Robert R. Hessler

Temporal change in megafauna at the Rose Garden hydrothermal vent (Galapagos Rift; eastern tropical Pacific)

Hydrothermal vent communities must undergo substantial temporal change because of their dynamic physical milieu. This was verified in March 1985, when the Rose Garden hydrothermal vent on the Galapagos Rift was revisited after 514 years absence. Comparison of photographs from the two visits revealed considerable faunal change. Among the hosts to chemoautotrophic bacteria, vestimentiferans were reduced from dominance to very low numbers. The mytilid was now extremely abundant and dominated vent openings. Vesicomyids also were more abundant. In general, vent-field suspension feeders had declined; anemones were distinctly less abundant, and siphonophores and enteropneusts were virtually absent. The decline of serpulids was likely, but less obvious. Of the mobile scavengers and carnivores, both galatheids and whelks were distincly more common. n nThese community changes appear to result from both continuing recruitment and changes in the physical milieu. While the growth of some populations could have resulted from expanding opportunities, the population of at least one, the vesicomyid, had not achieved carrying capacity in 1979, and this could have pertained to others as well. The decrease of vestimentiferans may have been caused by declining vent-water flux, a process that would favor mytilids, or more complete vent-water filtration by the mytilids themselves. The same factors also could explain the reduction of vent-field filter feeders. These observations suggest that early stages in the cycle of Galapagos-type vent communities are likely to be dominated by vestimentiferans, but that bivalves will replace them with time.

Scavenging amphipods from the floor of the Philippine trench

Abstract Free vehicle, baited camera, and baited trap stations at 9600 and 9800 m on the bottom of the axis of the Philippine Trench demonstrate the presence of only one mobile scavenger, the amphipod Hirondellea gigas. This amphipod congregates in large numbers, consumes the bait in a matter of hours, and then disperses. Similar concentrations of amphipods have been recorded for all bait studies at hadal depths to date. In none of these did fish appear, even though they are characteristic of all samplings at shallower depths. The reason for this is obscure. Specimens of H. gigas can be classified according to developmental stage and sex. Except for stages ♂1 and ♀2, the stages are equivalent to instars, of which there are seven to eight for females and four for males. Brooding females were not trapped. The growth ratio is relatively constant among instars, except in the female where it decreases markedly during the period when there is a strong increase in the accumulation of reproductive products. Evidence favors the view that females breed only once. If this is so, total fecundity averages about 97 oocytes per female. Many H. gigas individuals had material in their guts that was ingested before being trapped. The fraction of such individuals decreased linearly with increasing maturity. A major fraction of the volume of this ingested material often consisted of bacteria. Several hypotheses are consistent with this result.

Distribution and behavior of scavenging amphipods from the central North Pacific

Deep-sea scavenging amphipod distributions, abundance, and behavior were studied with five-vehicle baited traps. The species composition and numerical abundances were comparable in the areas sampled. All size classes of the four predominant species (Paralicella caperesca, P. tenuipes, Orchomene gerulicorbis, and Eurythenes gryllus) were trapped including sexually mature individuals, but no females were ovigerous. The central North Pacific E. gryllus is morphologically distinct from Atlantic E. gryllus and may represent a different species. n nPelagic and demersal amphipod guilds exist, as determined by body size and vertical and horizontal distributional patterns. The demersal guild is composed of P. caperesca, P. tenuipes, and O. gerulicorbis, all of which are <2 cm in total body length and occur within 1 m of the sediment. E. gryllus, the only representative of the pelagic guild, ranges between 1.7 and 14 cm in total body length and has its greatest abundance several to tens of meters above the sediment. n nMembers of the demersal guild probably can detect and exploit both large and small food falls or autochthonous organic particles because of their proximity to the sediment. They reside within the area of the benthic boundary layer where currents are slowest, and the diffusion of odor is slower than higher in the water column, thus reducing the area from which they can be attracted to a food item. n nThe vertical distribution of E. gryllus correlates well with several physical features of the benthic boundary layer. Their vertical abundance increases an order of magnitude near the top of the Ekman layer, correlating with the increase in current velocities. It continues to increase up to 20 m, corresponding with the probable vertical extent of more than two-thirds of an odor trace. Their abundance decreases significantly above 20 m. The above correlations imply that the vertical distribution of E. gryllus is an adaptation that provides a wide chemosensory overview of the sediment, and that they may be adapted to use the swifter background currents above the Ekman layer in their search for food. Finally, their vertical distribution implies that they feed primarily on relatively large food particles. Individuals found above 50 m are likely to be unaware of food falls and probably feed by predation. n nSpecies in the demersal guild are very patchy and probably occur in groups even when not feeding; film data indicate that E. gryllus occurs singly when not at bait. Near an odeor source all species tend to choose bait with other amphipods on it, neglecting similar bait nearby. When given a choice of traps, individual E. gryllus prefer to enter traps containing individuals of similar age and sex. At least at close range, locating a food item is probably based more on information received from other amphipods rather than food odor. It is unknown how this communication might occur.

CENTRAL NERVOUS SYSTEM OF HUTCHINSONIELLA MACRACANTHA (CEPHALOCARIDA)

The central nervous system of the cephalocarid Hutchinsoniella macracantha is well developed, although missing several structures which are characteristic of the general crustacean plan. There are no signs of eyes either in the adult or in the larva. The organ of Bellonci, central body, protocerebral bridge, and paracentral lobes are absent. The mushroom bodies occur in the central nervous system in a form different from that found in malacostracan crustaceans and are unexpectedly well developed. They are connected to the olfactory lobes, which in this species are displaced ventrally into the clypeus (anterior part of the so-called labrum), extremely large, and of hitherto unseen construction. The central nervous system of Hutchinsoniella does not conform to a paradigm of overall primitiveness and must have evolved separately in the cephalocarid line for a long period.

REPRODUCTIVE SYSTEM OF HUTCHINSONIELLA MACRACANTHA (CEPHALOCARIDA)

Hutchinsoniella macracantha is a simultaneous hermaphrodite. The small, paired ovaries are cephalic. From each, a long oviduct runs posteriorly to the eighteenth trunk segment, where it loops forward and runs toward the sixth trunk segment. The paired, sausage-shaped testes are dorsal to the midgut in the anterior end of the abdomen. A vas deferens runs down from the anterior end of each testis to join the oviduct. A common gonoduct exits on the posterior face of the sixth thoracopod. The ovaries contain no nurse or follicle cells, although projections from the epithelial cells and oocytes intertwine in the core of the ovary. The mechanism for translocating oocytes along the oviduct is problematical; a conveyor belt hypothesis is offered. Vitellogenesis does not begin until the posterior loops of the oviduct. Only 1 pair of oocytes matures at a time. Spermatogonia are scattered over the length of the testes. Each multiplies to form a small cluster of simultaneously developing spermatocytes; there are no cytoplasmic bridges or nurse cells. Within the testes, sperm formation is asynchronous and continuous. In addition to the acrosome, nucleus, and aflagellar central rod, each sperm cell forms voluminous vacuoles in the remaining cytoplasm; the associated volume increase probably causes mature sperm to enter and move through the testicular central lumen into the vas deferens. There is no obvious morphological obstacle to self fertilization. Each huge (approximately 0.4 mm long) ovum is attached to the ninth thoracopod with cement secreted by glands located midventrally in the ninth thoracic segment (Less)

Digestive System of the Cephalocarid Hutchinsoniella Macracantha

The digestive tract of the cephalocarid Hutchinsoniella macracantha begins with an atrium oris, posterior to the mouth. The esophagus loops up through the head and ends with a valve to the midgut. The epithelial cells and cuticle of both these structures are connected with long, winding, apically distended microvilli. The midgut is a straight tube with a pair of diverticula anteriorly. Midgut epithelial cells have microvilli, light vesicles, and a peculiar endoplasmic reticulum that is produced into a palisade of extensions toward the apical surface. Outside the basal lamina are muscle and peri-intestinal cells which send fingerlike projections into the basal portion of the midgut epithelial cells. The rectum joins the midgut near the end of the eighteenth segment and consists of unspecialized epithelial cells. In spite of the simple gross morphology the digestive tract has a complicated musculature. Circular muscles follow the whole tract including the diverticula. Radial muscles attach to the esophagus and rectum. Many longitudinal muscles are found inside the circular muscles in the anterior part of the midgut but only a few outside posteriorly. Gland cells occur in the labrum and diverticula. Absorption of nutrients seems to be limited to the midgut epithelial cells. Metabolites are transported via their endoplasmic reticulum to that of the peri-intestinal cells. (Less)

The excretory system of Hutchinsoniella macracantha (Crustacea, Cephalocarida)

The primary excretory organ of adult Hutchinsoniella macracantha is the maxillary gland, which is located laterally in the posterior portion of the cephalon. It consists of an end sac and a tubule which is divided into proximal and distal sections. The end sac is composed of podocytes filled with endocytotic vesicles which grade in size upward to a large residual body. A large valve controls the opening connecting the end sac to the proximal tubule. The proximal tubule is distinguished by a dense lining of long microvilli. The distal tubule is lined with a thin cuticle. The excretory pore is a valvelike slit on the base of the posteromedial surface of the protopod of the second maxilla. In the nauplius, an antennal gland with similar morphology is located within the most proximal segment of the second antenna. In the adult, this gland appears to be nonfunctional. (Less)

Sensory structures associated with the cuticle of Hutchinsoniella macracantha (Crustacea, Cephalocarida)

The body cuticle of the cephalocarid crustacean Hutchinsoniella macracantha Sanders, 1955, carries two types of small setae which differ in their external shape and number of ciliated sensory cells. It also has two types of pores, one being a gland opening, the other containing the tip of a cilium. The setae and the pore type containing a ciliated sensory cell are considered chemosensory organs.

Tegumental glands of Hutchinsoniella macracantha (Cephalocarida)

Three types of tegumental glands of Hutchinsoniella macracantha body, limb, and caudal, are described for the first time. Previously described salivary and cement glands also belong to this category. All have the same fundamental construction. They consist of 1 or 2 secretory cells opening into a short cuticular channel ending in a pore. The channel is Formed by a sheath or canal cell and an intermediate cell. The intermediate cell has an actin-like filament bundle encircling the duct. If is a type of tricellular tegumental gland common in crustaceans. The secretory cells can be very large, the caudal gland in the cerci being 0.5-0.6 mm in length. They are, at least distally, completely filled with secretory granules. Some contain enormous Golgi-like bodies. The secretory cells vary in cytological details and granular secretion, both within a pair and depending on body position. It is concluded that a common gross morphology of the tegumental glands of Hutchinsoniella houses different functions. (Less)

SEGMENTAL PODOCYTIC EXCRETORY GLANDS IN THE THORAX OF HUTCHINSONIELLA MACRACANTHA (CEPHALOCARIDA)

All thoracic limbs of Hutchinsoniella except for the ninth contain entirely of podocytes. At the periphery of the gland, each podocyte with the investing basal lamina; centrally, there is no conspicuous podocytes, either in compact glands or in a more diffuse arrangement, decapods, mysids, and isopods, and perhaps occur in other crustaceans known to filter hemolymph and to pinocytotically extract constituents ification and storage of residues. Histologically, the thoracic limb identical to the end sac of the maxillary and antennal glands, but or exit duct. The second maxilla lacks this gland, even though in every other way. On the basis of podocyte histology and conclude that these glands are serially homologous to the end sac glands, and are descendant from the segmentally from the repetitious ancestralexcretory system. (Less)