The component community structure of larval trematodes in the pulmonate snail Helisoma anceps

Factors that affected the component community structure of larval trematodes in the pulmonate snail Helisoma anceps in Charlie's Pond, North Carolina, were studied over a 15-mo period using a multiple mark-recapture protocol. Patent infections of 8 species were observed in 1,485 of 4,899 snails examined. Reproductive activity, population size, and survival rate of the snail population were estimated to evaluate the extent of resource availability for the parasites. Antagonistic interactions between trematode species that occurred at the infracommunity level had a neglible effect on the composition and structure of the component community. The patterns observed at this level were related to temporal heterogeneity in the abundance of infective stages (mostly miracidia), differential responses of trematode species to the diverse and constantly changing distribution of snail size and abundance, differential mortality of snails infected with certain trematode species, constant recruitment of 1 trematode species over time, and the existence of predictable disturbances such as the complete mortality of the host population and recruitment of a replacement population during a 6-8 wk period. The last factor operated as a reset mechanism for this snail-trematode system once each year. A model of patch dynamics, with snails as patch resources, best explains the organization and dynamics of this system.     

Fine structure of the retinal photoreceptors of the ranch mink Mustela vison

The structure of the retinal photoreceptors of the ranch mink (Mustela vison) has been investigated by light and electron microscopy. In this mammalian species, the photoreceptors can be readily differentiated and adequately described by the classical terminology of rods and cones, with the rods being the more numerous. Rods are long slender cells while cones are shorter and stouter in appearance. Both rods and cones are highly differentiated and extremely polarized cells consisting of an outer segment, a non-motile connecting cilium, an inner segment, a nuclear region and a synaptic process extending to an expanded synaptic ending. Morphological differences are noted between rods and cones for most of the various regions of these cells. While rods reach to the cell body of the retinal pigment epithelial (RPE) cells, larger apical processes from the RPE extend to the shorter cone cells, so that both photoreceptor types are in intimate contact with the retinal epithelial cells.     

Fine structure of comparable synapses in a mature and larval lobster muscle

Neuromuscular synapses from the single excitor axon to the proximal accessory flexor muscle (PAFM) was studied by serial section electron microscopy in a 1st stage larval (< 0.1 g) and a large adult (6.8 kg) lobster. The adult innervation of a lateral and a medial fiber, physiologically identified as low- and high-output respectively, was similar in the number and mean size of synapses but had significantly larger pre-synaptic dense bars for the high-output synapses. This correlation between quantal transmitter output and pre-synaptic dense bars and the appearance of exocytotic profiles along the dense bars strongly implicates the bars as active sites of transmitter release. Moreover the mature innervation is differentiated on the basis that the percentage of dense bar area to synaptic area is 9% for the low-output type compared to 22% for its high-output counterpart. In the larval PAFM the excitatory axon has not proliferated many branches and the innervation is therefore localized to groups of fibers in the lateral, medial and central regions of the muscle rather than to individual fibers. The lateral and medial sites of innervation representing putative low- and high-output types respectively (because of their location) do not differ in the size and number of pre-synaptic dense bars thereby suggesting a similarity in quantal synaptic transmission. However the percentage of dense bar area to synaptic area is 40% for the lateral site compared to 67% for the medial site. Since this is a trend mimicking the mature innervation it shows an early stage in the differentiation of low-and high-output synapses. Furthermore the main axon provides half of the total innervation in the larval PAFM but none in the adult thereby demonstrating a restructuring of multiterminal innervation.     

Spatial heterogeneity in recruitment of larval trematodes to snail intermediate hosts

Spatial variation in parasitism is commonly observed in intermediate host populations. However, the factors that determine the causes of this variation remain unclear. Increasing evidence has suggested that spatial heterogeneity in parasitism among intermediate hosts may result from variation in recruitment processes initiated by definitive hosts. I studied the perching and habitat use patterns of wading birds, the definitive hosts in this system, and its consequences for the recruitment of parasites in snail intermediate hosts. Populations of the mangrove snail, Cerithidea scalariformis, collected from mangrove swamps on the east coast of central Florida are parasitized by a diverse community of trematode parasites. These parasites are transmitted from wading birds, which frequently perch on dead mangrove trees. I tested the hypothesis that mangrove perches act as transmission foci for trematode infections of C. scalariformis and that the spatial variation of parasitism frequently observed in this system is likely to emanate from the distribution of wading birds. On this fine spatial scale, definitive host behaviors, responding to a habitat variable, influenced the distribution, abundance and species composition of parasite recruitment to snails. This causal chain of events is supported by regressions between perch density, bird abundance, bird dropping density and ultimately parasite prevalence in snails. Variation between prevalence of parasites in free-ranging snails versus caged snails shows that while avian definitive hosts initiate spatial patterns of parasitism in snails through their perching behaviors, these patterns may be modified by the movement of snail hosts. Snail movement could disperse their associated parasite populations within the marsh, which may potentially homogenize or further increase parasite patchiness initiated by definitive hosts.     

Guild structure of larval trematodes in the snail Helisoma anceps: patterns and processes at the individual host level

Factors that influenced the infracommunity structure of trematodes parasitizing the pulmonate snail Helisoma anceps were studied over a 15-mo period; the guild included 8 species of parasites. Infracommunities were depauperate, with double patent infections observed in only 7 of 1,485 infected snails; a total of 4,899 was examined. Halipegus occidualis-Haematoloechus longiplexus was the most common dual infection. Both species share the same definitive host and, in both cases, eggs are the infective stage for the snail. Switches and losses of infections in individual snails were observed, suggesting the occurrence of dynamic interactions within the guild. A dominance hierarchy was constructed based on field observations and experimental infections. Echinostomatids were dominant; species without rediae in their life cycles were subordinates. Halipegus occidualis (which has rediae) was intermediate in dominance. Spatial and temporal heterogeneity in the distribution and abundance of trematode infective stages indicate that not all the snails have the same probability of becoming infected. Habitat structure, behavior of the definitive host, the nature of the infective stages, and snail population dynamics (mortality, recruitment, and size structure) generated spatial and temporal heterogeneity in this system. As a consequence, predictions of the probabilities of interspecific interactions based on an analysis of observed and expected frequencies of multiple infections could be inappropriate unless the potential sources of heterogeneity are considered.     

Rapid colonisation of Lymnaea stagnalis by larval trematodes in eutrophic ponds in central Europe

High recruitment rates of multiple species and hierarchical competition are the keys to a competitive exclusion model of community assembly in larval trematode communities in molluscs. Eutrophic environments provide conditions for accelerating trematode transmission and this would increase the strength of interspecific interactions. To test these predictions, we provide the first known assessment for a pulmonate snail host, and for highly productive aquatic environments, of the rates of colonisation and extinction at the level of individual snail host patches, of a large guild of trematode species. Using a uniquely large dataset from a relatively long-term mark-recapture study of Lymnaea stagnalis in six eutrophic fishponds in central Europe, we demonstrate extraordinarily rapid colonisation by trematodes of a snail host, thus meeting the assumptions of the competitive exclusion model. Overall annual colonisation rates ranged from 243% to 503% year⁻¹ so that the odds of trematode establishment in an individual snail in these ponds are two to five times per year. Extinction rates were substantially lower than colonisation rates and, therefore, would not result in turnover rates high enough to significantly affect prevalence patterns in the snail populations. At the species level, analyses of sample-based estimates of probabilities of colonisation revealed that shared species traits associated with transmission and competitive abilities determined the limits of colonisation abilities. Colonisation rates were exceedingly high for the species transmitted to the snails passively via eggs. There was a significant effect of species competitive abilities on colonisation rates due to subordinate species being substantially better colonisers than both strong and weak dominants, a pattern consistent with the predictions of the competition-colonisation trade-off hypothesis. Our results suggest that, with the extraordinarily high trematode colonisation potential in the area studied, the spatial and temporal patterns of intraspecific heterogeneity in recruitment may provide conditions for intensification of interspecific interactions so that complex community assembly rules may be involved.     

Fine structure of the liver in the larval lamprey, Petromyzon marinus L.; hepatocytes and sinusoids

The ultrastructure of hepatocytes, bile canaliculi, and hepatic sinusoids of the larval lamprey, Petromyzon marinus, was examined using thin-sectioned and freeze-fractured tissues. The liver is a "tubular gland" with hepatocytes arranged in a tubular fashion around large bile canaliculi. Hepatocytes are roughly conical in shape, with their tapered apices facing a bile canalicular lumen. They possess extensive rough and smooth endoplasmic reticulum, a well-developed Golgi complex, abundant mitochondria, and varying numbers of large secondary lysosomes. Both secondary lysosomes and the Golgi complex are concentrated in the apical or peribiliary cytoplasm, indicating a possible role in bile secretion. The apical surfaces of the hepatocytes bear numerous elongate microvilli and occasional cilia, which project into the bile canaliculi. The hepatocytes are joined, apically, by junctional complexes composed of zonulae occludentes and adhaerentes. In freeze-fracture, the zonulae occludentes are of variable apicobasal depth and consist of honeycomb-like meshworks of fibrils. Spaces of variable width frequently appear in the P-face grooves, indicating that the zonulae occludentes are "leaky." Numerous communicating (gap) junctions join the hepatocytes laterally. Varying numbers of lateral microvilli project into the intercellular spaces and, basally, the plasma membrane is deeply infolded, resulting in the formation of apparently interdigitating basal processes resting upon a thin basal lamina. Sinusoids are composed of both a heavily-fenestrated, continuous endothelium, and phagocytic reticulo-endothelial (Kupffer) cells. Depsite the difference in arrangement of their hepatocytes, the mammalian and lamprey livers show similar ultrastructural features.     

Fine structure of the larval midgut of the fly Rhynchosciara and its physiological implications

The midgut of Rhynchosciara americana larvae consists of a cylindrical ventriculus from which protrudes two gastric caeca formed by polyhedral cells with microvilli covering their apical faces. The basal plasma membrane of these cells is infolded and displays associated mitochondria which are, nevertheless, more conspicuous in the apical cytoplasm. The anterior ventricular cells possess elaborate mitochondria-associated basal plasma membrane infoldings extending almost to the tips of the cells, and small microvilli disposed in the cell apexes. Distal posterior ventricular cells with long apical microvilli are grouped into major epithelial foldings forming multicellular crypts. In these cells the majority of the mitochondria are dispersed in the apical cytoplasm, minor amounts being associated with moderately-developed basal plasma membrane infoldings. The proximal posterior ventriculus represents a transition region between the anterior ventriculus and the distal posterior ventriculus. The resemblance between the gastric caeca and distal posterior ventricular cells is stressed by the finding that their microvilli preparations display similar alkaline phosphatase-specific activities. The results lend support to the proposal, based mainly on previous data on enzyme excretion rates, that the endo-ectoperitrophic circulation of digestive enzymes is a consequence of fluid fluxes caused by the transport of water into the first two thirds of midgut lumen, and its transference back to the haemolymph in the gastric caeca and in the distal posterior ventriculus.     

Fine structure of the galeal styloconic sensilla of larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised of a smaller cone, which is inserted into a style. They are thought to play an important role in host-plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each approximately 70 um in length and 30 um in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.     

Fine structure of the spiracular glands in larval Drosophila melanogaster (Meig.) (Diptera : Drosophilidae)

The spiracular glands in the third-instar larvae of Drosophila melanogaster (Diptera : Drosophilidae) were examined with light and electron microscopy. Three club-shaped, unicellular glands are associated with each spiracle. Each gland has a large nucleus with a well-developed nucleolus and polytene chromosomes. Cytoplasmic features include a prominent plasma membrane reticular system (PMRS), which contains electron-dense material and gives rise to endocytotic vesicles. Numerous lipid droplets, mitochondria, free ribosomes, glycogen, lysosomes, and vesicles are scattered throughout the cytoplasm. The smooth endoplasmic reticulum is abundant, but rough endoplasmic reticulum and Golgi complexes are sparse. Small lipid droplets appear to fuse with one another to form larger droplets. In late third-instar glands, normal mitochondria decrease in number; they appear to degenerate and become converted into dense bodies with finely granular interiors. Several microvillous channels containing lipid are present within the cytoplasm. In the lumina of most of these channels are found the distal tips of cuticular ductules. The cuticular ductules merge with one another dorsally to form the main duct that carries the lipoid secretion to the spiracle cleft. The ultrastructure of the spiracular glands is consistent with their roles in the synthesis of lipoid secretion, which is used to provide hydrophobicity of the surface and to trap small particulate matter.     

Temporal variations in the infection of a population of Cerithidea cingulata by larval trematodes in Kuwait Bay

The prosobranch gastropod Cerithidea cingulata (Gastropoda: Potamididae) in Kuwait Bay was examined for larval trematode infections over a 17-month period. A total of 2537 snails were examined and 1265 (49.9%) found to be infected with one or more species of trematodes. The component community in the snail comprised 12 species representing the families Cyathocotylidae (2), Echinostomatidae (2), Haplosplanchnidae (1), Heterophyidae (2), Microphallidae (1), Philophthalmidae (2), Plagiorchiidae (1) and Schistosomatidae (1). Cyathocotylid II (41.6%) was by far the most prevalent species followed by the microphallid (3.9%), the two species comprised 90% of the total trematode fauna. The prevalence of infection increased with shell size and was significantly higher in male (47%) than female (33%) snails. Multiple infections were observed in only 15 (1.2%) of the infected snails; cyathocotylid I and cyathocotylid II combination occurred 14 times and heterophyid I and the microphallid occurred once. Trematode species were more diverse and prevalent in winter, and cercarial shedding peaked in summer. Behaviour of the definitive host and snail population dynamics were probably the major contributors to the detected temporal pattern in the infections.     

Nitrogenous compounds of the body fluids of usual and unusual gastropod hosts of larval trematodes

Body fluid was analyzed for nitrogenous compounds in four selected fresh water gastropods. Striking differences were noted in the levels of total nitrogen (TN), protein nitrogen (PN), and nonprotein nitrogen (NPN) among the four snails studied.