No abstract

The investigation of neurogenesis in polychaetes not only facilitates insights into the developmental biology of this group, but also provides new data for phylogenetic analyses. This should eventually lead toward a better understanding of metazoan evolution including key issues such as the ontogenetic processes that underlie body segmentation. We here document the development of the larval nervous system in the polychaete Sabellaria alveolata using fluorescence-coupled antibodies directed against serotonin, FMRFamide, and tubulin in combination with confocal laser scanning microscopy and 3D reconstruction software. The overall pattern of neurogenesis in S. alveolata resembles the condition found in other planktonic polychaete trochophores where the larval neural body plan including a serotonergic prototroch nerve ring is directly followed by adult features of the nervous system such as circumesophageal connectives and paired ventral nerve cords. However, distinct features are also found in S. alveolata, such as the innervation of the apical organ with ring-shaped neurons, the low number of immunoreactive perikarya, and the lack of a posterior serotonergic cell. Moreover, in the larvae of S. alveolata, two distinct modes of neuronal development are expressed, viz. the simultaneous formation of the first three segmental neurons of the peripheral nervous system on the one hand versus the sequential appearance of the ventral commissures on the other. This highlights the complex mechanisms that underlie annelid body segmentation and indicates divergent developmental pathways within polychaete annelids that lead to the segmented nervous system of the adult.

We systematically revise here the Australian taxa of the Thiaridae, a group of freshwater Cerithioidea with pantropical distribution and “marsupial” (i.e. viviparous) reproductive modes. On this long isolated continent, the naming of several monotypic genera and a plethora of species have clouded both the phylogenetical and biogeographical relationships with other thiarids, in particular in Southeast Asia, thus hampering insight into the evolution of Australian taxa and their natural history. Based on own collections during five expeditions to various regions in Australia between 2002 and 2007, the study of relevant type material and the comparison with (mostly shell) material from major Australian museum collections, we describe and document here the morphology (of adults and juveniles) and radulae of all relevant thiarid taxa, discussing the taxonomical implications and nomenclatural consequences. Presenting comprehensive compilations of the occurrences for all Australian thiarid species, we document their geographical distribution (based on over 900 records) with references ranging from continent‐wide to drainage‐based patterns. We morphologically identify a total of eleven distinct species (also corroborated as distinct clades by molecular genetic data, to be reported elsewhere), of which six species are endemic to Australia, viz. “Thiara australis ”, Plotiopsis balonnensis, and “Stenomelania ” denisoniensis with wide distribution and Melasma onca, Sermyla venustula, and Ripalania queenslandica with more restricted ranges. In contrast, Thiara amarula and Stenomelania cf. aspirans as well as Melanoides tuberculata, Plotia scabra, and Sermyla riqueti are widely distributed also outside Australia, in particular in the Malay Archipelago and the Indo‐West Pacific, respectively. The occurrences especially of the latter three species are discussed, concluding on quite distinct historical contingencies. Three thiarids species, viz. Stenomelania cf. aspirans, Sermyla riqueti and Plotia scabra, are recorded here for the first time for Australia. Based on a new taxonomic framework for the Thiaridae we point out some of the pertinent problems with naming and artificial delineation of species, revealing why typology and earlier practice of splitting was misleading in case of these truly “Darwinian snails”. We also report on finding two clearly distinct viviparous modes in Australian thiarids, discussing their distribution in certain fluvifaunal provinces and major drainage systems in concert with these reproductive features of life history tactics. While the live‐bearing T. amarula, S. cf. aspirans, and R. queenslandica, that release veligers (ovo‐viviparity), are found to have very restricted occurrences in rivers and streams in the Jardinian province of NE Queensland only, the five more widely distributed Australian endemics Plotiopsis balonnensis and “Stenomelania ” denisoniensis as well as “Thiara australis ”, Sermyla venustula and Melasma onca in the Leichhardtian province all brood and release shelled juven...

Aspects of muscle development are still widely neglected in studies on invertebrate ontogeny, which is probably at least partly due to the inherent complexity of animal myoanatomical bodyplans. This has resulted in significant gaps in our understanding of the evolutionary and ontogenetic origin of this crucial mesoderm-derived organ system, particularly in indirect developing representatives of the Lophotrochozoa. Here, we document the temporal and spatial patterns of muscle formation and cell proliferation in the polychaete Sabellaria alveolata during planktotrophic larval development in order to assess the presumed generation of muscle units and segments from a posterior growth zone. In addition, we address the question whether the three primary segments differ in their mode of myogenesis from the subsequently forming segments. We found that in the first three segments the ventral transverse muscles differentiate from anterior to posterior, whereas the ventral oblique muscles develop simultaneously. Hence, subsequent and simultaneous developmental processes of specific muscle groups are possibly regulated in different ways, thus emphasizing the plasticity of the formation of metamerically arranged organ systems in polychaetes. The occurrence of three clusters of proliferating cells in the trunk region of the metatrochophore indicates synchronous subdivision of the larval mesoderm in the first three segments. Assuming a polychaete-like ancestor at the base of the annelid tree, comparative analysis suggests that the bodywall of the last common annelid ancestor might have been devoid of circular muscles and consisted of four separate longitudinal muscle strands that develop from anterior to posterior.

We document a case of shell convergence in sympatric freshwater cerithioidean gastropods that has caused confusion in traditional classifications emphasizing shell features. Based on the comparative study of the operculum, radula, and embryonic shell obtained from dry remains of soft bodies, we present evidence that "Melania" pisum is not а thiarid species closely related to Balanocochlis glans (Busch, 1842), as has been supposed with respect to the very similar shells of both species. The species is transferred to the family Pachychilidae, because it shows various typical character states for the family. It is tentatively placed within the genus Sulcospira, which is endemic to Java. We assume that a similar shell shape has evolved in both species of not closely related gastropods through convergence, which once more reveals that purely shell-based classifications are particularly problematic.