Densovirus associated with sea-star wasting disease and mass mortality.

Populations of at least 20 asteroid species on the Northeast Pacific Coast have recently experienced an extensive outbreak of sea-star (asteroid) wasting disease (SSWD). The disease leads to behavioral changes, lesions, loss of turgor, limb autotomy, and death characterized by rapid degradation ("melting"). Here, we present evidence from experimental challenge studies and field observations that link the mass mortalities to a densovirus (Parvoviridae). Virus-sized material (i.e., <0.2 µm) from symptomatic tissues that was inoculated into asymptomatic asteroids consistently resulted in SSWD signs whereas animals receiving heat-killed (i.e., control) virus-sized inoculum remained asymptomatic. Viral metagenomic investigations revealed the sea star-associated densovirus (SSaDV) as the most likely candidate virus associated with tissues from symptomatic asteroids. Quantification of SSaDV during transmission trials indicated that progression of SSWD paralleled increased SSaDV load. In field surveys, SSaDV loads were more abundant in symptomatic than in asymptomatic asteroids. SSaDV could be detected in plankton, sediments and in nonasteroid echinoderms, providing a possible mechanism for viral spread. SSaDV was detected in museum specimens of asteroids from 1942, suggesting that it has been present on the North American Pacific Coast for at least 72 y. SSaDV is therefore the most promising candidate disease agent responsible for asteroid mass mortality.

Mitochondrial phylogeny of the brittle star genus Ophioderma.

We reconstructed the mitochondrial phylogeny of the species of the brittle star genus Ophioderma, using sequences of the Cytochrome Oxidase I gene (COI) to address four questions: (i) Are the species of Ophioderma described on morphological evidence reflected in mitochondrial genealogy? (ii) Which species separated from which? (iii) When did speciation events occur? (iv) What is the rate of COI evolution in ophiuroids? We found that most of the 22 described species we sampled coincide with monophyletic clusters of COI sequences, but there are exceptions, most notably in the eastern Pacific, in which three undescribed species were indicated. The COI phylogeny lacks resolution in the deeper nodes, but it does show that there are four species pairs, the members of which are found on either side of the central American Isthmus. Two pairs with a genetic distance of ~ 4% between Atlantic and Pacific members were probably split during the final stages of Isthmus completion roughly 3 million years ago. The rate of divergence provided by these pairs allowed the calibration of a relaxed molecular clock. Estimated dates of divergence indicate that the lineages leading to extant species coalesce at times much older than congeneric species in other classes of echinoderms, suggesting that low extinction rates may be one of the reasons that ophiuroids are species-rich. The mean rate of COI substitution in Ophioderma is three times slower than that of echinoids. Conclusions of previous mitochondrial DNA studies of ophiuroids that relied on echinoid calibrations to determine divergence times need to be revised.

Coherently aligned nanoparticles within a biogenic single crystal: A biological prestressing strategy.

In contrast to synthetic materials, materials produced by organisms are formed in ambient conditions and with a limited selection of elements. Nevertheless, living organisms reveal elegant strategies for achieving specific functions, ranging from skeletal support to mastication, from sensors and defensive tools to optical function. Using state-of-the-art characterization techniques, we present a biostrategy for strengthening and toughening the otherwise brittle calcite optical lenses found in the brittlestar Ophiocoma wendtii This intriguing process uses coherent nanoprecipitates to induce compressive stresses on the host matrix, functionally resembling the Guinier-Preston zones known in classical metallurgy. We believe that these calcitic nanoparticles, being rich in magnesium, segregate during or just after transformation from amorphous to crystalline phase, similarly to segregation behavior from a supersaturated quenched alloy.

Ophiuroid Skeleton Ontogeny Reveals Homologies Among Skeletal Plates of Adults: A Study of Amphiura filiformis, Amphiura stimpsonii and Ophiophragmus filograneus (Echinodermata).

The characteristic oral papillae of the Family Amphiuridae are shown to have conservative patterns of ontogenesis, even among congeners with differing modes of reproduction such as Amphiura stimpsonii, a brooder, and A. filiformis which has free-living juveniles. Homologous oral papillae can be identified by tracing the distinctive ontogenetic transformations of individual skeletal elements. This method shows that the oral papillae of adults are not serially homologous, and that homologies cannot necessarily be inferred from the relative positions of papillae in any particular ontogenetic stage. For example, the most proximal oral papilla develops like a tooth on the dental plate and later moves to the proximal oral plate; a distal papilla grows as a spine on the adoral shield and moves to the distal oral plate. Based on the development of the oral papillae of Amphiura, Amphioplus, and Ophiophragmus species, it appears that post-larval ontogenesis of the amphiurids can be more reliable than larval morphology as an indicator of phylogenetic affinity. However, there are striking differences in postlarval skeletal ontogenesis among congeners, such as in formation of the adoral shield spines and primary plates of the disc, which may be related to modes of reproduction and post-larval biology. It can take over a year for adult oral armature to develop in free-living amphiurid juveniles, and the process occurs before hatching in brooded young. Specializations in the oral armature of postlarvae are probably critical to their survival.

DEVELOPMENT OF AMPHIOPLUS ABDITUS (VERRILL) (ECHINODERMATA: OPHIUROIDEA). II. DESCRIPTION AND DISCUSSION OF OPHIUROID SKELETAL ONTOGENY AND HOMOLOGIES.

1. Amphioplus abditus has a vestigial two-piece larval skeleton that has portions with different crystallographic orientations. The larval skeleton is resorbed and, unlike that of echinoids, it does not act as a center of formation of the plates of the adult. The major skeletal elements of the adult develop from single (usually triradiate) spicules, and there is a uniform crystallographic orientation within each plate. 2. The radial shields, adoral shields, genital plates and genital scales are ophiuroid specializations without homologues in the asteroids. Ophiuroids can regenerate radial shields but not the apical primary plates (the latter are probably atavistic structures). 3. The madreporite and oral plates, generally thought to migrate from the dorsal surface of the disc, originate in situ on the ventral surface of A. abditus. A dorsolateral plate, probably confused with the madreporite in past studies, is a precociously formed interradial-1. The formation of a "precocious interradial plate" could be a vestige of the primitive ophiuroid madreporite. In fact, the madreporites of asteroids, ancient ophiuroids, and recent ophuroids may not be homologous. 4. The origin of each of the oral papillae is described. Buccal scales, previously (and incorrectly) thought to develop into peristomial plates, form the second oral papillae in A. abditus. Consequently, the second oral papillae of amphiurids should not be considered "oral tentacle scales". The true tentacle scales are cryptic structures within the buccal cavity. 5. The oral papillae of the different amphiurid genera are probably homologous. Judging from differences in the oral frame, there are probably two major amphiurid groups composed of taxa which retain the buccal scales as oral papillae (Amphioplus and possibly Amphiura), and those like Axiognathus (and possibly Amphipholis and Amphiodia) which resorb the buccal scales. 6. A new system of homologues is suggested for the plates of the ophiuroid oral skeleton. The proximal oral plate is considered the ambulacral portion of the first modified arm-segment and buccal scales may be the first pair of adambulacrals. The distal oral plates (ambulacral), adoral shields (adambulacral), and the first ventral arm-plate (within the buccal slit) compose the second transformed arm-segment of the oral frame. This pattern of homology, together with the dissimilarities between ophiuroid and asteroid discs constitute important differences between the ophiuroids and asteroids.