, are valuable for recognizing live-live interactions in Silurian and Carboniferous reefs and interpreting the types of spatial competition represented. Comparison of overhead (plan) views of live-live coral competition in Polynesian reefs with vertical sections of Silurian and Carboniferous sponge-dominated reefs and biostromes suggests that direct-aggressive competition is more common among extant than among Paleozoic reef-builders. Stand-offs showing clone margin distortion and overwhelming with minor skeletal distortion are most common in our fossil examples and probably relate to the dominance of these reefs by sponges. Success by extant sponges in spatial competition is largely due to allelochemical deterrence which may explain the predominance of stand-off and overwhelming confrontations in fossil sponges rather than tentacle-mesentery based direct aggression among extant corals and bryozoans.     

Effects of two marinas on the composition of fouling assemblages

The composition of fouling assemblages was surveyed inside and near two fully enclosed marinas using settlement plates. The location of a plate, inside or outside the marina, influenced the abundances of four functional groups of fouling organisms (solitary ascidians, arborescent bryozoans, encrusting bryozoans and colonial ascidians). Transplantation of mature assemblages revealed that reductions in the abundance of arborescent bryozoans inside marinas might be explained by increased growth and recruitment of these bryozoans outside the marina. Surveys of settlement revealed decreased recruitment of encrusting bryozoans inside the marinas, a result consistent with patterns of adult abundance. It is proposed that an increased abundance of solitary ascidians inside marinas may be due to decreased competition. A second survey of adult assemblages was performed with multiple ‘Outside’ sites per marina. Effects of location consistent with the first survey were found for arborescent bryozoans, and in one marina area for solitary ascidians and encrusting bryozoans, but not in the other. Although mechanisms can be proposed to explain the effects of the marina (inside or outside) on the abundances of solitary ascidians, arborescent bryozoans and encrusting bryozoans, the second survey revealed that the effects may vary among marinas.     

Larger foraminifera as a substratum for encrusting bryozoans (Late Oligocene,Tethyan Seaway,Iran)

Considering the diversity and abundance of larger foraminifera examined from a wide range of Late Oligocene to Early Miocene palaeoenvironments in the Tethyan Seaway, encrusting bryozoans make extremely little use of their tests as substratum. Significant encrustations by bryozoans were exclusively found on large (ø c. 6 cm), undulating tests of Lepidocyclina spp., on which, however, a remarkable 34 taxa of encrusting bryozoans were recorded. This shallow-water fauna of Chattian age was analyzed in respect of the bryozoan taxa present, colony growth type, and mode of budding, colony size, as well as onset of reproduction. Taxic and morphological similarities between the fossil assemblage and modern faunas encrusting mobile substrata indicate a long history of bryozoans as part of the interstitial habitat, while the tests of certain larger foraminifera may have played a significant role in the evolution of shallow-water bryozoans by providing substrata for encrusting species in otherwise unfavorable environments.     

How competitive intransitivity and niche overlap affect spatial coexistence

Competitive intransitivity is mostly considered outside the main body of coexistence theories that rely primarily on the role of niche overlap and differentiation. How the interplay of competitive intransitivity and niche overlap jointly affects species coexistence has received little attention. Here, we consider a rock–paper–scissors competition system where interactions between species can represent the full spectra of transitive–intransitive continuum and niche overlap/differentiation under different levels of competition asymmetry. By comparing results from pair approximation that only considers interference competition between neighbouring cells in spatial lattices, with those under the mean-field assumption, we show that 1) species coexistence under transitive competition is only possible at high niche differentiation; 2) in communities with partial or pure intransitive interactions, high levels of niche overlap are not necessary to beget species extinction; and 3) strong spatial clustering can widen the condition for intransitive loops to facilitate species coexistence. The two mechanisms, competitive intransitivity and niche differentiation, can support species persistence and coexistence, either separately or in combination. Finally, the contribution of intransitive loops to species coexistence can be enhanced by strong local spatial correlations, modulated and maximised by moderate competition asymmetry. Our study, therefore, provides a bridge to link intransitive competition to other generic ecological theories of species coexistence.     

Paleoecology of Sphenothallus on an Upper Ordovician hardground

A hardground from the Upper Ordovician Dillsboro Formation near Dillsboro, Indiana, U.S.A., preserves an assemblage of encrusting and boring fossils on both top and bottom surfaces. The slab is inferred to have been an undercut ledge, and the dominant fossils of the assemblage, holdfasts of the tube-building worm Sphenothallus and trepostome bryozoans, are prevalent on both sides. The clumping of Sphenothallus holdfasts has been statistically demonstrated using a nearest-neighbor technique. Sphenothallus has also been shown to withstand overgrowth in interactions with bryozoans.     

Operculate cyclostome bryozoans (Eleidae) from the Bohemian Cretaceous

Eleidae (‘melicerititids’) are unusual cyclostome bryozoans possessing operculate zooids and mandibulate polymorphs (‘eleozooids’) resembling the avicularia of cheilostome bryozoans. Here we describe 13 eleid species from the Cenomanian-Turonian of the Bohemian Cretaceous basin in the Czech Republic. Three species of Meliceritites are new: Meliceritites kankensis, M. stillativa and M. upohlavyensis. Reptoforicula gen. nov. is introduced for free-walled eleids with multilamellar encrusting colonies, containing the type species Reptoforicula zbyslavensis sp. nov. and R. vodrazkai sp. nov. Type specimens of three species erroneously assigned to Eleidae are refigured and their true affinities discussed. This study underscores the value of eleozooids in distinguishing between otherwise closely similar eleid species. The Bohemian late Cenomanian-early Turonian eleid fauna is dominated by encrusting species, a pattern that is typical of nearshore localities and contrasts with offshore chalks where erect, vincularian species are more numerous.     

Dissecting the role of transitivity and intransitivity on coexistence in competing species networks

It is well established that intransitively assembled interaction networks can support the coexistence of competing species, while transitively assembled (hierarchical) networks are prone to species loss through competitive exclusion. However, as the number of species grows, the complexity of ecological interaction networks grows disproportionately, and species can get involved simultaneously in transitive and intransitive groups of interactions. In such complex networks, the effects of intransitivity on species persistence are not straightforward. Dissecting networks into intransitive/transitive components can help us to understand the complex role that intransitivity may play in supporting species diversity. We show through simulations that those species participating in the largest group of intransitive interactions (the core of the network) have high probabilities of persisting in the long term. However, participation in a group of intransitive interactions other than the core does not always improve persistence. Likewise, participating in transitive interactions does not always decrease persistence because certain species (the satellites) transitively linked to the core have also a high persistence probability. Therefore, when networks contain transitive and intransitive structures, as it can be expected in real ecological networks, the existence of a large intransitive core of species can have a disproportionate positive effect on species richness.     

Competition within and between encrusting clonal invertebrates

Colonies of encrusting marine invertebrates are tractable models for the study of competition, because of the relative ease with which observations can be made on the frequency and outcome of overgrowth interactions. Studies of intraspecific competition have found that competition is predicated upon a genetically controlled recognition event, which results in either fusion or rejection. Data are rapidly accumulating in two model systems showing that fusion is associated with somatic cell parasitism and that rejection is associated with overgrowth. Thus, encounters between conspecifics define a choice: to compete at the level of the cell lineage or to compete at the level of the colony. Fusion-rejection genes act to control the units (or targets) of selection.     

A new encrusting interstitial marine fauna from Brazil

Abstract. This paper reports the second occurrence of a sand‐grain encrusting interstitial epifauna dominated by bryozoans and polychaetes at a site thousands of kilometers from the first described occurrence of such a fauna 20 years ago. Such faunas seem to have gone almost unrecorded in the marine ecological literature, but they are potentially geographically widespread and ecologically significant, deserving recognition and further study by benthic ecologists. Although rooted‐erect and free‐living lunulitiform bryozoans can be abundant in soft‐bottom habitats, the presence of encrusting forms was, until recently, considered to be limited to patches of hard substrata. In 1985 and 1988, a new and seemingly unique habitat for encrusting bryozoans and other organisms on single grains of shell or sand was reported from the coastal waters of Florida, USA. Here we report a second discovery of an interstitial encrusting fauna from the continental shelf off the state of São Paulo, Brazil. In addition to the cupuladriid Discoporella umbellata, several species of bryozoans (9 cheilostomes, 3 ctenostomes, and 1 cyclostome) were found encrusting on or boring into sand grains from the 4 stations examined. Four species were found exclusively on sand to gravel size grains. The most abundant colonies, with ～1300–1500 colonies m^?2, belonged to a new species of Cleidochasma. New species of Trypostega and Reginella, each with up to 200–300 colonies m^?2, were also discovered. The grain‐encrusting bryozoans were characterized by their small size, and by the fact that sexual reproduction was initiated very early in colony growth; brood chambers (for the development of embryos into larvae) occurred in colonies having only a few zooids. Colonies of boring ctenostome and cheilostome bryozoans were even more abundant than those of grain encrusting forms, being present in almost every piece of shell (～5000–5500 colonies m^?2). The fauna also included representatives of other groups of encrusting organisms, especially tubeworms (11,000–13,000 tubes m^?2). Planned work on samples from additional stations on the São Paulo shelf will no doubt yield a larger number of species from various taxa and perhaps show some overlap in sand fauna species between the Brazilian and Floridian sites. In addition to the unique species of single grain encrusters, colonies of bryozoan species characteristic of larger subtidal hard substrata were also found on sand or gravel size grains, indicating that an interstitial refuge may be available to some epifaunal taxa and suggesting that this interstitial refuge, which remains almost completely unknown to benthic ecologists, may play a large role in determining distributions of those taxa.     

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.     

Bryozoan (Stenolaemata) records from the upper Callovian (Middle Jurassic) of the Moscow region

Encrusting bryozoans (Stenolaemata, Tubuliporida) discovered from upper Callovian deposits (Middle Jurassic) near the town of Kolomna in the Moscow region are described. They belong to two new species: Microeciella kolomnensis sp. nov. and Diplosolen akatjevense sp. nov. These bryozoans encrust a fragment of the large shell of cephalopod mollusk (ammonite), which is an unusual substrate not only for bryozoans but also for the other encrusting organisms.     

Settlement patterns of species in a marine fouling community and some mechanisms of succession

The succession of fouling organisms that settled on asbestos plates immersed at Garden Island. Western Australia, was partially caused by the settlement patterns of the species involved. Both temporal variations in the abundance and continuity of settlement of each species and the age of the communities in which each species settled abundantly contributed to producing the succession. Additionally, temporal variations in the settlement of each species probably resulted in variability in the pattern of succession.Balanus and Spirorbis, the two earliest species in the succession, settled continuously and abundantly so that they immediately occupied a substantial area. The species which predominated later in the succession, Anomia, Ostrea, encrusting bryozoans, and Mytilus, settled discontinuously so that they were unlikely to settle immediately on a newly immersed surface. Additionally, Anomia, Ostrea, and the encrusting bryozoans did not settle abundantly and did not occupy a substantial area while small. Thus, there was also a delay between the time when these species settled and the time when they had grown sufficiently large to occupy a substantial area.Mytilus was the only species that settled abundantly in established communities and thus could persist indefinitely in a community. Settlement of the other species which occurred earlier in the succession was largely restricted to newly immersed plates and consequently these species were limited in their duration in the succession by the life-spans of the individual organisms.     

a new ichnogenus for etchings made by cheilostome bryozoans into calcareous substrates

Some encrusting cheilostome bryozoans etch a pattern of small pits into hard calcareous substrates, especially calcitic and aragonitic shells of molluscs. These patterns, herein described as Leptichnus ichnogen. nov., comprise pits which are sub-circular to elongate in cross section and are found in either uniserial ( L. dromeus isp. nov.) or multiserial arrangements ( L. peristroma isp. nov., the type species). Each pit corresponds to the location of a single zooid in the bryozoan colony. The oldest known Leptichnus is Late Cretaceous (Maastrichtian), the trace fossil first becomes common in the Cenozoic, and at least nine modern cheilostome genera produce incipient Leptichnus. Leptichnus can be the only evidence remaining of encrusting cheilostomes following taphonomic or diagenetic loss of their calcareous skeletons. The mechanism by which bryozoans etch into their calcareous substrates is unknown but is almost certain to be chemical and necessitates having windows in the basal walls of the zooids which permit contact with the substratum beneath. Etching may result in better adherence to the substrate, giving protection from abrasion and bioerosion.     

The sequential analysis of aggressive acts during hierarchy formation: an application of the ‘jigsaw puzzle’ approach

The ‘jigsaw puzzle’ approach is a general method for investigating how interactions among individuals cumulate to form the overall patterns of dominance behaviour in groups. Here, the model is used to discover how aggressive interactions between pairs of individuals modify subsequent interactions with bystanders or third parties. The model indicates that four sequences of successive, aggressive acts are possible in component triads of larger groups: two ensure transitive attack relationships and two can lead to either transitive or intransitive relationships. An application of the model to 14 groups of four hens demonstrates that the two sequences guaranteeing transitivity make up 77% of all sequences. More specifically, hens attacking one group member usually go on to attack a second member, and hens receiving one attack frequently receive a second attack from a bystander. In contrast, an attacked hen rarely ‘redirects’ an attack to a bystander, and a bystander rarely attacks a group member who has just attacked another individual. The application of the jigsaw puzzle approach to aggressive sequences in other species is discussed. Data available for several primate species corroborate the findings in hens and provide support for the method as a general tool for investigating the proximate mechanisms of hierarchy formation.