Reef-bryozoans and bryozoan-microreefs: Control factor evidence from the philippines and other regions

Summary In this paper, a preliminary concept on the interplay of local, regional and global control factors of bryozoan diversity and distribution pattern is introduced. Recent bryozoans from the Philippines, New Zealand and the Gulf of Aqaba are compared to the selected fossil specimens from the Oxfordian and Santonian. Reef bryozoan skeletons are studied in order to separate local control within the substrate-water interface from regional control. The latter originate mainly from the transport function of the water column (e.g. sediment load, wave energy, vagile predators). This is true especially for erect (tree-like) and massive (multilaminar) bryozoans which are subjected to the dynamics of the water body in the littoral area. This regional control, affects simultaneously several structural and substrate zones of a reef. Early life history of vertically growing reef bryozoans reflect local control, while older zoarial structures reflect the signals of regional influence. Three types of multiserial nodular bryozoans are cited: self-overgrowing sheets (‘S-Nodule’, derived from ‘S-Sheet’), circumlaterally budding colonies (‘C-Nodules’), and fungiform bryozoans. Bryozoan growth form selection allows the separation of two types of regional controls, long range control which favours the selection of specialized sheets, and slow rate control documented in the growth form modification of sheets changing into nodules. In the domain of local control epibiontic microorganisms, microbial mats and biofilms on hard substrata represent probably the most important elements, aside from the limited substrate space. Symbiotic and/or competitive bryozoan-microorganism interrelationships result in the distinct adaptations of bryozoan growth. It is apparent that bryozoan modifications of substrate micro-topography influence the character of epibiontic microbial settlement. The peak occurrence of microbial settlement on prominent external bryozoan skeleton parts is discussed as an effect of feeding currents. In contrast, smooth and unelevated exoskeletons are less favourable for micro-epibiontic colonization. Due to the high level of order in the spatial zonation patterns of epibionts on calcifying bryozoans, the term ‘bryozoan-microreef’ is introduced. Bryozoan-microreefs are locally controlled reefs. They are e.g. characterized by higher competitive abilities than bryozoans without ‘reefdwellers’. Local control has a fast rate of change and is reflected in morphologies of individual zooids and/or single zooid generations. Regional control has a slower rate but a higher range. It is important for growth modifications of the whole zoaria. The very slow rate of global control cannot be recorded within the life span of bryozoan zoaria unless it is expressed through regional control (such as monsoons). Nevertheless, global control is paleoecologically important because it is traceable in bryozoan (paleo-)biogeography. For practical purpose, we suggest to define those control factors of bryozoans as global which affect simultaneously at least one tropical and one non-tropical bryozoan community. This reportis dedicated to the memory of our friend and research colleague Mr. Joselito G. Santiago of the University of San Carlos, Marine Biology Section. Tiago (as we fondly call him) has started within the scope of the Philippine Bryozoan Project an applied research on fish abundance and productivity around a new artificial reef model. This prototype model was conceptualized with him. His promising study remained unfinished; he died May 13, 1993, while sampling inside the reef cave off Marigondon.     

The oldest bryozoan reefs: a unique Early Ordovician skeletal framework construction

Adachi, N., Ezaki, Y. & Liu, J. 2011: The oldest bryozoan reefs: a unique Early Ordovician skeletal framework construction. Lethaia, Vol. 45, pp. 14–23. The oldest bryozoan reefs occur in the Lower Ordovician (late Tremadocian) Fenhsiang Formation of the Three Gorges area, South China. These reefs show a unique type of bryozoan (Nekhorosheviella) framework, and were constructed as follows: the first stage involved colonization by lithistid sponges, which acted as a baffler to trap sediments, providing bryozoans with a stable substrate for attachment. The bryozoans then grew as an encruser on the surfaces of sponges, showing a preferential downwards and lateral growth within the sponge scaffolding to avoid biological and physical disturbance. Finally, these biotic combinations among skeletal organisms formed a rigid, three‐dimensional skeletal framework. This mode of bryozoan growth in association with lithistid sponges is remarkable and unique in its growth direction, and the appearance of such reefs, just prior to the widespread development of skeletal‐dominated reefs as part of the Great Ordovician Biodiversification Event, provides an excellent example of the earliest attempts by skeletal organisms to form frameworks by themselves. This find significantly enhances our understanding of the initial stages of skeletal‐dominated reef evolution and the ensuing development of reefs during the Middle–Late Ordovician. □Bryozoa, Early Ordovician, lithistid sponge, Ordovician radiation, reef.     

Bryozoans and microbial communities of cool-temperate to subtropical latitudes—paleoecological implications

The environmental distribution of encrusting bryozoans settling on disarticulated and living bivalve shells has been recorded from five stations in Japan and New Zealand. Some insight into the observed distribution patterns emerges from information on the interaction of bryozoans with microbial mats. Advancing existing classifications, we have subdivided the encrusting bryozoan morphotypes into seven different growth types that largely reflect the biological potentials of bryozoans in competition for space on substrate surfaces. The frequency distribution of these types (s-/c-/m-/z-laminae, runners, spots, bryostromatolites) reveals the influence of microbial mats as a control factor of bryozoan substrate coverage. Microbial mats in turn are correlated with latitudinal gradients in Japan and New Zealand from cool-temperate to subtropical and tropical waters. Unlike erect bryozoans, laminar ones are probably underrated as facies fossils. Accordingly, laminar bryozoan growth types are reconsidered as a tool for paleoecological interpretation of marine hard substrate communities.     

The role of bryozoans in fossil reefs—an example from the Middle Devonian of the Western Sahara

Stenolaemate bryozoans with their stable calcitic skeletons play a significant role in reef building. In the Middle Devonian Sabkhat Lafayrina reef complex (Western Sahara), bryozoans are abundant and diverse. Although they do not form part of the principal framework of reefs, bryozoans are involved significantly in reef growth, especially in the initial stage. In this way, bryozoans are important with respect to initiating reef growth. They contribute greatly to sediment stabilization, making it possible for principal reef builders to grow on hardened and stabilized substrates, and also play sediment-baffling and sediment-filling roles. The aim of this study is to document the diversity of bryozoans in a Middle Devonian reef complex and to estimate their potential for initiation and contribution to reef structures.     

Biomineralization in bryozoans: present,past and future

Many animal phyla have the physiological ability to produce biomineralized skeletons with functional roles that have been shaped by natural selection for more than 500 million years. Among these are bryozoans, a moderately diverse phylum of aquatic invertebrates with a rich fossil record and importance today as bioconstructors in some shallow‐water marine habitats. Biomineralizational patterns and, especially, processes are poorly understood in bryozoans but are conventionally believed to be similar to those of the related lophotrochozoan phyla Brachiopoda and Mollusca. However, bryozoan skeletons are more intricate than those of these two phyla. Calcareous skeletons have been acquired independently in two bryozoan clades – Stenolaemata in the Ordovician and Cheilostomata in the Jurassic – providing an evolutionary replicate. This review aims to highlight the importance of biomineralization in bryozoans and focuses on their skeletal ultrastructures, mineralogy and chemistry, the roles of organic components, the evolutionary history of bimineralization in bryozoans with respect to changes in seawater chemistry, and the impact of contemporary global changes, especially ocean acidification, on bryozoan skeletons. Bryozoan skeletons are constructed from three different wall types (exterior, interior and compound) differing in the presence/absence and location of organic cuticular layers. Skeletal ultrastructures can be classified into wall‐parallel (i.e. laminated) and wall‐perpendicular (i.e. prismatic) fabrics, the latter apparently found in only one of the two biomineralizing clades (Cheilostomata), which is also the only clade to biomineralize aragonite. A plethora of ultrastructural fabrics can be recognized and most occur in combination with other fabrics to constitute a fabric suite. The proportion of aragonitic and bimineralic bryozoans, as well as the Mg content of bryozoan skeletons, show a latitudinal increase into the warmer waters of the tropics. Responses of bryozoan mineralogy and skeletal thickness to oscillations between calcite and aragonite seas through geological time are equivocal. Field and laboratory studies of living bryozoans have shown that predicted future changes in pH (ocean acidification) combined with global warming are likely to have detrimental effects on calcification, growth rate and production of polymorphic zooids for defence and reproduction, although some species exhibit reasonable levels of resilience. Some key questions about bryozoan biomineralization that need to be addressed are identified.     

Coevolution of post-Palaeozoic arthropod basibiont diversity and encrusting bryozoan epibiont diversity?

We hypothesize that the diversification of motile marine arthropods with hard carapaces resulted in a concurrent increase in the diversity of encrusting marine bryozoans whose larvae exploited those substrates through the Mesozoic and Cenozoic. To test this, family-level data were tabulated from the literature on the post-Palaeozoic diversity of motile marine arthropod basibionts and sessile marine bryozoan epibionts. We found strong temporal correlation from general to more specific basibiont-epibiont relationships (i.e. arthropods and bryozoans in general to decapods and encrusting gymnolaemates to robust decapods and encrusting cheilostomes in particular). We compared the diversification of bryozoans to another common group of basibionts (i.e. molluscs) and found weaker correlations. This suggests that the diversification of motile arthropods with hard carapaces (e.g. brachyuran crabs) may have played a role in the diversification of sessile bryozoans (e.g. encrusting cheilostomes) in the post-Palaeozoic.     

Calcified epibionts as palaeoecological tools: examples from the Recent and Pleistocene reefs of Barbados

Calcified epibionts (crustose coralline algae, bryozoans, foraminiferans and serpulid worms) which colinize primary framebuilders of Recent Barbados reefs exhibit a well-defined zonation of species and morphological growth forms in response to environmental factors such as water turbulence and light. Exposed environments are characterized by thick crusts of coralline algae whereas cryptic environments are dominated by thin crusts of algae, bryozoans, foraminiferans and serpulid worms. A model, based on this zonation, was used to decipher the environments of growth and early burial of Pleistocene reefs. Lagoonal corals possess an assemblage of encrusters which document prolonged growth in a uniform environment. Reef crest corals support a mixed succession of shallow water encrusters which record a gradual decrease in light as substrates are smothered by accumulating debris. Sequences such as these represent growth under stable conditions. The model can also be used to interpret sequences formed by catastrophic events and fluctuations in sea level.     

Early Ordovician reefs in South China (Chenjiahe section, Hubei Province): deciphering the early evolution of skeletal-dominated reefs

The Lower Ordovician (late Tremadocian–early Floian) Fenhsiang and the overlying Hunghuayuan Formations at the Chenjiahe section in the Three Gorges area of Hubei Province, South China, include four types of reef: microbe-dominated (lithistid sponge–stromatolite and lithistid sponge–calcimicrobial) reefs, and skeletal-dominated (lithistid sponge–bryozoan and bryozoan–pelmatozoan) reefs. The microbe-dominated reefs are characterized by the dominance of microbial sediments that encrusted and bound the surfaces of sponges to reinforce the reef frameworks. In contrast, the skeletal-dominated reefs are distinguished by bryozoans that encrusted frame-building sponges and pelmatozoans, and that grew downward to fill the open spaces available within the frameworks. A series of these reefs shows a temporal succession in reef type, with a decline in the lithistid sponge–stromatolite reefs and an increase in the lithistid sponges and receptaculitids within the lithistid sponge–calcimicrobial reefs in the Hunghuayuan Formation; the lithistid sponge–bryozoan reefs are common in both the Fenhsiang and Hunghuayuan Formations. These features of the Chenjiahe reefs are in marked contrast to other coeval reefs on the Yangtze Platform and elsewhere. Skeletal-dominated reefs first developed in the Three Gorges and adjacent areas, located on the central part of the platform. Likewise, lithistid sponges and receptaculitids first developed in the Three Gorges area and then expanded their range. In contrast, stromatolites declined over time, but remained abundant on a marginal part of the platform. The spatial–temporal distributions of these reefs on the Yangtze Platform reflect the initiation of the Great Ordovician Biodiversification Event and its consequences, although influenced by local environmental conditions. The Three Gorges area was a center for the development of skeletal-dominated reefs, which were established earlier here than elsewhere in the world. These reef types and their spatial–temporal successions provide invaluable clues to the earliest evolution of skeletal-dominated reefs and their ensuing development during the Middle–Late Ordovician.     

新疆西准噶尔晚泥盆世法门期的苔藓虫动物群及其形态特异性和古环境意义*

本文重新研究了产自新疆西准噶尔晚泥盆世法门期洪古勒楞组和铁列克提组的苔藓虫动物群, 重点关注其形态学特征和古生态学意义。该苔藓虫动物群由隐口目(Cryptostomata)、窗格苔藓虫目(Fenestrata), 变口目 (Trepostomata)和泡孔目(Cystoporata)组成。优势类群隐口目苔藓虫是具有中轴或中板的细枝状和双叶状群体。 它们的虫室具有加厚的外区体壁, 弯曲强烈的自虫室, 数量众多的大虫室(metazooecia)或隐蔽虫室(tectizooecia), 以及径向刺突和/或棘状刺突。多样化的异虫室、刺突、自虫室等结构, 以及不同生长环境中的不同生态组合是新疆西准噶尔法门期苔藓虫成功演化的基础。洪古勒楞组下段和铁列克提组下部的苔藓虫动物群可以划分为三类生态组合(即: No. 1, No. 2和No. 3), 代表前滨—近岸—远岸的沉积环境。其中, 近岸环境的枝状—双叶状—网状苔藓虫组合的属种丰度、分异度最高。     

新疆西准噶尔晚泥盆世法门期的苔藓虫动物群及其形态特异性和古环境意义*

本文重新研究了产自新疆西准噶尔晚泥盆世法门期洪古勒楞组和铁列克提组的苔藓虫动物群, 重点关注其形态学特征和古生态学意义。该苔藓虫动物群由隐口目(Cryptostomata)、窗格苔藓虫目(Fenestrata), 变口目 (Trepostomata)和泡孔目(Cystoporata)组成。优势类群隐口目苔藓虫是具有中轴或中板的细枝状和双叶状群体。 它们的虫室具有加厚的外区体壁, 弯曲强烈的自虫室, 数量众多的大虫室(metazooecia)或隐蔽虫室(tectizooecia), 以及径向刺突和/或棘状刺突。多样化的异虫室、刺突、自虫室等结构, 以及不同生长环境中的不同生态组合是新疆西准噶尔法门期苔藓虫成功演化的基础。洪古勒楞组下段和铁列克提组下部的苔藓虫动物群可以划分为三类生态组合(即: No. 1, No. 2和No. 3), 代表前滨—近岸—远岸的沉积环境。其中, 近岸环境的枝状—双叶状—网状苔藓虫组合的属种丰度、分异度最高。     

Microbial mats associated with bryozoans (Coorong Lagoon, South Australia)

Summary Bryostromatolites are laminated carbonate rocks composed of bryozoan zoarial laminae. The laminated texture is frequently caused by patterns of bryozoan self overgrowth as a regular defensive tactic against microbial fouling. In the Coorong Lagoon (South Australia), another type of bryostromatolite is present where the laminated growth of the weakly calcifying bryozoan speciesConopeum aciculata is postmortally stabilized by cyanobacterial mats at the surface, and fungal mats settling in the zooecial cavities. A tough extracellular slime network produced by benthic cyanobacteria is a trap for sediment particles, provides a method of adhesion to the bryozoan substrate, and produces a biological lamination by the vertical stratification of dead bryozoan skeletons. These slimes are also important for the preservation of cell structures and for their fossilization. Seasonal fluctuations in salinity and water level are the most important regional control factors, causing a phase displacement in the growth optima of microbial mats and bryozoans, thereby resulting in a rigid bryostromatolitic fabric.     

Mineralogy of Arctic bryozoan skeletons in a global context

Bryozoans are major carbonate producers in some ancient and Recent benthic environments, including parts of the Arctic Ocean. Seventy-six species of bryozoans from within the Arctic Circle have been studied using XRD to determine their carbonate mineralogies and the Mg content of the calcite. The majority of species were found to be calcitic, only four having bimineralic skeletons that combined calcite and aragonite, and none being entirely aragonitic. In almost all species, the calcite was of the low- (<4 mol% MgCO₃) or intermediate-Mg (4–11.99 mol% MgCO₃) varieties. Previous regional studies of bryozoan biomineralogy have found higher proportions of bimineralic and/or aragonitic species in New Zealand and the Mediterranean, with a greater number of calcitic species employing intermediate- and high-Mg calcite. The Antarctic bryozoan fauna, however, has a similar mineralogical composition to the Arctic. The lesser solubility of low-Mg calcite compared to both Mg calcite and aragonite in cold polar waters is most likely responsible for this latitudinal pattern. However, it is unknown to what extent environmental factors drive the pattern directly through eliciting an ecophenotypic response from the bryozoans concerned or the pattern reflects genetic adaptations by particular bryozoan clades.     

Evaluation of malacosporean life cycles through transmission studies

Myxozoans, belonging to the recently described Class Malacosporea, parasitise freshwater bryozoans during at least part of their life cycle, but no complete malacosporean life cycle is known to date. One of the 2 described malacosporeans is Tetracapsuloides bryosalmonae, the causative agent of salmonid proliferative kidney disease. The other is Buddenbrockia plumatellae, so far only found in freshwater bryozoans. Our investigations evaluated malacosporean life cycles, focusing on transmission from fish to bryozoan and from bryozoan to bryozoan. We exposed bryozoans to possible infection from: stages of T. bryosalmonae in fish kidney and released in fish urine; spores of T. bryosalmonae that had developed in bryozoan hosts; and spores and sac stages of B. plumatellae that had developed in bryozoans. Infections were never observed by microscopic examination of post-exposure, cultured bryozoans and none were detected by PCR after culture. Our consistent negative results are compelling: trials incorporated a broad range of parasite stages and potential hosts, and failure of transmission across trials cannot be ascribed to low spore concentrations or immature infective stages. The absence of evidence for bryozoan to bryozoan transmissions for both malacosporeans strongly indicates that such transmission is precluded in malacosporean life cycles. Overall, our results imply that there may be another malacosporean host which remains unidentified, although transmission from fish to bryozoans requires further investigation. However, the highly clonal life history of freshwater bryozoans is likely to allow both long-term persistence and spread of infection within bryozoan populations, precluding the requirement for regular transmission from an alternate host.     

Diversity and systematics of the Malacosporea (Myxozoa)

Abstract. Myxozoans belonging to the recently described class Malacosporea parasitize freshwater bryozoans during at least part of their life cycle. There are at present only two species described in this class: Buddenbrockia plumatellae and Tetracapsuloides bryosalmonae . The former can exist as vermiform and sac-like stages in bryozoan hosts. The latter, in addition to forming sac-like stages in bryozoans, is the causative agent of salmonid proliferative kidney disease (PKD). We undertook molecular and ultrastructural investigations of new malacosporean material to further resolve malacosporean diversity and systematics. Phylogenetic analyses of 18S rDNA sequences provided evidence for two new putative species belonging to the genus Buddenbrockia , revealing a two-fold increase in the diversity of malacosporeans known to date. One new malacosporean is a vermiform parasite infecting the bryozoan Fredericella sultana and the other occurs as sac-like stages in the rare bryozoan, Lophopus crystallinus . Both bryozoans represent new hosts for the genus Buddenbrockia . Our results have established that the malacosporean which infected F. sultana was not a vermiform stage of T. bryosalmonae , although it was collected from a site endemic for PKD. Ultrastructural investigation of new material of B . plumatellae revealed the presence of numerous external tubes associated with developing polar capsules, confirming that the absence of external tubes should no longer be considered as a character of the class Malacosporea.     

Threshold effects of food concentration on the skeletal morphology of the bryozoan Electra pilosa (Linnaeus, 1767)

Many palaeontological studies rely heavily on characteristics of the preserved phenotype, i.e. the morphology of skeletal hard parts. Although the potential for environmental influences on the phenotype is expected, rarely is the magnitude of the effects quantifiable relative to genetic factors. The clonal/colonial body plan of Bryozoa allows for the partitioning of morphological variance into its genetic and environmental factors addressing the question of, ‘how much phenotypic variation is induced in a population by changing a single environmental factor?’ The effects of variation of food concentration on whole‐colony growth rate and on zooid size/morphology can be profound in bryozoans. Here we test experimentally food effects on the skeletal phenotype of the bryozoan Electra pilosa (Linnaeus, 1767), an encrusting sheet‐like bryozoan. A threshold effect was observed for the relationship between zooecium size and food concentration. Very low concentrations resulted in stunted colonies with small zooecia, but at low to intermediate concentrations a close relationship existed with zooecium size. Maximum zooecium size occurred at submaximal food concentration and submaximum zooecium size occurred at higher food concentrations. Previous studies that have reported no effect of food availability on zooecium size assessed food concentration effects at higher concentrations than were effective in the present study. In the absence of other factors, variation in zooecium size is minimal and unchanging at moderate to high food concentrations. Greater variation in zooecium size is expected at and below threshold food concentrations. We show that the preservable phenotype of these specimens subjected to controlled and induced environmental variation also records information with genetic significance.     

Bryozoans as taphonomic engineers,with examples from the Upper Ordovician (Katian) of Midwestern North America

A combination of encrusting calcitic bryozoans and early seafloor dissolution of aragonitic shells recorded in the Cincinnatian Series of the upper Midwest of North America allowed the preservation of abundant moulds of mollusc fossils bioimmured beneath the attachment surfaces of the bryozoans. We here call this preservational process ‘bryoimmuration’, defined as a bryozoan‐mediated subset of bioimmuration. The bryozoans moulded very fine details of the mollusc shells, usually with more accuracy than inorganic sediment moulds. Most of the bryozoans are heterotrypid trepostomes with robust low‐Mg calcite skeletons. The molluscs are primarily bivalves, gastropods, nautiloids and monoplacophorans with their originally aragonitic shells now dissolved. Many of the encrusting bryozoans are so thin and broad that they give the illusion of calcitic mollusc shells clinging to the moulds. Some molluscs in the Cincinnatian, especially monoplacophorans and epifaunal bivalves, would be poorly known if they had not been bryoimmured. Unlike internal and external moulds in sediment, bryoimmured fossils could be transported and thus record aragonitic faunas in taphonomic assemblages (e.g. storm beds) in which they would otherwise be rare or absent. In addition, bryoimmurations of aragonitic shells often reveal the ecological succession of encrustation on the shells by exposing the earliest encrusters and borings that were later overgrown. Bryoimmuration was common during the Late Ordovician because the calcite sea at the time quickly dissolved aragonitic shells on the seafloor before final burial, and large calcitic bryozoans very commonly used molluscs as substrates. Bryoimmuration is an important taphonomic process for preserving aragonitic faunas, and it reveals critical information about sclerobiont palaeoecology. Several Cincinnatian mollusc holotypes are bryoimmured specimens. Bryozoans involved in bryoimmuration enhance the preservation of aragonitic fauna and thus act as taphonomic engineers.     

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.     

Bryozoans of the genera Stomatopora Bronn and Stoporatoma gen. nov. (Stenolaemata) from the Middle Jurassic of Moscow City and the Moscow Region

Encrusting uniserial bryozoans are recorded from the Middle Callovian of Moscow city and the Moscow Region: Stoporatoma gerasimovi gen. et sp. nov. and three species of the genera Stomatopora Bronn; i.e., S. dichotoma (Lamouroux), S. waltoni Haime, and S. bajocensis (Orbigny). Some morphological characters and environmental conditions of these bryozoans are discussed. A new bryozoan that is characterized by a large size and distinctive circular reptozooecia is described.     

Faunal evidence for a late quaternary trans‐Antarctic seaway

Collapse of the West Antarctic Ice Sheet (WAIS) would raise global sea level by ～3.3–5 m. Ice‐sheet models and geological data suggest at least one collapse has happened during the last 1.1 Ma, and some scenarios of future climate change predict a collapse within the next two centuries. A complete WAIS collapse would open shallow seaways across West Antarctica, potentially enabling exchange of animals between West Antarctic seas. We investigated biological evidence for past connectivity between different regions of Antarctica by comparing the composition of modern bryozoan assemblages from the continental margin around Antarctica. Surprisingly, we found most similarity between two areas which are not currently connected – the shelves of the Weddell Sea (WS) and Ross Sea (RS). We evaluated three hypotheses to explain this and conclude that bryozoans most likely dispersed through a trans‐Antarctic seaway that opened in response to a WAIS collapse and connected the WS and RS shelves. These bryozoans must have survived glaciations(s) during subsequent ice ages in refuges, whereas they were wiped out in most other regions of the Antarctic shelf. After the last glacial period, bryozoan assemblages could freely disperse between many of the regions we examined (e.g. Antarctic Peninsula and South Shetland Islands), which has allowed recolonization of areas in which bryozoans had been eradicated during the last ice age. For the bryozoans on the WS and RS shelves to be more similar than those which are in close proximity means the trans‐Antarctic seaway may have been as late as the last few interglacials. Current rates of warming are exceptional compared with the near past glacial cycles so our study, the strongest faunal evidence of WAIS collapse during the recent geological past, thus supports predictions of a near future WAIS collapse (with considerable global sea level implications) and resultant future major faunal exchanges.     

Bryozoan populations reflect nutrient enrichment and productivity gradients in rivers

1. The hypothesis that nutrient enrichment will affect bryozoan abundance was tested using two complementary investigations; a field‐based method determining bryozoan abundance in 20 rivers of different nutrient concentrations by deploying statoblast (dormant propagule) traps and an experimental laboratory microcosm study measuring bryozoan growth and mortality. These two methods confirmed independently that increased nutrient concentrations in water promote increases in the biomass of freshwater bryozoans. 2. Statoblasts of the genus Plumatella were recorded in all rivers, regardless of nutrient concentrations, demonstrating that freshwater bryozoans are widespread. Concentrations of Plumatella statoblasts were high in rivers with high nutrient concentrations relative to those with low to moderate nutrient concentrations. Regression analyses indicated that phosphorus concentrations, in particular, significantly influenced statoblast concentrations. 3. Concentrations of Lophopus crystallinus statoblasts were also higher in sites characterised by high nutrient concentrations. Logistic regression analysis revealed that the presence of L. crystallinus statoblasts was significantly associated with decreasing altitude and increasing phosphorus concentrations. This apparently rare species was found in nine rivers (out of 20), seven of which were new sites for L. crystallinus. 4. Growth rates of Fredericella sultana in laboratory microcosms increased with increasing nutrient concentration and high mortality rates were associated with low nutrient concentrations. 5. Our results indicate that bryozoans respond to increasing nutrient concentrations by increased growth, resulting in higher biomasses in enriched waters. We also found that an important component of bryozoan diets can derive from food items lacking chlorophyll a. Finally, bryozoans may be used as independent proxies for inferring trophic conditions, a feature that may be especially valuable in reconstructing historical environments by assessing the abundance of statoblasts in sediment cores.