THE UROPHYSIS AND THE CAUDAL NEUROSECRETORY SYSTEM OF FISHES

1. The caudal neurosecretory system is defined in teleosts as a complex of secretory neurones (Dahlgren cells) in the caudal spinal cord leading by a tract to neurohaemal tissue organized as a typical neurosecretory storage-release organ: the urophysis. 2. The teleost urophysis is generally a distinct, easily recognizable, lobate structure of variable external form. Significant morphological variations lie in the organization of the neurosecretory fibres in relation to the vascular bed and in the degree of penetration of the meninx by the neurosecretory fibres to form an organ external to the spinal cord proper. 3. The elasmobranch caudal system is composed of large cells with short axons projecting to a diffuse vascular bed; there is no organized urophysis. 4. The caudal neurosecretory system and its urophysis appear late in post larval development by comparison with the hypothalamic neurosecretory system. The Dahlgren cells originate from the ependyma in development and also during regeneration of the caudal system in adult life. 5.The elasmobranch system may represent the more primitive condition, and stages in the evolution of the advanced urophysial types can be visualized. The particular histology shown by the caudal system appears to have taxonomic significance. 6.The cytology of the Dahlgren cell and its neurosecretory material is described. The proteinaceous neurosecretory material has an affinity for acid stains but not for the Gomori stains or reagents demonstrating SH/SS groups. The inclusions visible at the light-microscope level are aggregates of elementary neurosecretory granules, 800–2500A diameter, which originate from Golgi centres. The possible participation of preterminal axonal regions–and tubular systems evident therein—in the formation of neurosecretory material is considered. 7.The structure of the axon terminals raises questions about the way in which neurohormone may be released into the blood. Small vesicles have been variously interpreted as cholinergic synaptic vesicles and as products of the fragmentation of membranes of elementary neurosecretory granules. Evidence for the release of ‘neuro-secretion centripetally’ into the cerebrospinal fluid also exists. 8.Functional analysis of the caudal neurosecretory system has proven most difficult, The bulk of earlier data and more recent information indicate a role in ionic regulation. Increased sodium uptake by the gills of goldfish has been reported, as a result of administration of urophysial extract, and electrophysiological studies indicate a responsiveness of the system to variations in blood sodium ion concentration. The urophysis also has a definite pressor effect in eels and will stimulate water retention in anurans. The early claim of Enami that the system was involved in buoyancy regulation has never been substantiated. It must be admitted that the function of this system, virtually ubiquitous in teleost and elasmobranch fishes at least, has been anything but established and still represents a major challenge to comparative physiologists.     

High carriage of Endolimax nana in diarrhoeal specimens from homosexual men

Diarrhoeal stool specimens from patients attending clinics for homosexuals were more frequently positive for Endolimax nana (31-6 and 39%) than specimens from the general population (9%).     

Scale‐dependent longitudinal patterns in mussel communities

1. Species richness and assemblage patterns of organisms are dictated by numerous factors, probably operating at multiple scales. Freshwater mussels (Unionidae) are an endangered, speciose faunal group, making them an interesting model system to study the influence of landscape features on organisms. In addition, landscape features that influence species distributions and the scale at which the factors have the greatest impact are important issues that need to be answered to conserve freshwater mussels. 2. In this study, we quantified freshwater mussel communities at 16 sites along three mid‐sized rivers in the south‐central United States. We addressed the following questions: (i) Are there predictable longitudinal changes in mussel community composition? (ii) What landscape variables best explain shifts in community composition? and (iii) At what scale do landscape variables best predict mussel community composition? 3. After controlling for the influence of longitudinal position along the stream, we compared mussel distributions to a suite of hypothesised explanatory landscape variables across multiple scales – catchment scale (entire drainage area), buffer scale (100‐m riparian buffer of the entire catchment) and reach scale (100‐m riparian buffer extending 1 km upstream from the sampling site). 4. We found a significant and consistent longitudinal shift in dominant mussel species across all three rivers, with community composition strongly related to distance from the headwaters, which is highly correlated with stream size. After accounting for stream size, variables at the buffer scale were the best predictors of mussel community composition. After accounting for catchment position, mean channel slope was the best explanatory variable of community composition and appeared in all top candidate models at the catchment and buffer scales. Coverage of wetland and urban area were also correlated with community composition at the catchment and buffer scales. 5. Our results suggest that landscape‐scale habitat factors influence mussel community composition. Landscape features at the buffer scale performed best at determining community composition after accounting for position in the catchment; thus, further protection of riparian buffers will help to conserve mussel communities.     

Context-dependent effects of freshwater mussels on stream benthic communities

1. We asked whether unionid mussels influence the distribution and abundance of co‐occurring benthic algae and invertebrates. In a yearlong field enclosure experiment in a south‐central U.S. river, we examined the effects of living mussels versus sham mussels (shells filled with sand) on periphyton and invertebrates in both the surrounding sediment and on mussel shells. We also examined differences between two common unionid species, Actinonaias ligamentina (Lamarck 1819) and Amblema plicata (Say 1817). 2. Organic matter concentrations and invertebrate densities in the sediment surrounding mussels were significantly higher in treatments with live mussels than treatments with sham mussels or sediment alone. Organic matter was significantly higher in the sediment surrounding Actinonaias than that surrounding Amblema. Actinonaias was more active than Amblema and may have increased benthic organic matter through bioturbation. 3. Living mussels increased the abundance of periphyton on shells and the abundance and richness of invertebrates on shells, whereas effects of sham mussels were similar to sediment alone. Differences in the amount of periphyton growing on the shells of the two mussel species reflected differences in mussel activity and shell morphology. 4. Differences between living and sham mussel treatments indicate that biological activities of mussels provide ecosystem services to the benthic community beyond the physical habitat provided by shells alone. In treatments containing live mussels we found significant correlations between organic matter and chlorophyll a concentrations in the sediment, organic matter concentrations and invertebrate abundance in the sediment and the amount of chlorophyll a on the sediment and invertebrate abundance. There were no significant correlations among these response variables in control treatments. Thus, in addition to providing biogenic structure as habitat, mussels likely facilitate benthic invertebrates by altering the availability of resources (algae and organic matter) through nutrient excretion and biodeposition. 5. Effects of mussels on sediment and shell periphyton concentrations, organic matter concentrations and invertebrate abundance, varied seasonally, and were strongest in late summer during periods of low water volume, low flow, and high water temperature. 6. Our study demonstrates that freshwater mussels can strongly influence the co‐occurring benthic community, but that effects of mussels are context‐dependent and may vary among species.     

Development of Intracellular Membrane Systems in the Columnar Epithelium of the Urinary Bladder of the Euryhaline Goby Gillichthys mirabilis

The membrane systems in "columnar cells" of the goby urinary bladder were studied after staining with ferrocyanide-reduced osmium tetroxide (K arnovsky ). In addition to the endoplasmic reticulum, two distinct systems of membranes were observed: 1) a vesiculotubular system made up of vesicles and short tubules located between the Golgi area and the apical membrane and 2) well-developed infoldings of the laterobasal plasma membrane which form either complete or fenestrated sheets. Adaptation to 5% seawater or prolactin exposure of seawater fish induces a proliferation of these membrane systems and, in particular, of the complete infoldings of the laterobasal plasma membrane. These observations suggest high activity of these bladder cells in osmoregulatory adjustments to hypotonic environments. The divergence between cytological and physiological indicators of activity is considered.