COVER ILLUSTRATION: A cuticular scale (foreground) and a normal seta (background) from the prosoma of the anyphaenid spider, Aysha velox. This micrograph illustrates two of the most important morphological differences between scales and other types of spider setae: most scales have a reduced socket morphology and a pedicel bent at an obtuse angle, whereas other setae have large, well developed sockets, and a straight or slightly bent pedicel. SEM, 5000X. (See Townsend & Felgenhauer.)
A redescription of Rhabdamoeba marina, an inconspicuous
marine amoeba from benthic sediments
Andrew Rogerson, Fiona J. Hannah, and O. Roger Anderson
Embryogenesis, planulae longevity, and competence in the octocoral
Dendronephthya hemprichi
Maimon Dahan and Yehuda Benayahua
The infection mechanism of the cystophorous cercariae of Halipegus occidualis
(Digenea: Hemiuridae).
Derek A. Zelmer and Gerald W. Esch
Extracellular matrix and muscle fibers in the gills of freshwater
bivalves
Scott Medler and Harold Silverman
External gestation in exogonine syllids (Annelida: Polychaeta): dorsal egg
attachment by means of epitokous chaetae.
Michael Kuper and Wilfried Westheide
Ultrastructural evidence of detoxification in the alimentary canal of
Urechis caupo
Jaishri Menon and Alissa J. Arp
Cuticular scales of spiders
Victor R. Townsend, Jr. and Bruce E. Felgenhauer, pp 318-330
The ecology of an assemblage dominant: the encrusting bryozoan Fenestrulina
rugula
David K A Barnes and Andrew Clarke
Spicule formation in the solitary ascidian Bathypera feminalba (Ascidiacea,
Pyuridae)
Gretchen Lambert
A redescription of Rhabdamoeba marina DUNKERLY 1921, an inconspicuous marine amoeba from benthic sediments
Abstract. Rhabdamoeba marina was isolated from coastal waters off the Isle of Cumbrae, Scotland. It is worthy of attention for two reasons. Firstly, it is an example of a marine amoeba that is easily overlooked in fresh or enriched samples despite its widespread distribution within these waters. Secondly, the validity of the genus Rhabdamoeba has been questioned mainly because the original description by Dunkerly (1921) was brief and based solely on preserved material. The present redescription of this isolate, based on light and electron microscopy, removes the confusion surrounding the status of the genus. Amoebae are between 5.0 and 25.0 µm in length, and are essentially stationary apart from the occasional extension of a long, active, feeding pseudopodium up to 50 µm from the cell body. The pseudopodia of stationary cells are distinctive, appearing as knob-like or foot-like extensions around the cell body. Small papillae at the tips of these pseudopodia are evident at high magnifications by light and scanning electron microscopy. Diagnostic features of note at the ultrastructural level include a cell surface without a glycocalyx and a nucleus with a central nucleolus (i.e., vesicular nucleus) which is common in small amoebae. The cytoplasm contains Golgi cisternae and the mitochondria have tubular cristae. The life cycle of this protist includes a flagellated stage that has not previously been reported.
Additional key words: naked amoebae, taxonomy, flagellates, flagellated buds, ultrastructure
Embryogenesis, planulae longevity, and competence in the octocoral Dendronephthya hemprichi
Abstract. The development of the azooxanthellate octocoral Dendronephthya hemprichi from spawned gametes to planula larvae has been described, using scanning electron microscopy (SEM). Development of fertilized eggs is holoblastic, occurring until the 32-cell stage either by superficial divisions or by total cleavage. The pseudospiral development from the eight-cell stage leads to round stereoblastulae, followed by the appearance of embryos with bizarre forms. All developmental stages until the mature planulae are negatively buoyant. The planulae have a densely flagellated surface, and swim actively. The duration of development to mature planulae followed a seasonal pattern, ranging between 27—48 h from summer (at 26°C) to winter (at 20°—21°C), and probably reflected temperature effects. Similarly, in summer the first cleavage furrow was observed 2 h after mixing the gametes, whereas in winter this did not occur for at least 4 h. A significant variation was noted in egg diameter and planulae size from different colonies collected at the same time (December 1990). Longevity and maximal competency periods of 100 and 65 days, respectively, were found for the planulae during July—September 1990, suggesting a potential for long distance dispersal in the reef environment.
Additional key words: Cnidaria, Octocorallia, development, Red Sea
The infection mechanism of the cystophorous cercariae of Halipegus occidualis (Digenea: Hemiuridae).
Abstract. Cystophorous cercariae of Halipegus occidualis perforate the gut wall of feeding microcrustaceans via the explosive expulsion of a delivery tube from the cercariocyst, resulting in the transfer of the body of the cercaria into the hemocoel of the second intermediate host. The hypothesis that this discharge is associated with osmotic changes within the cercariocyst was tested. Increasing the osmolarity of the solution bathing the cercariocysts inhibited normal excystment. However, ~80% of cercariocysts subsequently placed in distilled water excysted normally. Excystment was sufficiently slowed in 0.25 Osm solutions to reveal at least two separate stages of the process. First, water drawn into the space between the inner and outer cyst walls upon release of the cercariocyst from the snail host creates sufficient turgor pressure to evert the delivery tube when the integrity of the caudal appendage is breached. Second, asymmetric osmotic influx of water into the cercaria forces the swelling body through the delivery tube. Contrary to earlier reports, the delivery tube is everted through the caudal appendage of the cercariocyst, and bears no spine at its tip. The fact that the infection mechanism is osmotically driven has both physiological and evolutionary implications in fresh-water hemiurids, whose origin is thought to be the marine environment.
Additional key words: parasite, trematodes, cercariocyst
Extracellular matrix and muscle fibers in the gills of freshwater bivalves
Abstract. The supportive tissue in the gills of three species of freshwater bivalves (Corbicula fluminea, Dreissena polymorpha, and Toxolasma texasensis) is composed of a similar extracellular matrix with interspersed muscle fibers. These matrices consist of a periodic acid Schiff-positive tissue supported by small collagen fibrils as determined by morphological and biochemical examination. The periodicity observed in fibrils and the electrophoretic migration patterns are consistent with other collagens. The ultrastructure of the muscle fibers associated with the connective tissue corresponds with a known type of smooth muscle. The high degree of similarity between the tissues of these species suggests a similar, conserved function.
Additional key words: collagen, smooth muscle, extracellular matrix
External gestation in exogonine syllids (Annelida: Polychaeta): dorsal egg attachment by means of epitokous chaetae
Abstract. Sexually mature individuals of three exogonine syllid species carry eggs or embryos on the dorsolateral body surface above the parapodia. They are attached to the body by chaetae that either penetrate the egg envelope (Sphaerosyllis hermaphrodita) or are closely apposed to the eggs (Brania sp. and B. subterranea). The chaetae are delicate capillaries packed together in bundles; they originate in epidermal thickenings or sacs situated dorsally and slightly anterior to the dorsal cirri. These chaetae are present only in sexually mature animals, and thus are interpreted as epitokous structures, but they are not homologous with the epitokous swimming chaetae situated below the dorsal cirri occurring in several syllids.
Additional key words: brood protection, epitoky, meiofauna, chaetae, Syllidae.
Ultrastructural evidence of detoxification in the alimentary canal of Urechis caupo
Abstract. The marine worm Urechis caupo exhibits specialized secondary lysosomes termed sulfide-oxidizing bodies throughout the epithelial tissues of the alimentary canal which appear to concentrate environmental toxins such as aluminum, silica and sulfur, as revealed by X-ray microanalysis. Ultrastructural examination of the alimentary tract of U. caupo reveals a digestive tube consisting of an inner layer of epithelium lining the central lumen, a layer of connective tissue with embedded muscle fibers, and a peritoneal covering. The thickness, ciliation and morphology of the innermost epithelium show regional variation that correlate with functional specialization, and the main cell types are ciliated, microvillate, and mucus- and enzyme-secreting cells. In the anterior part of alimentary canal (esophagus, gizzard, and stomach), the epithelium is almost entirely composed of ciliated and mucus cells, serving to move particulate matter and fluids, and the gizzard epithelial morphology reflects comminution of food particles. In the midgut, the epithelium is composed of microvillate and enzyme-secreting cells, suggesting involvement in digestive and absorptive functions. The hindgut, which acts as a water lung and is devoid of any digestive function, is lined by microvillate cells involved in gaseous exchange. The epithelium of the body wall and hindgut have been previously reported to contain numerous sulfide-oxidizing bodies, similar in ultrastructure and elemental composition to those of the alimentary canal described here (Menon & Arp 1992; 1993). These unusual organelles, which form a peripheral defense on all exposed surfaces of the worm, are part of an extensive suite of sulfide detoxification and tolerance mechanisms which enable the worm to flourish in a sulfide-rich environment.
Additional key words: worm, alimentary canal, sulfide, sulfide-oxidizing bodies, Urechis caupo
Cuticular scales of spiders
Abstract. Because the morphology of the cuticular scales of spiders is extremely variable both within and between families, any study attempting to use cuticular scales as a systematic character must first have a formal definition that differentiates scales from other types of setae. The purpose of our study was to evaluate the characters used previously in the literature to distinguish cuticular scales from other types of setae and, if necessary, to provide a new, comprehensive definition for this setal type in spiders. The results of our SEM survey of the surface morphology of the scales of 23 species of spiders representing 10 families do not support the morphology of the socket as a reliable character for distinguishing scales from other types of setae. Our results indicate that cuticular scales should be defined as flattened setae that have a pedicel bent so that the scale overlays the surface of the cuticle. Our results also suggest that the urticating hairs of theraphosid spiders should not be considered to be a type of cuticular scale. Instead, we propose the recognition of three main types of scales: lanceolate, spatulate, and plumose. In addition to qualitative comparisons, we measured and compared the cuticular scales for several species and found that differences in scale width were directly related to the morphotype of the scales being examined.
Additional key words: Araneae, morphology, SEM, macrosetae, urticating hairs
The ecology of an assemblage dominant: the encrusting bryozoan Fenestrulina rugula
Abstract. The cheilostome bryozoan Fenestrulina rugula is a major component of the encrusting fauna of physically disturbed shallow water habitats in Antarctica. On rocks collected from Rothera Point, Adelaide Island, F. rugula was the dominant occupier of space (88% of all bryozoans, 76% of all fauna), though spirorbid polychaetes showed more colonisation events per unit area. Growth rate was relatively rapid in comparison with other polar bryozoans, but slower than temperate or tropical species. In fully reproductive colonies almost 80% of zooids carried ovicells. Colonies whose growth brought them into contact with other encrusting fauna (usually another colony of F. rugula) produced smaller zooids and initiated ovicell production earlier than unrestrained colonies. Analysis of overgrowth interactions showed that F. rugula was a relatively poor competitor compared with other encrusting bryozoans, and most within-species interactions were indeterminate. Population mortality was relatively high, averaging 89% per annum, although not as high as in some other species from ephemeral habitats. F. rugula is thus a typical early coloniser in being relatively fast-growing, quick to mature, short-lived and a relatively poor competitor. The population dynamics and ecology of this assemblage dominant appear to have been influenced primarily by the ephemeral nature of its habitat.
Additional keywords: population dynamics, growth, competition, reproduction, mortality.
Spicule formation in the solitary ascidian Bathypera feminalba (Ascidiacea, Pyuridae)
Abstract. Bathypera feminalba is a solitary deep-water ascidian inhabiting rocky areas of the NE Pacific. The low mound-shaped body is sheathed by a thin fibrous tunic densely studded with calcitic spicules ~150 µm in length. Under the tunic, the ascidian body is covered by an epithelium bound to the inner edge of the tunic. New tunic and spicules form extracellularly in the epithelium inside the incurrent and excurrent siphons and around the periphery of the animal where it is attached to the substratum. Each spicule has a round flattened base, a narrow collared neck or sulcus that anchors it in the tunic, and a ring of distal spines. The central spine is the longest and usually directed outward from the central axis. A thin acellular organic envelope closely covers each spicule. A dense fibrous layer forms between the envelope and epithelium. Additional organic material remains in thin sections of demineralized spicules. The spicule base forms first; then, as the epithelial cells produce additional tunic, the spicules are pushed out of the epithelium into the ever-thickening tunic. Beyond this stage, no cells can be found in the tunic near the spicule base, and probably no further mineralization of the base occurs. As the spicule base is forming, secondary highly pseudopodial sclerocytes aggregate in the tunic overlying the distal part of the spicules. These sclerocytes are apparently responsible for production of the spines, which are comprised of numerous overlapping "shingles" of calcite. Mineralization of the spines probably continues throughout the life of the animal, because secondary sclerocytes remain numerous in the distal regions of all spicules.
Additional key words: biomineralization, calcite, tunicate
aTo whom correspondence should be addressed
