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Echinoderm skeletons are composed of calcium carbonate ossicles that join in a variety of ways to form flexible or, more rarely, fixed joints. Ossicle "fusion" in echinoderms has been widely reported in the literature to form various types of fixed joint, but fusion in the sense of chemical union (ankylosis) of the calcitic ossicles has rarely been demonstrated. The arm skeleton of ophiuroids is primarily composed of a series of vertebral ossicles; each vertebra is a compound ossicle that consists of paired ambulacral ossicles united by a fixed joint, often reported to be fused. Development of vertebral ossicles in the amphiurid brittlestar Ophiophragmus filograneus from the arm tip to the oral frame was examined using scanning electron microscopy to follow ontogeny of the vertebra. As an ambulacral ossicle grew, its stereomal trabeculae interdigitated and interlocked by hooking around those of its paired ambulacral, forming the characteristic sinuous suture line central to ophiuroid vertebrae. This three‐dimensional interlocking of stereom formed the joint between paired ambulacrals. With further growth of the vertebra, limited fusion of trabeculae of the paired ambulacrals added to the structure of the joint, primarily at the articular surfaces between successive vertebrae. The joint found here with interlocked trabeculae between ambulacral ossicles of O. filograneus appears to be the same type described in the literature in some echinoids and other ophiuroids. This unique type of fixed joint is described here and named the "campylogomphosis" (Greek: campylos, bent; gomphos, bolt). This newly recognized joint might have implications in echinoderm phylogeny, comparative biology, medicine, and materials science. [ABSTRACT FROM AUTHOR]

The Florida seagrass brittlestar, Ophiophi·agmus flograneus, is endeinic to shallow estuarine bays and lagoons of Florida, where it is exposed to variations in salinity that are beyond the ambient range for most echinoderms, Whereas the physiological basis for its salinity tolerance has not been studied, the species seems to have other aspects of its biology that help with its estuarine life. The full spectrum of its life-history pattern is unknown. Still, the presence of planktonic juveniles in an ichthyoplankton study of the Indian River Lagoon System indicates that it can be dispersed even if it lacks a pluteus lana Occurrence in tlie plankton ranges from spring through fall, although variations in timing and density differ among basins of the Indian River Lagoon System. Planktonic juveniles are smaller than those reported from benthic samples, although some demonstrate the unusual pattern of arm growth reported for new bentl-tic recruits of this species. The presence of two long non-adjacent arms of the larger specimens might serve to gi·asp blades of seagrass upon settlement to the benthos. [ABSTRACT FROM AUTHOR]

Echinoderm pedicellariae are groups of specialized opposable spines (valves) that articulate on a baseplate. The valves of a pedicellaria act together to grasp items for prey capture, surface cleaning, or defense. Pedicellariae have long been known to occur in sea urchins and seastars, and many types for different functions have been described. Here we describe a type of pedicellaria in basketstars and snakestars in the euryalid family Gorgonocephalidae. Valves of gorgonocephalous pedicellariae have been described and illustrated as independent "hooklets" since the 1800s. Aside from one illustration in 1930, baseplates have been illustrated only since the 1980s, after the advent of the scanning electron microscope. The structure, arrangement, and operation of valves on baseplates warrant their recognition as a pedicellaria, analogous to those of sea urchins and seastars. The gorgonocephalous pedicellaria is a multi-valved, unstalked, crossed pedicellaria. It is described and illustrated here with recommended morphological terminology to facilitate future studies on biomechanics, histology, paleontology, and phylogenetics. [ABSTRACT FROM AUTHOR]

New records of the Indo-Pacific seastar Mithrodia clavigera (Lamarck, 1816) include the north-central Gulf of Mexico, southeastern Florida, and Ascension Island. Material includes in-situ photographs, specimens from our own field collections, and museum specimens. This species was previously reported in the Atlantic basin in the 1880s off Brazil and in the western Caribbean Sea in the late 1960s and early 1970s. More recent findings are attributable to the advent of SCUBA, seafloor photography, and genetic analysis. The presence of M. clavigera in the Atlantic Ocean and adjacent waters probably represents a former connection with populations in the eastern Pacific Ocean before the rise of the Isthmus of Panama. Our observations here of specimens collected off Palm Beach and Fort Lauderdale, Florida, and the Gulf of Mexico add to our understanding of this species' behavior and ecology. [ABSTRACT FROM AUTHOR]

Adult specimens of the West Indian basketstar Astrophyton muricatum lack spines on basal branches of the arms. This character has been used in a number of taxonomic treatments to distinguish several genera of basketstars from others that bear arm spines on basal branches. Examination of a growth series revealed that juvenile A. muricatum bear arm spines on basal regions and lose them progressively farther out on the arm over time. At all sizes, spines on more basal spine-bearing branches lose the morphology that is typical of most spine-bearing segments. Individual spines seem to be resorbed from the imperforate claw-like tip to the bulbous perforated base. Spine bosses on spineless segments lose the characteristic morphology that supports the attachment and operation of spines. Of four possible hypotheses to explain the absence of spines from basal arm segments, we reject the hypothesis that juveniles fail to form spines on those segments. Data predominately support the hypothesis that basal spines are eventually resorbed, but the hypothesis that spines are released by histolysis at some point in life does not conflict with the evidence on spine loss. There is no evidence that spines are abraded and not regenerated. A taxonomic key to basketstars of the West Indian Province is provided that does not rely on presence or absence of arm spines. [ABSTRACT FROM AUTHOR]

Sea cucumbers (holothuroids) lack the only known echinoderm immune organ, the axial organ. Holothuroids of the families Synaptidae and Chiridotidae have coelomic organs, known as ciliary urns, that gather and excrete waste and, therefore, might function in immunity. Although ciliary urns are widely reported and illustrated in the literature, the process and histology of urn development remain unknown. Development and structure of ciliary urns were examined in Synaptula hydriformis using scanning electron, brightfield, and scanning laser confocal microscopy. Mature urns occurred on all three mesenteries in 10‐tentacled young and later growth stages, and developing urns were found in post‐pentactulae, 10‐tentacled young, and released juveniles. Developing urns were circular clusters of ciliated collar cells protruding from the mesentery. The cells increased in number to form the sessile cushion stage with a shallow lumen. The subsequent spoon‐shaped stage had a stalk and a deepened lumen with an extensive ciliary field where coelomocytes began to accumulate. Mature urns had a thin stalk and cornucopia‐shaped body with an abluminal epithelium of squamous cells and an adluminal epithelium of densely packed ciliated collar cells. Cell boundaries of the rim of mature urns and of the stalk and body of developing urns were outlined on one or both sides by microvilli and an elevated cell membrane. Ciliary urns resembling the cushion‐stage urns of S. hydriformis have been described in the sea star Archaster typicus. If urns in these groups are homologous, it is likely that cushion urns are plesiomorphic and that they are present and have been overlooked in other echinoderms. [ABSTRACT FROM AUTHOR]