Evolutionary systematics of the viviparous gastropod Sermyla (Gastropoda: Cerithioidea: Thiaridae), with the description of a new species

While most Cerithioidea are marine, some occur in brackish and freshwater habitats. Many members are systematically problematic due to variability or homoplasy in conchological characters, which has led to taxonomic redundancy, but also because of discrepancies between phylogenetic trees and morphologically distinguishable units as revealed in recent molecular genetic studies. We have chosen an evolutionary systematic approach and combine analyses of shell biometry and geometric morphometrics with the analyses of reproductive traits and molecular genetics based on mtDNA and AFLP markers in order to resolve the relationships among species of the genus Sermyla. We describe a new species from Sulawesi, Sermyla kupaensis sp. nov., which is characterized by a distinct reproductive strategy. This unique reproductive strategy corresponds with its distinct molecular genetic signal. However, it is not possible to distinguish S. kupaensis from S. riquetii based on shell morphology alone. We also provide data on the population structure of the endemic Australian species Sermyla carbonata, for which we found a drainage-based population structure. Overall, we present a new concept of the relationships among the species within the genus Sermyla based on morphological and genetic data.

Survey of Stenomelania Fisher, 1885 (Cerithioidea, Thiaridae): The potential of trematode infections in a newly-recorded snail genus at the coast of Andaman Sea, South Thailand

Stenomelania snails (Fisher 1885) have been reported from the coastal regions of the Indian Ocean and the Pacific Ocean, spanning India to Australia. Here, the species diversity and distribution of these snails in the south of Thailand are recorded. The snails were also examined for trematode infections in 13 locations in three Provinces, viz. Krabi, Trang and Satun, along the coast of the Andaman Sea. A total of 1,551 snails were in five morphs tentatively identified as Stenomelania aspirans, S. crenulata, S. punctata, S. torulosa and the closely-related Neoradina prasongi. With 10 infected snails, the trematode infection rate was 0.64%. The cercariae were categorised into three species from two morphologically-distinguishable types, viz. parapleurolophocercous cercariae (Haplorchis taichui and Procerovum cheni) and xiphidiocercariae (Loxogenoides bicolor), through the morphological characterisation of the larval stage. These trematodes were also analysed using the internal transcribed spacer subunit II region to confirm the species identity at generic and infrageneric levels.

A glimpse in the dark? A first phylogenetic approach in a widespread freshwater snail from tropical Asia and northern Australia (Cerithioidea, Thiaridae)

Thiaridae are a speciose group of freshwater snails in tropical areas including a high number of described nominal taxa for which modern revisions are mostly lacking. Using an integrative approach, the systematic status of a group of thiarids from the Oriental region, including the nominal speciesMelaniaasperaandM.rudis, is reassessed on the basis of shell morphology and biometry, radula dentition patterns, and reproductive biology along with molecular genetic methods. Our results suggest that populations from the Oriental region cannot be distinguished on the basis of shell morphology, radula characters and their reproductive mode and are monophyletic based on mitochondrial sequences. Hence,M.rudiswithM.asperaare regarded as belonging to the same species along with several other nominal taxa that were previously included inM.rudis. Moreover, populations from Thailand and Australia, from where the species was not previously recorded, could be shown to form a monophyletic group together with samples from Indonesia. However, a generic affiliation withThiara, in which the investigated taxa were often included in the past, was not supported in our phylogenetic analyses, highlighting the need for a comprehensive revision of the genus-group systematics of Thiaridae as a whole.

Molecular phylogeography and reproductive biology of the freshwater snail Tarebia granifera in Thailand and Timor (Cerithioidea, Thiaridae): morphological disparity versus genetic diversity

The freshwater thiarid gastropod Tarebiagranifera (Lamarck, 1816), including taxa considered either congeneric or conspecific by earlier authors, is widespread and abundant in various lentic and lotic water bodies in mainland and insular Southeast Asia, with its range extending onto islands in the Indo-West-Pacific. This snail is, as one of the most frequent and major first intermediate host, an important vector for digenic trematodes causing several human diseases. As a typical thiarid T.granifera is viviparous and parthenogenetic, with various embryonic stages up to larger shelled juveniles developing within the female’s subhemocoelic (i.e non-uterine) brood pouch. Despite the known conchological disparity in other thiarids as well as this taxon, in Thailand Tarebia has been reported with the occurrence of one species only. In light of the polytypic variations found in shell morphology of freshwater snails in general and this taxon in particular, the lack of a modern taxonomic-systematic revision, using molecular genetics, has hampered more detailed insights to date, for example, into the locally varying trematode infection rates found in populations of Tarebia from across its range in Thailand as well as neighboring countries and areas. Here, we integrate evidence from phylogeographical analyses based on phenotypic variation (shell morphology, using biometry and geometric morphometrics) with highly informative and heterogeneous mtDNA sequence data (from the gene fragments cytochrome c oxidase subunit 1 and 16 S rRNA). We evaluate both the morphological and molecular genetic variation (using several phylogenetic analyses, including haplotype networks and a dated molecular tree), in correlation with differences in the reproductive biology among populations of Tarebia from various water bodies in the north, northwest, central, and south of Thailand, supplementing our respective analyses of parasite infections of this thiarid by cercaria of 15 trematode species, reported in a parallel study. Based on the comparison of topotypical material from the island of Timor, with specimens from 12 locations as reference, we found significant, albeit not congruent variation of both phenotype and genotype in Tarebiagranifera, based on 1,154 specimens from 95 Thai samples, representing a geographically wide-ranging, river-based cross-section of this country. Our analyses indicate the existence of two genetically distinct clades and hint at possible species differentiation within what has been traditionally considered as T.granifera. These two lineages started to split about 5 mya, possibly related to marine transgressions forming what became known as biogeographical barrier north of the Isthmus of Kra. Grounded on the site-by-site analysis of individual Tarebia populations, our country-wide chorological approach focussing on the conchologically distinct and genetically diverse lineages of Tarebia allows to discuss questions of this either reflecting subspecific forms versus being distinct species within a narrowly delimited species complex. Our results, therefore, provide the ground for new perspectives on the phylogeography, evolution and parasitology of Thai freshwater gastropods, exemplified here by these highly important thiarids.

Exploring the evolutionary potential of parasites: Larval stages of pathogen digenic trematodes in their thiarid snail host Tarebia granifera in Thailand

Minute intestinal flukes from several distinct families of endoparasitic platyhelminths are a medically important group of foodborne trematodes prevalent throughout Southeast Asia and Australasia. Their lifecycle is complex, with freshwater snails as primary intermediate hosts, with infecting multiple species of arthropods and fish as second intermediate hosts, and with birds and mammals including humans as definitive hosts. In Southeast Asian countries, the diversity of snail species of the Thiaridae which are frequently parasitized by trematode species is extremely high. Here, the thiarid Tarebiagranifera in Thailand was studied for variation of trematode infections, by collecting the snails every two months for one year from each locality during the years 2004–2009, and during 2014–2016 when snails from the same localities were collected and new localities found. From ninety locations a total of 15,076 T.granifera were collected and examined for trematode infections. With 1,577 infected snails the infection rate was found to be 10.46 %. The cercariae were categorized into fifteen species from eight morphologically distinguishable types representing several distinct families, viz. (i) virgulate xiphidiocercariae (Loxogenoidesbicolor, Loxogenesliberum and Acanthatriumhistaense), (ii) armatae xiphidiocercariae cercariae (Maritreminoidescaridinae and M.obstipus); (iii) parapleurophocercous cercariae (Haplorchispumilio, H.taichui and Stictodoratridactyla); (iv) pleurophocercous cercariae (Centrocestusformosanus); (v) megarulous cercariae (Philophthalmusgralli); (vi) furcocercous cercariae (Cardicolaalseae, Alariamustelae and Transversotremalaruei); as well as (vii) echinostome-type cercariae, and (viii) gymnocephalous-type cercariae. In addition, a phylogenetic marker (internal transcribed spacers 2, ITS2) was employed in generic and infrageneric level classifications of these trematodes, using sequences obtained from shed cercariae isolated from T.granifera specimens of the second study period collected in various regions in Thailand. We obtained ITS2 sequences of cercariae from nine species (of seven types): Loxogenoidesbicolor, Loxogenesliberum, Maritreminoidesobstipus, Haplorchistaichui, Stictodoratridactyla, Centrocestusformosanus, Philophthalmusgralli, as well as from one species each of echinostome cercariae and gymnocephalous cercariae. Thus, this analysis combines the parasites’ data on morphology and geographical occurrence with molecular phylogeny, aiming to provide the groundwork for future studies looking into more details of the parasite-snail evolutionary relationships.

Morphospace saturation in the stem-gnathostomes pteraspidiformes heterostracans: an early radiation of a 'bottom' heavy clade

Ostracoderms (fossil armoured jawless fishes) shed light on early vertebrate evolution by revealing the step-wise acquisition of jawed vertebrate characters, and were important constituents of Middle Palaeozoic vertebrate faunas. A wide variety of head shield shapes are observed within and between the ostracoderm groups, but the timing of these diversifications and the consistency between different measures of their morphospace are unclear. Here, we present the first disparity (explored morphospace) versus diversity (number of taxa) analysis of Pteraspidiformes heterostracans using continuous and discrete characters. Patterns of taxic diversity and morphological disparity are in accordance: they both show a rise to a peak in the Lochkovian followed by a gradual decline in the Middle-Late Devonian. Patterns are largely consistent for disparity measures using sum of ranges or total variance, and when using continuous or discrete characters. Pteraspidiformes heterostracans can be classified as a "bottom-heavy clade", i.e., a group where a high initial disparity decreasing over time is detected. In fact, the group explored morphospace early in its evolutionary history, with much of the subsequent variation in dermal armour occurring as variation in the proportions of already evolved anatomical features. This Early Devonian radiation is also in agreement with the paleobiogeographic distribution of the group, with a maximum of dispersal and explored morphospace during the Lochkovian and Pragian time bins.

Discrete and continuous character-based disparity analyses converge to the same macroevolutionary signal: a case study from captorhinids

The relationship between diversity and disparity during the evolutionary history of a clade provides unique insights into evolutionary radiations and the biological response to bottlenecks and to extinctions. Here we present the first comprehensive comparison of diversity and disparity of captorhinids, a group of basal amniotes that is important for understanding the early evolution of high-fiber herbivory. A new fully resolved phylogeny is presented, obtained by the inclusion of 31 morphometric characters. The new dataset is used to calculate diversity and disparity through the evolutionary history of the clade, using both discrete and continuous characters. Captorhinids do not show a decoupling between diversity and disparity, and are characterized by a rather symmetric disparity distribution, with a peak in occupied morphospace at about the midpoint of the clade's evolutionary history (Kungurian). This peak represents a delayed adaptive radiation, identified by the first appearance of several high-fiber herbivores in the clade, along with numerous omnivorous taxa. The discrete characters and continuous morphometric characters indicate the same disparity trends. Therefore, we argue that in the absence of one of these two possible proxies, the disparity obtained from just one source can be considered robust and representative of a general disparity pattern.

A preliminary checklist of the freshwater snails of Sabah (Malaysian Borneo) deposited in the BORNEENSIS collection, Universiti Malaysia Sabah

Sabah, a Malaysian state at the north-eastern tip of Borneo, is situated in one of the Earth’s biodiversity hotspots yet its freshwater gastropod diversity remains poorly known. An annotated checklist of the freshwater gastropods is presented, based on specimens deposited in the BORNEENSIS collection of the Institute for Tropical Biology and Conservation at Universiti Malaysia Sabah, Malaysia. A KMZ file is also provided, which acts as a repository of digital images and complete collection data of all examined material, so that it can be shared and adapted to facilitate future research.

Morphological and Genetic Variation Between the Japanese Populations of the Amphidromous Snail Stenomelania crenulata (Cerithioidea: Thiaridae)

Freshwater gastropods often have limited dispersal capability and small geographic ranges, and face severe threats from habitat loss and degradation. However, in addition to the scarcity of knowledge on their life history traits, species taxonomy has not been adequately resolved and boundaries between intra- and interspecific variation remain unclear for many taxa. One such example of an indeterminate species boundary with implications for conservation issues is the relationship between the thiarid snails Stenomelania crenulata in Okinawa and southwards (ranked as CR+EN in the 2012 Japanese Red List) and S. rufescens in mainland Japan (VU). The results of our multi-disciplinary investigation into variation in the shell morphology and mitochondrial (COI) and nuclear (ITS-1) gene sequences suggest that S. rufescens represents a geographic variant and a junior synonym of S. crenulata. The widespread geographic range of S. crenulata, spanning a few thousand kilometers north to south, is possible due to an amphidromous life cycle that involves a marine planktotrophic larval phase and upstream migration after settlement in estuaries. Nevertheless, there is recognizable morphological and genetic differentiation between distant populations, probably reflecting a relatively short pelagic duration and possibly also infrequent transoceanic dispersal; metamorphic competence is achieved in two weeks in full seawater and even more rapidly in brackish water. The Okinawan population, with only a few known localities, therefore deserves the high conservation priority; conservation efforts need to involve the proper maintenance of migration pathways including all marine, brackish and freshwater environments.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.