Population dynamics, biomass and productivity of Limapontia depressa (Gastropoda, Opisthobranchia) at Bull Island, Dublin, Ireland

Limapontia depressa was present in the saltmarsh pools at Bull Island, Dublin from October to June each year. Numbers peaked in December, with a secondary peak in April, and the maximum density recorded was just under 23000 indiv m^–2. The strong population seasonal cycle was not linked to salinity or chlorophyll-a cycles but did coincide with the drying of the pools. Average production (P) was 233 kJ m^–2 y^–1, with a P:B ratio of just 2:1, which is rather low for such a small animal and may be a function of the low growth rate. These conclusions must be treated with caution due to uncertainty over recruitment mechanisms. Gamete output (G) was calculated at a maximum of 28.6 kJ m^–2 y^–1. This is a higher secondary production than previously reported for saltmarsh herbivores, and may be an important link into the wider estuarine trophic system and a vital food resource for small waders. The low G:P ratio suggests that not all semelparous species necessarily adopt an r-strategy of 'reproductive recklessness'.     

Eastern Australian Pleurobranchomorpha (Gastropoda, Opisthobranchia)

Collections of pleurobranchomorph gastropods made in 1968 are described, and compared with material in the British Museum (Natural History) and with live Mediterranean specimens examined in the Naples Aquarium. All the eastern Australian species obtained are illustrated by line diagrams. Three species new for Australia are described, one is transferred to another known species, and six others are re-described. A list of the 12 species (with synonyms) reliably reported for eastern Australian waters concludes the paper, together with a key for their identification.     

Dorid nudibranchs from eastern Australia (Gastropoda, Opisthobranchia)

Collections of nudibranch gastropods made in 1968 are described. All the 18 eastern Australian species of doridaceans (excluding chromodoridiform species) obtained are illustra-ed and investigated anatomically, often for the first time since their original discovery over a century ago. A list of the dorid nudibranches reliably recorded from Australian waters (compiled by R. Burn) supplements the paper.     

Population dynamics,productivity and biomass allocation ofZizania latifolia in an aquatic-terrestrial ecotone

The population and production ecology of aZizania latifolia stand at a sheltered shore of the Hitachi-Tone River were investigated. Shoot emergence was observed twice a year; the fist was a synchronized shoot emergence in April and the second was from August to October. Aboveground biomass was mostly occupied by leaves and peaked at 1500 g dry weight m⁻² in August. The belowground biomass also reached its peak, 750 g dry weight m⁻², in August. The secondary shoots were small in spite of their high density. Leaves were produced continuously throughout the season. The leaf life span was as short as 55.6 days for cohorts that emerged from May through to September. Total annual net production ofZ. latifolia could be more than 3400 g dry weight m⁻². Shoot clusters of several centimeters were observed in April. The following self-thinning caused a regular distribution of the remaining shoots in August. Most shoots produced in August to October were found near a shoot persisting since April. They showed more concentrated distribution than shoots in April. A large biomass allocation to leaves and the ability to produce many clump shoots during the late growing period may facilitate dominance ofZ. latifolia in relatively sheltered sites.     

Particulate inputs to Dublin Bay and to the South Lagoon,Bull Island

There are substantial riverine and sewage particulate inputs into Dublin Bay. The main river, the R. Liffey, accounts for almost 85% of the riverine input, which amounts to 37.1 t d^–1 of SPM. The sewage input, which is discharged into the estuary, is slightly less at 35.3 t d^–1. The combined inputs deliver 17.4 t d^–1, 2.9 t d^–1and 1.2 t d^–1of particulate C, N and P, respectively, to the Bay as a whole. Although the particulate N load accounts for just 20% of the total N input, the retention of particulates within the system, and the subsequent remobilisation of N from the sediments is heavily implicated in the macroalgal blooms. A particulate mass balance of SPM, and of C, N and P for the South Lagoon showed no consistent pattern of import/export except that the fluxes of the individual elements reflected SPM load and varied with quantity rather than quality. The molar C:N ratio was around 6:1 although on occasion values approaching 3:1 were obtained, indicating very high quality and potentially labile SPM. The controlling factors in the budget appeared to be the water velocities and to a lesser extent, the SPM load, which surprisingly was no greater on a spring tide than on a neap. In this shallow, and primarily intertidal, system, wind- or wave-driven resuspension may exert more influence than predictable factors such as tidal range.     

Ultrastructure and Phylogenetic Significance of Notaspidean Spermatozoa (Mollusca, Gastropoda, Opisthobranchia)

Spermatozoa of five notaspidean opisthobranchs [Berthellina citrina, Berthella ornata, Pleuro-branchus peroni, Pleurobranchaea maculata, Umbruculum sinicum] were examined using TEM. In all five species, the acrosome (sensu lato) consists of an apical vesicle (the acrosomal vesicle) and acrosomal pedestal. The acrosomal pedestal overlaps the nuclear apex, and in P. peroni (and possibly B. ornata) is periodically banded—-the first reported incidence of this type of substructure in any euthyneuran acrosome. Although sperm nuclei of P. peroni, B. ornata and B. citrina differ in length and also the number of keels present (nucleus 7 μm long with four/five keels present in Pleurobranchus; 17 μm long with one keel in Berthella; 15 μm long with a very weak keel in Berthellina), the basal invagination to which the centriolar derivative, axoneme and coarse fibres are attached is always poorly developed, and very little overlap between nucleus and midpiece occurs. In P. maculata and U. sinicum, the nucleus forms a helical cord around the axoneme and mitochondrial derivative such that it is not possible to recognize exclusively ‘nuclear’ and ‘midpiece’ regions of the spermatozoon. In all notaspideans investigated, (1) the axoneme, coarse fibres and glycogen helix are enclosed by the paracrystalline and matrix components of the mitochondrial derivative and (2) a dense ring structure (attached to the plasma membrane) and glycogen piece are observed. While the glycogen piece is very short (0.85–1.43 μm) with a very degenerate axoneme in B. citrina, B. ornata and P. peroni, this region of the spermatozoan is well developed (30–35 μm long) in U. sinicum and exhibits a fully intact 9 + 2 axoneme. The ‘glycogen piece’(or its presumed homologue) in P. maculata spermatozoa is very short (0.65 μm), devoid of any axonemal remnant and constructed of a hollow, internal cylinder attached to an outer (incomplete) shell, and contains scattered (glycogen) granules. Spermatozoal structure supports a close relationship between the genera Berthellina, Berthella and Pleurobranchus. These three genera have more distant links with Pleurobranchaea, while Umbraculum maintains an isolated, specialized position within the Notaspidea.     

Substratwahlversuche mit Microhedyle milaschewitchii Kowalevsky (Gastropoda,Opisthobranchia: Acochlidiacea)

Zusammenfassung Microhedyle milaschewitchii Kow. aus einem sublitoralen Schellsandvorkommen bei Rovinj (Jugoslawien) bevorzugte in Zweifachwahlversuchen eine Korngröße von 0,5–2 mm, die mit 73,1% im natürlichen Substrat vorherrscht. Die Tiere verhielten sich zum Licht neutral und zeigten im Vergleich zu Protodrilus keine extreme Thigmotaxis. Sterile Sande werden ebenso wie getrockneter Sand gegenüber natürlichem Substrat abgelehnt, sind aber nach zehntägiger Inkubation mit Sandbakterien diesem gleichwertig. Zwischen bakterienhaltigem Quarzsand und Kalksand zeigte sich keine signifikante Präferenz. Für die Besiedlung eines Substrates scheinen optimale Korngröße und lebende Mikroorganismen maßgebend zu sein.

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Summary Microhedyle milaschewitchii Kow. obtained from a sublittoral shellsand near Rovinj (Yugoslavia) prefers a grain size range of 0.5 to 2 mm in simple-choice experiments. This fraction predominates in the natural substrate (73.1%). Light did not affect the behaviour of the animals which are much less thigmotactical than Protodrilus. Sterile sands are rejected when offered together with natural substrate, but are equally attractive after having been incubated with sand bacteria for ten days. Similarly dried sand is not accepted by the animals. There is no significant preference between quartz and chalk sands, both containing bacteria. Optimal grain size and living microorganisms apparently are essential for the colonization of a substrate.

     

Potential key characters in Opisthobranchia (Gastropoda, Mollusca) enhancing adaptive radiation

Five potential key characters which might have enhanced species radiation in the Opisthobranchia (Gastropoda) are discussed. These are: 3–4 cuticular plates in the gizzard of Cephalaspidea s.str., kleptoplasty in Sacoglossa, kleptocnides in Aeolidoidea, a symbiotic relationship with unicellular algae in Phyllodesmium Ehrenberg, 1831, and mantle dermal formations in Chromodorididae. Interpretation of the characters as key innovations is based on phylogeny and/or comparison of species numbers in subgroups. Possible adaptive zones are discussed, and alternative interpretations indicated.     

Comparative immunohistochemistry of the cephalic sensory organs in Opisthobranchia (Mollusca,Gastropoda)

The structure and function of the central nervous systems of opisthobranch gastropods have been studied extensively. However, the organisation and function of the peripheral nervous system are poorly understood. The cephalic sensory organs (CSOs) are known to be chemosensory structures in the head region of opisthobranchs. In the present study, we used immunohistochemical methods and confocal laserscanning microscopy to comparatively examine the CSOs of different opisthobranchs, namely Acteon tornatilis, Aplysia punctata, Archidoris pseudoargus and Haminoea hydatis. We wanted to characterise sensory epithelia in order to infer the function of sensory structures and the organs they constitute. Immunoreactivity against the three antigens tyrosine hydroxylase, FMRFamide and serotonin was very similar in the CSOs of all investigated species. Tyrosine hydroxylase-like immunoreactivity was detected primarily in subepidermal sensory cell bodies, which were much more abundant in the anteriorly situated CSOs. This observation indicates that these cells and the respective organs may be involved in contact chemoreception and mechanoreception. The dominant features of FMRFamide-like immunoreactivity, especially in the posterior CSOs, were tightly knotted fibres which reveal glomerulus-like structures. This suggests an olfactory role for these organs. Serotonin-like immunoreactivity was detected in an extensive network of efferent fibres, but was not found within any peripheral cell bodies. Serotonin-like immunoreactivity was found in the same glomerulus-like structures as FMRFamide-like immunoreactivity, indicating a function of serotonin in the efferent control of olfactory inputs. Besides this functional implication, this study could also add some knowledge on the doubtful homology of the CSOs in opisthobranch gastropods.     

Die Gattung Bathydoris Bergh, 1884 (Gnathodoridacea) im phylogenetischen System der Nudibranchia (Opisthobranchia, Gastropoda)

The genus Bathydoris Bergh, 1884 (Gnathodoridacea) in the phylogenetic system of the Nudibranchia (Opisthobranchiu, Gastropoda) The position of the genus Bathydoris Bergh, 1884 in the phylogenetic system of the Nudibranchia is discussed. Doridoxa Bergh, 1900, a genus which is assigned to the taxon Gnathodoridacea together with the genus Bathydoris, is taken out from this taxon, lacking snapomorphies. Doridoxa is considered to be a sister grou of the Euctenidiacea, with the transrormation of the right digestive gland into a caecum as the only synaomorphy with the latter. The Gnathodoridacea with the one enus Bathydoris is removed from the taxon Doridacea and considered to be the sister group or the latter. Gnathodoridacea and Doridacea form the taxon Euctenidiacea with the synapomorphy “dorsomedial position of anus and nephroproct”.     

Origins and relationships of primitive members of the Opisthobranchia (Mollusca: Gastropoda)

An examination of possible prosobranch precursors of opisthobranchs and pulmonates suggests that archaeogastropods are poor candidates, as they lack the complex female glands which characterize all higher gastropods. Similarities between members of the Rissoacea and Cerithiacea with the Opisthobranchia and Pulmonata are here considered to be a result of parallel evolution. Much of the basis for suggesting phylogenetic aRinities between members of these taxa hinges on the supposition that pyramidellids are opisthobranchs. This contention is questioned, as the euthyneurous nervous system and hermaphroditic reproductive system share weak structural and positional homologies with opisthobranchs. The range of morphological variation within the Pyramidellidae should be more thoroughly investigated, as this taxon is poorly known. The reproductive system of members of the Littorinacea is largely homologous with that found in the least derived opisthobranchs and pulmonates. This, when considered in conjunction with other morphological and palaeontological evidence, suggests that the least derived mesogastropods, the Littorinacea, provide a more precise model of the opisthobranch-pulmonate ancestor among living prosobranchs. The specialization of the digestive system in littorinaceans suggests that morphological divergence has taken place following the cladogenesis of these taxa, and that a direct ancestor/descendent relationship cannot be implied from extant littorinaceans. The status of Acteon and other acteonids as archetypal opisthobranchs is questioned. Moditication 01 all morphological systems, with the exception of the shell and mantle complex, diminish the position of the Acteonidae as ancestral opisthobranchs. Members of the Ringiculidae more closely approach the ancestral form, but have undergone modification, as well. No extant opisthobranch retains all plesiomorphic character states. For this reason, in addition to the fact that contusion exists as to the plesiomorphic conditions of some characters within the Opisthobranchia, a hypothetical ancestral opisthobranch is characterized. It appears that parallel evolution, which confounds the relationships of certain prosobranchs and opisthobranchs, is also evident within the Opisthobranchia. Much of the confusion that has surrounded the determination of phylogenetic relationships within the Opisthobranchia relates directly to the high incidence of parallelism throughout the subclass.     

On the phylogeny of the Arminidae (Gastropoda, Opisthobranchia, Nudibranchia) with considerations of biogeography

In the past, the different genera of the family Arminidae have been diagnosed (mostly) on the basis of plesiomorphic characters and therefore their monophyly is questionable. The Arminidae are characterized by the autapomorphies 'possession of marginal sacs' and 'rhinophores with longitudinal lamellae'. The genus Heterodoris is excluded from the family. Two possible hypotheses about the phylogenetic relationships within the Arminidae are presented.
The Arminidae probably originated in the western Pacific, near Japan and have distributed in a westerly direction. The most primitive species ( Dermatobranchus ) are restricted mainly to the western Pacific region while the more derived species ( Armina ) have a world-wide distribution.     

Ultrastructural, histochemical and cytochemical characterization of intestinal epithelial cells in Aplysia depilans (Gastropoda, Opisthobranchia)

To improve the current knowledge about the digestive system in opisthobranchs, light and electron microscopy methods were used to characterize the epithelial cells in the mid‐intestine of Aplysia depilans. This epithelium is mainly formed by columnar cells intermingled with two types of secretory cells, named mucous cells and granular cells. Columnar cells bear microvilli on their apical surface and most of them are ciliated. Mitochondria, multivesicular bodies, lysosomes and lipid droplets are the main components of the cytoplasm in the region above the nucleus of these cells. Peroxisomes are mainly found in middle and basal regions, usually close to mitochondria. Mucous cells are filled with large secretory vesicles containing thin electron‐dense filaments surrounded by electron‐lucent material in which acidic mucopolysaccharides were detected. The basal region includes the nucleus, several Golgi stacks and many dilated rough endoplasmic reticulum cisternae containing tubular structures. The granular cells are characterized by very high amounts of flat rough endoplasmic reticulum cisternae and electron‐dense spherical secretory granules containing glycoproteins. Enteroendocrine cells containing small electron‐dense granules are occasionally present in the basal region of the epithelium. Intraepithelial nerve fibres are abundant and seem to establish contacts with secretory and enteroendocrine cells.     

A comparative ultrastructural investigation of the cephalic sensory organs in Opisthobranchia (Mollusca, Gastropoda)

Cephalic sensory organs (CSOs) are specialised structures in the head region of adult Opisthobranchia involved in perception of different stimuli. The gross morphology of these organs differs considerably among taxa. The current study aims at describing the cellular morphology of the CSOs in order to reveal cellular patterns, especially of sensory epithelia, common for opisthobranchs. Transmission electron microscopy was used to characterise the fine structure of the organs and to compare the CSOs of four different opisthobranch species. The cellular composition of the sensory system is conserved among taxa. The epidermal cells in sensory regions are always columnar and ciliated cells are frequently apparent. The sensory cells are primary receptors arranged in subepidermal cell clusters. They extend dendrites which penetrate the epithelium and reach the surface. Some of the dendrites bear cilia, whereas others only build a small protuberance. Processing of sensory information takes place in the peripheral glomeruli of all species. Moreover, few taxa possess additional peripheral ganglia at the base of their CSOs. The results of the present study might support other investigations indicating that the posterior CSOs are primarily involved in distance chemoreception, whereas the anterior CSOs might be used for contact chemoreception and mechanoreception.