Mary E. Rice

Asexual reproduction in a sipunculan worm.

The sipunculan worm Aspidosiphon brocki reproduces asexually by transverse fision into two unequal parts, the smaller part comprising the posterior fifth of the animal. Prior to fission each part regenerates the structures essential to the formation of a new individual. The smaller posterior part (daughter) regenerates an anterior body, including introvert, anterior gut, retractor muscles, and nephridia, whereas the larger anterior part (parent) regenerates only the posterior body wall.

Musculature in sipunculan worms: ontogeny and ancestral states.

SUMMARY Molecular phylogenetics suggests that the Sipuncula fall into the Annelida, although they are morphologically very distinct and lack segmentation. To understand the evolutionary transformations from the annelid to the sipunculan body plan, it is important to reconstruct the ancestral states within the respective clades at all life history stages. Here we reconstruct the ancestral states for the head/introvert retractor muscles and the body wall musculature in the Sipuncula using Bayesian statistics. In addition, we describe the ontogenetic transformations of the two muscle systems in four sipunculan species with different developmental modes, using F‐actin staining with fluorescent‐labeled phalloidin in conjunction with confocal laser scanning microscopy. All four species, which have smooth body wall musculature and less than the full set of four introvert retractor muscles as adults, go through developmental stages with four retractor muscles that are eventually reduced to a lower number in the adult. The circular and sometimes the longitudinal body wall musculature are split into bands that later transform into a smooth sheath. Our ancestral state reconstructions suggest with nearly 100% probability that the ancestral sipunculan had four introvert retractor muscles, longitudinal body wall musculature in bands and circular body wall musculature arranged as a smooth sheath. Species with crawling larvae have more strongly developed body wall musculature than those with swimming larvae. To interpret our findings in the context of annelid evolution, a more solid phylogenetic framework is needed for the entire group and more data on ontogenetic transformations of annelid musculature are desirable.

Structure, ultrastructure, and function of the terminal organ of a pelagosphera larva (Sipuncula)

SummaryThe terminal organ, a structure enabling pelagosphera larvae of Sipuncula to form temporary attachments to substrata, was examined behaviorally and with light and electron microscopy for larvae of Golfingia misakiana, collected from the Florida Current. The terminal organ appears as a retractile rounded knob with a short neck joining the posterior extremity of the trunk. It can attach larvae directly to substratum or can secrete a tether-like mucus strand about which the organism moves. In unattached larvae, the terminal organ is often placed in the mouth. The terminal organ of a 5.5 day old larva consists of 29 cells: 8 epidermal, 3 mucus, 2 tension-bearing, 5 sensory, 10 retractor muscles and 1 unknown cell. The mucus cells are presumed to release the adhesive material while the microvilli on the tension-bearing cells, with their dense cores of microfilaments, bear the strain. The latter are joined directly to the retractor muscles which originate on the dorsal body wall near the anus. Two of the sensory cells terminate within the cuticle flanking the adhesive pore and are assumed to be cuticle strain receptors. Three sensory cells terminate in cilia that extend posteriorly from the pore. These may function in substratum evaluation prior to temporary attachment, or settlement preceding metamorphosis. The terminal organ is compared to adhesive organs in other soft-bodied metazoans and although it approximates the structure found in some rotifers, it is considered to be independently evolved within the Sipuncula. The terminal organ can be understood as an adaptation in young larvae for protective attachment and facilitation of feeding whereas, in older larvae, it may only function in substrate evaluation prior to settlement.

Sipuncula: an emerging model of spiralian development and evolution

Sipuncula is an ancient clade of unsegmented marine worms that develop through a conserved pattern of unequal quartet spiral cleavage. They exhibit putative character modifications, including conspicuously large first-quartet micromeres and prototroch cells, postoral metatroch with exclusive locomotory function, paired retractor muscles and terminal organ system, and a U-shaped digestive architecture with left-right asymmetric development. Four developmental life history patterns are recognized, and they have evolved a unique metazoan larval type, the pelagosphera. When compared with other quartet spiral-cleaving models, sipunculan development is understudied, challenging and typically absent from evolutionary interpretations of spiralian larval and adult body plan diversity. If spiral cleavage is appropriately viewed as a flexible character complex, then understudied clades and characters should be investigated. We are pursuing sipunculan models for modern molecular, genetic and cellular research on evolution of spiralian development. Protocols for whole mount gene expression studies are established in four species. Molecular labeling and confocal imaging techniques are operative from embryogenesis through larval development. Next-generation sequencing of developmental transcriptomes has been completed for two species with highly contrasting life history patterns, Phascolion cryptum (direct development) and Nephasoma pellucidum (indirect planktotrophy). Looking forward, we will attempt intracellular lineage tracing and fate-mapping studies in a proposed model sipunculan, Themiste lageniformis. Importantly, with the unsegmented Sipuncula now repositioned within the segmented Annelida, sipunculan worms have become timely and appropriate models for investigating the potential for flexibility in spiralian development, including segmentation. We briefly review previous studies, and discuss new observations on the spiralian character complex within Sipuncula.

Two New Species of Nephasoma (Sipuncula: Golfingiidae) from the Western Atlantic Ocean

Abstract Two new species of the genus Nephasoma, phylum Sipuncula, are described from subtropical waters of the western Atlantic. Nephasoma (Nephasoma) columbaris is described from the central east coast of Florida where it is found along the edge of the continental shelf at depths of 61 to 104 m associated with the rubble from the reefs of the deep water coral Oculina varicosa. A second species, Nephasoma (Nephasoma) ditadii, is described from Brazilian waters in sand habitats at 150 m on the continental shelf off the coast of the state of São Paulo. Morphology of the two species is compared with allied species in the genus and the phylum. Usage in the taxonomic literature of the diagnostic characters “shield” and “pseudoshield” is reviewed and evaluated.