New data on echiuran anatomy and histology: the case of Lissomyema mellita (Annelida: Thalassematidae).

Echiura is small group of unsegmented marine worms that are sometimes abundant in the benthos of all areas of the World Ocean. The study of echiuran morphology and anatomy is important for understanding echiuran biology and the function of benthic communities where echiurans dominate. The current study used paraffin histology, scanning electron microscopy, and 3D reconstruction to describe the anatomy of organ systems in Lissomyema mellita, which is within the tribe Thalassematini of the subfamily Thalassematinae. The body of L. mellita consists of a trunk and a proboscis. The trunk terminates at the anal lobe, which contains many large glands, the secretions of which may be used for processing of the hard substratum where studied animals were found. The proboscis has lateral ciliated grooves that are < 7 µm wide and are probably used for collecting and separating food particles. The coelom is divided into the coelom of the proboscis and the coelom of trunk. The location of proboscis coelom may suggest that the proboscis is derived from the oral segment of the body of a hypothetical segmented echiuran ancestor. The shortness and histological uniformity of the foregut indicates that L. mellita feeds on soft food particles that do not require mechanical processing. The circulatory system has a complex organization and contains several vascular elements that have not been previously described in echiurans: voluminous plexus around the foregut, voluminous dorsal and ventral lacunes, and additional vessels between neurointestinal and circular blood vessels. The dorsal blood vessel is very short; the circumchaetael vessel is absent. Most of the anatomical peculiarities of L. mellita appear to be related to its biology. The newness of the data on the organization of the circulatory system reflects the general scarcity of knowledge on echiuran anatomy. The newly discovered features should be incorporated into general schemes of echiuran organization.

Cryptic diversity of the rocky crab genus Glebocarcinus Nations, 1975 (Crustacea: Decapoda: Cancridae): description of a new species from Russian coastal waters of the Sea of Japan based on morphology, DNA and distribution.

A new cancrid crab species Glebocarcinus kashini sp. nov. (Decapoda: Brachyura: Cancridae) is described from Russian coastal waters of the Sea of Japan. The new species can be clearly separated from relative and sibling, Glebocarcinus amphioetus (Rathbun, 1898), by a less prominent and granulated dorsal surface of the carapace and dorsal surface of the cheliped propodus, less prominent sculpture of the carapace front and bluntly triangular anterolateral teeth of the carapace. Comparison of COI gene sequences supports the subdivision of these two species and shows their clear genetic separation from a related species, the American pygmy rock crab, G. oregonensis (Dana, 1852). Along the mainland coast of the Sea of Japan, G. kashini and G. amphioetus possibly overlap only in Posyet Bay and adjacent areas; the new species is distributed to the north from the bay while G. amphioetus is distributed to the south. Previous and new records of G. amphioetus from the area are discussed.

Sipunculan larvae and "cosmopolitan" species.

Sipuncula is a relatively small taxon with roughly 150 recognized species. Many species are geographically widespread or "cosmopolitan." The pelagosphera larvae of some species are estimated to spend several months in the plankton. However, recent molecular evidence suggests that many of the "cosmopolitan" species actually represent species-complexes, some not even monophyletic. Herein, we present data on three sipunculan species with different developmental modes that occur both in the Sea of Japan and in the Northeast Pacific. The development of the three species-Phascolosoma agassizii, Thysanocardia nigra, and Themiste pyroides-is exceptionally well studied in both regions of the Pacific. Significant differences have been observed between the two regions with respect to egg size, developmental mode, and developmental timing. In general, eggs are larger and development slower in the Northeast Pacific when compared with the Sea of Japan. These differences have been explained as a result of phenotypic plasticity exhibited under different environmental conditions, in particular temperature, but we show that the populations of all three species are also remarkably distinct genetically and that gene flow between the two regions is extremely unlikely. In Thysanocardia nigra, we even found two very distinct genetic lineages within the same location in the Northeast Pacific. The amount of genetic divergence between populations from the Sea of Japan and those from the Northeast Pacific is not correlated with developmental mode. Themiste pyroides, the species with the most abbreviated development, actually has the least degree of genetic divergence between the regions. Analyses of molecular variance show that the majority of the observed variation in all three species is between the regions. We conclude that all three "cosmopolitan" species actually represent complexes of cryptic or pseudo-cryptic species. These examples demonstrate that a solid taxonomic framework based on molecular and morphological evidence is a prerequisite for evaluating relationships between dispersal capabilities, species' ranges, and the connectivity of populations.

Cellular and muscular growth patterns during sipunculan development.

Sipuncula is a lophotrochozoan taxon with annelid affinities, albeit lacking segmentation of the adult body. Here, we present data on cell proliferation and myogenesis during development of three sipunculan species, Phascolosoma agassizii, Thysanocardia nigra, and Themiste pyroides. The first anlagen of the circular body wall muscles appear simultaneously and not subsequently as in the annelids. At the same time, the rudiments of four longitudinal retractor muscles appear. This supports the notion that four introvert retractors were part of the ancestral sipunculan bodyplan. The longitudinal muscle fibers form a pattern of densely arranged fibers around the retractor muscles, indicating that the latter evolved from modified longitudinal body wall muscles. For a short time interval, the distribution of S-phase mitotic cells shows a metameric pattern in the developing ventral nerve cord during the pelagosphera stage. This pattern disappears close to metamorphic competence. Our findings are congruent with data on sipunculan neurogenesis, as well as with recent molecular analyses that place Sipuncula within Annelida, and thus strongly support a segmental ancestry of Sipuncula.

Development of the tentacular apparatus in sipunculans (Sipuncula): I. Thysanocardia nigra (Ikeda, 1904) and Themiste pyroides (Chamberlin, 1920).

The development and arrangement of the tentacular apparatus of Thysanocardia nigra (Ikeda, 1904) and Themiste pyroides (Chamberlin, 1920) are described and illustrated using scanning electron microscopy. In T. nigra, the tentacular apparatus is composed of two crowns: the nuchal arc enclosing the nuchal organ and a crown of numerous oral tentacles arranged in U-shaped festoons. In early juveniles, two dorsal horn-like protrusions develop into the first, or primary, pair of tentacles of the nuchal arc. The second pair of tentacles of the nuchal arc develops dorsolaterally on the bases of the primary tentacles. Two ventrolateral lobes of the oral disk grow and become subdivided by the longitudinal ciliary groove into anlages of one set of dorsal and one set of ventral tentacles, thus forming a first oral festoon. Later, a pair of dorsolateral lobes develop between the first festoons and the nuchal arc to form a second pair of oral festoons. The third and following pairs of oral festoons develop in the dorsolateral growth zones lateral to the borders of the nuchal arc, where they meet the oral crown. The growing festoons extend down the oral disk and run alongside the head. A new oral tentacle appears directly at/on the base of the previous tentacle, thus giving rise to a typical sympodium with an alternating arrangement of tentacles. In T. pyroides, a second pair of tentacles develops from two ciliary lobes that are ventrolateral outgrowths of the circumoral ciliary field around the terminal mouth opening. The third pair of tentacles appears from the dorsolateral lobes at the base of primary tentacles, between the first two pairs of tentacles. These six tentacles determine the position of six main stems of the tentacular apparatus designated the first tentacles in the corresponding stems. The second tentacle in every stem appears as a ventrolateral outgrowth at the base of the first tentacle. The third and following tentacles in the stem are developed between the two previous tentacles according to a sympodial pattern. In both species, the distinct sympodial pattern in the arrangement of tentacles in the tentacular apparatus is well evidenced by the outlines of the ciliary oral grooves. The branched stems of T. pyroides may be homologized structurally and functionally to the oral festoons of T. nigra. J. Morphol. (c) 2006 Wiley-Liss, Inc.