Metazoan Diversity Research Articles

The hypothesis of basic phenotype modules

AbstractThe evolutionary events that allowed the rapid occurrence of metazoa are still enigmatic. The presumably oldest metazoan fossils are microscopic and occur just above 635 Ma, at the beginning of the Ediacaran period. Upon condition that the lack of macrofossils in the lower Ediacaran strata is real, the assumption of a sudden appearance of already complex, but still small animals that flourished during the first half of the Ediacaran is a reasonable option. Consequently, the emergence of the first macrofossils with metazoan affinity in mid Ediacaran strata would indicate a second leap in animal evolution. Here, these apparent leaps are explained in terms of a new concept of evolvability that is based on well definable developmental modules: A system based on blast cell-induced cell division modules has paved the way for rapid evolution to small multicellular animals. The second module, an ancestral form of segments, allowed the construction of a new sort of metameric body plans that appeared some tenth of million years later and at a larger scale. Based on this new model of basic radiations, the lower Ediacaran strata are predicted to contain a sequence of exotic embryos that is followed by an explosion of small metazoan diversity. The upper Ediacaran biota are interpreted as representatives of an evolutionary succession that culminated in a maximally evolvable and segmented ancestor with an already complex archetype body plan. Then, around the Ediacaran-Cambrian boundary branching of most modern animal phyla would have taken place, with extensive secondary simplifications in many groups. This radically new hypothesis allows the formulation of clear predictions that are in agreement with available fossils and geochemical data from that period, however, it calls for a precise and chronologically well-definable sequence of Ediacaran events that can only be confirmed through future investigations.

Expression profiles of urbilaterian genes uniquely shared between honey bee and vertebrates

BackgroundLarge-scale comparison of metazoan genomes has revealed that a significant fraction of genes of the last common ancestor of Bilateria (Urbilateria) is lost in each animal lineage. This event could be one of the underlying mechanisms involved in generating metazoan diversity. However, the present functions of these ancient genes have not been addressed extensively. To understand the functions and evolutionary mechanisms of such ancient Urbilaterian genes, we carried out comprehensive expression profile analysis of genes shared between vertebrates and honey bees but not with the other sequenced ecdysozoan genomes (honey bee-vertebrate specific, HVS genes) as a model.ResultsWe identified 30 honey bee and 55 mouse HVS genes. Many HVS genes exhibited tissue-selective expression patterns; intriguingly, the expression of 60% of honey bee HVS genes was found to be brain enriched, and 24% of mouse HVS genes were highly expressed in either or both the brain and testis. Moreover, a minimum of 38% of mouse HVS genes demonstrated neuron-enriched expression patterns, and 62% of them exhibited expression in selective brain areas, particularly the forebrain and cerebellum. Furthermore, gene ontology (GO) analysis of HVS genes predicted that 35% of genes are associated with DNA transcription and RNA processing.ConclusionThese results suggest that HVS genes include genes that are biased towards expression in the brain and gonads. They also demonstrate that at least some of Urbilaterian genes retained in the specific animal lineage may be selectively maintained to support the species-specific phenotypes.

Strong coupling of predation intensity and diversity in the Phanerozoic fossil record

The importance of ecological interactions in driving the evolution of animals has been the focus of intense debate among paleontologists, evolutionary biologists, and macroecologists. To test whether the intensity of such interactions covaries with the secular evolutionary trend in global biodiversity, we compiled a species-level database of predation intensity, as measured by the frequency of common predation traces (drillings and repair scars ranging in age from Ediacaran to Holocene). The results indicate that the frequency of predation traces increased notably by the Ordovician, and not in the mid-Paleozoic as suggested by multiple previous studies. Importantly, these estimates of predation intensity and global diversity of marine metazoans correlate throughout the Phanerozoic fossil record regardless of corrections and methods applied. This concordance may represent (i) an ecological signal: long-term coupling of diversity and predation; (ii) a diversity-driven diffusion of predatory behaviors: an increased probability of more complex predatory strategies to appear at higher diversity levels; or (iii) a spurious concordance in signal capture: an artifact where rare species and less-frequent (e.g., trace-producing) predatory behaviors are both more detectable at times when sampling improves. The coupling of predation and diversity records suggests that macroevolutionary and macroecological patterns share common causative mechanisms that may reflect either historical processes or sampling artifacts.

Early Cambrian metazoan fossil record of South China: Generic diversity and radiation patterns

South China with its rich fossil record is an important region for studying Early Cambrian metazoan diversity patterns. Compilation of a database of metazoan genera from the traditional Lower Cambrian (Cambrian Series 1–2) of South China allows a quantitative analysis of biodiversity changes. The dataset shows that about 876 genus names have been reported from the Lower Cambrian of South China. Of these only about 582 genera are now considered valid after taxonomic scrutiny. Plotting generic diversity shows a two-part, first-order trend: 1, diversity increase through the most of the Meishucunian and Qiongzhusian stages (South China usage), but punctuated by a modest decline during the upper Meishucunian; followed by 2, diversity decline in the Canglangpuan and Longwangmiaoan stages (South China usage). About 155 genera of small shelly fossils (SSFs) occur in Meishucunian rocks. The Meishucunian fauna is dominated by helcionellids, orthothecimorph hyoliths and other enigmatic fossils and is very consistent with the pretrilobitic ‘Tommotian Fauna’. Diversity increases in the Qiongzhusian stage, reaching 304 genera, the peak value known for the Cambrian. The Qiongzhusian fauna is characterized mainly by the occurrence of dominant pan-arthropods and crown-group brachiopods, representing an initial phase of the Cambrian Evolutionary Fauna. A substantial decline in generic diversity through the Canglangpuan (ca 153 genera) and Longwangmiaoan (ca 69 genera) stages is due partly to a eustatic regression, and it approximates the global pattern, reflecting an extinction event at the end of the Lower Cambrian. The observed regional biodiversity through this interval has both evolutionary and taphonomic influences: the decline in SSF diversity beginning in the upper Meishucunian was not only related to the decimation of SSFs, but also to a reduction in phosphogenesis. An apparent peak in diversity during the Qiongzhusian was somewhat exaggerated by the exceptional Chengjiang Biota.

Cryptic animal species are homogeneously distributed among taxa and biogeographical regions

BackgroundCryptic species are two or more distinct but morphologically similar species that were classified as a single species. During the past two decades we observed an exponential growth of publications on cryptic species. Recently published reviews have demonstrated cryptic species have profound consequences on many biological disciplines. It has been proposed that their distribution is non-random across taxa and biomes.ResultsWe analysed a literature database for the taxonomic and biogeographical distribution of cryptic animal species reports. Results from regression analysis indicate that cryptic species are almost evenly distributed among major metazoan taxa and biogeographical regions when corrected for species richness and study intensity.ConclusionThis indicates that morphological stasis represents an evolutionary constant and that cryptic metazoan diversity does predictably affect estimates of earth's animal diversity. Our findings have direct theoretical and practical consequences for a number of prevailing biological questions with regard to global biodiversity estimates, conservation efforts and global taxonomic initiatives.

Microbial carbonate abundance compared with fluctuations in metazoan diversity over geological time

Secular variation in microbial carbonate abundance may be reflected by stromatolite morphotype diversity and reefal microbial carbonate abundance. These datasets reveal long-term changes over the past 3000 Myr that include a peak of abundance 1250 Myr ago, Late Proterozoic decline, Cambrian resurgence, and fluctuating decline during the remainder of the Phanerozoic. It is conceivable that Proterozoic metazoan diversification coincided with inception of stromatolite decline ∼1250 Myr ago, but microbial carbonate increase during Cambrian metazoan radiation together with failure of microbial carbonates to increase in the aftermaths of the End-Ordovician, End-Triassic and End-Cretaceous Mass Extinctions suggest that factors in addition to metazoan competition significantly influenced long-term changes in microbial carbonate abundance.

Summary of Early Triassic carbon isotope records

Much attention has been given to the negative δ 13C anomaly nearly coincident with the Permian–Triassic boundary. New data indicate a stepwise decline in δ 13C initiating before the Latest Permian extinction event followed by highly variable δ 13C values during the remaining Early Triassic. δ 13C values appear much less erratic as global metazoan diversity increased in the Middle Triassic. Given the previously unappreciated magnitude of isotopic change and the number of large δ 13C excursions that occurred during the Early Triassic, catastrophic mechanisms like methane release/bolide impact become less attractive to explain the Early Triassic carbon isotopic record as a whole. To cite this article: F.A. Corsetti et al., C. R. Palevol 4 (2005) .

Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna

This paper offers a new biotic interaction hypothesis for the Cambrian ‘explosion’ of mineralized, benthic, metazoan diversity. It proposes that organic-mineral composite structures (e.g. shells and muscle lever-arms) originated in Proterozoic lineages of primary larva-like, but reproductively competent, pelagic bilaterians because mineralization was both mechanically and energetically favourable, not because it provided armour against predation. Increased strength and rigidity of composite structures permitted growth to sizes incompatible with a continued pelagic existence, while the increased density resulting from massive mineralization facilitated settlement into, and stability in, a nutrient-rich, Proterozoic benthic zone that offered new ecological opportunities. Because evolutionary success is recognized by the formation of recoverable fossils, which requires large, enduring populations, successful lineages are those that responded to the new opportunities by achieving broad niche occupancy through the evolution of metamorphosis to larger, mineralized ‘adult’ body forms with more efficient food-collecting apparatus and higher fecundity. Niche modification (e.g. reef and shell-bed formation) by early mineralized benthic settlers may have increased the likelihood of further successful settlement, leading to the appearance of a period of ‘explosive’ increase in benthic, mineralized, metazoan diversity. Predator-prey arms races may then have followed, causing early faunal turnover and possible selection for improved armour. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 483‐490. ADDITIONAL KEYWORDS: benthic settlement - Cambrian explosion ‐ metamorphosis - organic-mineral composite.

Omnivory and predation impact of the calanoid copepod Boeckella poppei in a maritime Antarctic lake

The copepod Boeckella poppei is a major species in high latitude lakes of the Southern Hemisphere. In such lakes the reduced diversity of metazoans contrasts with a rich microbial assemblage, making these systems amenable to the study of predation controls on the microbial food web. However, the diet of B. poppei is subject to conflicting reports, with little information on feeding rates. We incubated this species in water from Sombre Lake, a much-studied maritime Antarctic Lake on the South Orkney Islands, in order to quantify its feeding rates and potential impact on the microbial assemblage. Overall, clearance rates were similar across 4 experiments spanning November 1999–March 2000, but increased with prey size over the range of 2.7–18 μm equivalent spherical diameter (esd). B. poppei fed omnivorously, although small phototrophic flagellates comprised the bulk of the diet because of their overwhelming dominance in the incubation water. Larger motile preys—heterotrophic ciliates of ~18 μm esd—were cleared fastest (mean 555 ml mg−1 dry mass day−1) and at equivalent rates to those found for freshwater and marine copepods of similar size and at similar temperatures. Estimated predation impact on the microbial food web varied with the abundance of copepods; these were ~30-fold greater in March than in December. In March even the relatively abundant B. poppei (1.7 adults l−1) had a negligible impact on nanoflagellates, due to the low clearance rate on these small cells. However, in March, B. poppei adults were estimated to clear 24% of the lake water of ciliates daily. Given the generation time of ciliates (1.6 days measured in a previous summer study), and the fact that other larval stages of B. poppei were not assessed, this species has the potential to control this part of the microbial assemblage in Sombre Lake.

Errata

Environmental conditions and overwintering strategies of planktonic metazoans in and below coastal fast ice in the Gulf of Finland (Baltic Sea) Sarsia 89: 102–116 The following error appears in the abstract of the print version of the above paper published in issue 2, 2004 of the journal. The abstract is reproduced correctly below: A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20–28 cm, snow cover 0–1 cm. The under‐ice water column was stratified with a cold (−0.3–0.2°C) and less saline (S = 2.1–4.9) interface layer. Concentrations of particulate organic carbon (0.5–5.8 mg POC l ) and algal pigments (0.3–18.2 μg chlorophyll a l ) were higher in the ice than in the water (0.2–0.5 mg POC l , 1.6–7.1 μg chlorophyll a l ) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice‐algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under‐ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia bifilosa in the under‐ice water showed some herbivorous feeding (<0.1–0.23 ng gut pigment individual ), but analysis of fatty acids, fatty alcohols and biomarker ratios indicated a more omnivorous/carnivorous diet. Despite low temperatures, this copepod showed growth and development below the ice, doubling in numbers (mainly CI, CII) from 118 to 230 individuals m during the third week of March.

Environmental conditions and overwintering strategies of planktonic metazoans in and below coastal fast ice in the Gulf of Finland (Baltic Sea)

A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20–28 cm, snow cover 0–1 cm. The under‐ice water column was stratified with a cold (−0.3–0.2°C) and less saline (S = 2.1–4.9) interface layer. Concentrations of particulate organic carbon (0.5–5.8 mg POC l ) and algal pigments (0.3–18.2 μg chlorophyll a l ) were higher in the ice than in the water (0.2–0.5 mg POC l , 1.6–7.1 μg chlorophyll a l ) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice‐algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under‐ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia longi...

A fundamental Precambrian–Phanerozoic shift in earth's glacial style?

It has recently been found that Neoproterozoic glaciogenic sediments were deposited mainly at low paleolatitudes, in marked qualitative contrast to their Pleistocene counterparts. Several competing models vie for explanation of this unusual paleoclimatic record, most notably the high-obliquity hypothesis and varying degrees of the snowball Earth scenario. The present study quantitatively compiles the global distributions of Miocene–Pleistocene glaciogenic deposits and paleomagnetically derived paleolatitudes for Late Devonian–Permian, Ordovician–Silurian, Neoproterozoic, and Paleoproterozoic glaciogenic rocks. Whereas high depositional latitudes dominate all Phanerozoic ice ages, exclusively low paleolatitudes characterize both of the major Precambrian glacial epochs. Transition between these modes occurred within a 100-My interval, precisely coeval with the Neoproterozoic–Cambrian “explosion” of metazoan diversity. Glaciation is much more common since 750 Ma than in the preceding sedimentary record, an observation that cannot be ascribed merely to preservation. These patterns suggest an overall cooling of Earth's longterm climate, superimposed by developing regulatory feedbacks involving an increasingly complex biosphere.

Metazooplankton dynamics and secondary production of Daphnia magna (Crustacea) in an aerated waste stabilization pond

The seasonal dynamics of metazooplankton biomass was monitored in an aerated waste stabilization pond during three consecutive years (1994–1996). The pond showed a low diversity of planktonic metazoans because of elevated pH, relatively high concentration of free dissolved ammonia and low oxygen concentration. The planktonic community was composed of the anomopod branchiopod Daphnia magna, and the cyclopoid copepods Cyclops vicinus and Cyclops strenuus. Both predation by cyclopoids and competition with D.magna excluded rotifers from the pond, except during a short period in spring 1996. Daphnia magna was the dominant organism from a biomass point of view. In parallel with biomass, demographic parameters, secondary production and the spatial distribution of D.magna were studied. A significant seasonal and interannual variation in the density, biomass and production of D.magna was observed. The maximum density of daphnids varied from 264 × 103 to 686 × 103 individuals m–2 and the maximum biomass from 4 to 30 g dry weight (DW) m–2. The annual net production was high compared with the production of Daphnia in natural environments, ranging from 288 to 593 g DW m–2 year–1. The annual net production of exuviae accounted for ~25% of the total annual net production. Harvesting of daphnids for commercial applications that took place during the productive period did not have any discernible effect on the population dynamics of D.magna. Sexual reproduction was not observed during the three studied years. Negative mortality rates, occurring during early spring, however, indicated that recruitment from ephippia was effective in the pond of Differdange and that sexual reproduction took place before 1994. Swarming was regularly observed in relation to high densities.

The plankton and the benthos: origins and early history of an evolving relationship

Modern marine plankton communities include a broad diversity of metazoans that are suspension-feeding or micropredatory as adults. Many benthic marine species have larval stages that reside, and often feed, in the plankton for brief to very long periods of time, and most marine benthic communities include large numbers of suspension-feeders. This has not always been the case. Cambrian benthic communities included relatively few suspension-feeders. Similarly, there were few metazoan clades represented in the plankton, either as adult suspension-feeders or as larvae. Review of the fossil record suggests that the diversification of the plankton and suspension-feeding marine animals began in the Late Cambrian and continued into the Ordovician. These changes were accompanied by, and probably influenced, concurrent major changes in the marine realm, including an increase in tiering within benthic communities, the replacement of the Cambrian fauna by the Paleozoic fauna, and a general taxonomic diversification. The ultimate cause of these changes is uncertain, but it appears likely that the plankton was and is a refuge from predation and bioturbation for adults and larvae alike. The expansion in plankton biomass thus provided increased ecological opportunities for suspension-feeders in the plankton and benthos.

Interactions Between Metazoans and Large, Agglutinating Protozoans: Implications for the Community Structure of Deep-Sea Benthos

Large, agglutinating protozoans belonging to the Foraminiferida (suborder Astrorhizina) and the Xenophyophorea are conspicuous, often dominant faunal elements in the deep sea. A review of known and suspected interactions between these forms and metazoans reveals a potentially significant role for the protozoans in structuring deep-sea metazoan assemblages. Direct interactions include provision to metazoans of (a) hard or stable substratum, (b) refuge from predators or physical disturbance, and (c) access to enhanced dietary resources. In some instances, rhizopod tests may provide a nursery function. Xenophyophore modification of flow regimes, particle flux, bottom skin friction and sediment characteristics appear likely and are believed to account for altered composition and abundance of meiofauna and macrofauna in the vicinity of rhizopod tests. Some analogous interactions are observed between metazoans and biogenic sediment structures in shallow water. However, metazoan-rhizopod associations are hypothesized to be more highly developed and complex in the deep sea than are comparable shallow-water associations, due to rhizopod abilities to enhance scarce food resources and to low rates of disturbance in much of the deep sea. Agglutinating rhizopods appear to be a significant source of heterogeneity on the deep-sea floor and large tests often represent “hotspots“ of metazoan activity. As such, they are hypothesized to have contributed to the origin and maintenance of metazoan diversity in the deep sea by providing distinct microenvironments in which species can specialize.