Ultrastructural features of oogenesis in some marine neoophoran turbellarians

An ultrastructural study of oogenesis has been undertaken in some marine species of neoophoran turbellarians belonging to the Tricladida, Proseriata, and Prolecithophora. Among marine triclads, Cercyra hastata has oocytes with a remarkable amount of what appears to be autosynthetic proteinaceous yolk, while Procerodes dohrni and P. lobata have alecithal oocytes. Among the Proseriata, several species of the primitive family Monocelididae (subfamily Monocelidinae) have oocytes with a discrete amount of what appears to be autosynthetic yolk, while Parotoplana macrostyla, of the derived family Otoplanidae, has alecithal eggs. Finally preliminary observations on Plagiostomum maculatum (order Prolecithophora) suggest that oocytes have autosynthetic yolk globules.These results support the hypothesis (previously formulated on the basis of similar ultrastructural investigations on freshwater triclads) that the presence of autosynthetic yolk in some neoophoran turbellarians can be interpreted as a primitive character inherited from an ancestor with archoophoran organization. This plesiomorphic character would still be maintained in some species while lost in others following differentiation of vitellaria which are characteristic of the Neoophora.     

Small-scale spatial distribution of marine meiobenthos: the effects of decaying macrofauna

Summary To evaluate the effects of decaying animals on small-scale horizontal distribution of meiobenthos in muddy habitats, a laboratory experiment was performed at the Askö Laboratory in the northwestern Baltic Sea. A microcosm (35×55×28 cm) containing a ca. 7-cm thick layer of sieved (0.5 mm) sublittoral mud was established in June 1990. Three months later specimens of the bivalve Macoma balthica were collected and killed in boiling water. The sediment inside the microcosm was implanted with empty shell, empty shell and dead animal or left alone. At the end of the experiment (17 days) visual examination of the microcosm revealed black spots at the sediment surface where dead animals had been implanted. The densities of nematodes, the most abundant group (98%), were not significantly different between areas. However total non-nematode fauna was found in much lower numbers (P<0.01) in the black spot areas. A multivariate analysis (detrended correspondence ordination) of nematode species abundance data separated samples from the black spot areas from the others. Of the 25 nematode species recorded in the microcosm, there was a significant difference between areas for four species. The decaying animals clearly attracted Monhystera disjuncta which was almost 6 times as abundant in the black spot areas compared to control and shell areas. Both the overall dominant species, Leptolaimus elegans and Calomicrolaimus honestus were found in lower numbers in the areas of dead Macoma than in control and shell areas. Sabatieria pulchra was found in lower numbers in the control areas compared to shell and dead animal areas. The overall structure of the nematode assemblage indicated a shift to lower dominance in the dead animal areas and it is speculated that decomposing animal tissue may be of primary importance regarding spatial distribution of meiobenthos.     

Septate junctions in the cephalic epidermis of turbellarians (Bipalium)

Summary In the epidermis of turbellarians septate junctions of the pleated sheet type have been demonstrated in conventional thin sections and freeze fractured preparations. The structure of these junctions entirely agrees with that found in molluscs and arthropods.Financially supported by DFG (Sto 75/3,4; We 380/5)     

Ultrastructure of muscle cells from a few selected turbellarians: possible correlations between form and function

A comparative ultrastructural study of muscle cells from several turbellarian species revealed a basic smooth type of construction, with thick and thin myofilaments. However, the distribution of contractile elements, "dense bodies", smooth reticulum, mitochondria, as well as junctional specializations was found to vary within different systems. This variability is probably related to greater or lesser physical demands on the musculature. From a mechanical viewpoint, the most ‘powerful’ muscular system studied was represented by the pharyngeal bulb of Geocentrophora applanata, in which muscle fibers were organized into a compact, square reticulum, hemidesmosomes occurred in large numbers, and myofilaments were grouped in patches reminiscent of A and I bands of striated muscles. At the other extreme, muscular fibrils of the catenulids studied were thin and regularly spaced along the body wall, and the branched ends of radial myofibrils in the parenchyma were connected to the epithelial layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental evidence for the origins of determinative development in the polyclad turbellarians

Cell-deletion experiments were carried out on the embryo of the polyclad turbellarian Hoploplana inquilina to examine further the nature of development in primitive spiralians. The polyclads are of particular interest because they provide a link between the regulative development of acoels and the determinative development of annelids and molluscs. Single blastomeres were deleted at the two- and four-cell stages by puncture through the eggshell membrane with tungsten needles. All deletions resulted in abnormal larvae with consistent characteristics representing half or three-quarter Müller's larvae. The number of larval eyes was a particularly useful character in revealing mosaicism. This study establishes the polyclad embryo as determinative, but with important cell interactions also occurring during early development, and provides evidence that mosaicism became associated with spiral cleavage in the quartet form during the evolution of the Turbellaria.     

Old and new records of turbellarians from the central areas of Spain

From the point of view of its biodiversity, the Iberian Peninsula is one of the most interesting areas. Its special geographical location and geological history have created a high degree of diversity within the invertebrates groups in general, and within the ecological group of the Turbellaria in particular. In the present work, old and new records of turbellarian species from the central areas of the Iberian Peninsula are presented. Data on their biology and distribution within the freshwater habitats, mainly limnic, are also provided. This study is mainly based on the project Fauna Iberica III. This revised version was published online in July 2006 with corrections to the Cover Date.     

Occurrence of symbiotic turbellarians in the oyster,Crassostrea virginica

Crassostrea virginica was collected from several locations where it is cultured, both along the Northumberland Strait of New Brunswick and Malpeque Bay on the coast of Prince Edward Island. The oysters were found with two turbellarians on their gills. Urastoma cyprinae (Graff) was found in the oysters mostly during the warmer months of the year in numbers averaging as high as 50 worms per host (N = 50) and with as much as 78% of the host population infected (N = 100). Paravortex gemellipara (Linton) was also found during warmer months, but much less frequently or abundantly.Both male and female oysters were found to have U. cyprinae. No eggs or recent young of U. cyprinae were found in hosts; female-mature individuals of P. gemellipara with young were found from June through August.     

Effects of the deposit-feeding benthic bivalve Macoma balthica on meiobenthos

To evaluate the effects of a deposit-feeding bivalve on meiobenthic assemblage structure in muddy habitats, a laboratory experiment was performed at the Askö Laboratory in the northwestern Baltic proper. Microcosms, surface area 104 cm², containing a c. 7-cm thick layer of sieved (0.5 mm) sublittoral mud were established in June 1990. Two months later the tellinid bivalve Macoma balthica was added in quantities varying from 0 to 40 individuals per microcosm. After 5 months the effects of the bivalves on the meiofauna were surprisingly small. The density of harpacticoid copepods was lowest (P <0.05) in microcosms containing a high density of M. balthica. It is suggested that competition for food resources was responsible for this pattern. For all other meiofaunal groups, including nematodes which were the most abundant taxon (99%), no significant differences (P>0.05) were observed among treatments. The assemblage structure of the nematodes was similar between treatments. The vertical distribution of both major taxa and nematode species appeared to be unaffected by the presence of the bivalve.     

The diet of three species of umagillid neorhabdocoel turbellarians inhabiting the intestine of echinoids

Three species of umagillid turbellarians were found to have different nutritional relationships with their echinoid hosts: Syndisyrinx franciscanus ingests host intestinal tissue and ciliates that are symbiotic in the intestine of the host; Syndesmis dendrastrorum consumes intestinal tissue and materials that have been ingested by the host; an umagillid that closely resembles Syndesmis echinorum subsists entirely on host intestinal tissue.     

An ultrastructural study of oogenesis in a marine triclad

The ultrastructural features of oocyte differentiation were studied in the marine triclad Cercyra hastata. Oocytes at several stages of maturation, each surrounded by follicle cell projections, are present within each of the two ovaries. A pre-vitellogenic and a vitellogenic stage have been detected in the oogenesis of C. hastata. The pre-vitellogenic stage is mainly characterized by an increase in the nuclear and nucleolar volume and activity, and the appearance and development of cortical granule precursors which are elaborated by the Golgi complex. In early phases of the vitellogenic stage, intense delamination and blebbing of the nuclear envelope occurs which probably contributes to an increase in number of cytoplasmic membranes and to transfer of nuclear material to the cytoplasm. The rough endoplasmic reticulum is extensively developed and often assumes a ‘whorl’ array. Several areas of yolk precursor formation appear in the whorls. Numerous 2–5 μm protein yolk globules are subsequently formed which appear surrounded by a double membrane (cisternae of the smooth endoplasmic reticulum) and become randomly distributed throughout the cytoplasm of mature oocytes. The peripheral ooplasm is occupied by a monolayer of electron-dense cortical granules. Finally, the evolutionary significance of the autosynthetic mechanism of yolk production is discussed.     

The habitat function of mangroves for terrestrial and marine fauna: A review

Mangroves are defined by the presence of trees that mainly occur in the intertidal zone, between land and sea, in the (sub) tropics. The intertidal zone is characterised by highly variable environmental factors, such as temperature, sedimentation and tidal currents. The aerial roots of mangroves partly stabilise this environment and provide a substratum on which many species of plants and animals live. Above the water, the mangrove trees and canopy provide important habitat for a wide range of species. These include birds, insects, mammals and reptiles. Below the water, the mangrove roots are overgrown by epibionts such as tunicates, sponges, algae, and bivalves. The soft substratum in the mangroves forms habitat for various infaunal and epifaunal species, while the space between roots provides shelter and food for motile fauna such as prawns, crabs and fishes. Mangrove litter is transformed into detritus, which partly supports the mangrove food web. Plankton, epiphytic algae and microphytobenthos also form an important basis for the mangrove food web. Due to the high abundance of food and shelter, and low predation pressure, mangroves form an ideal habitat for a variety of animal species, during part or all of their life cycles. As such, mangroves may function as nursery habitats for (commercially important) crab, prawn and fish species, and support offshore fish populations and fisheries. Evidence for linkages between mangroves and offshore habitats by animal migrations is still scarce, but highly needed for management and conservation purposes. Here, we firstly reviewed the habitat function of mangroves by common taxa of terrestrial and marine animals. Secondly, we reviewed the literature with regard to the degree of interlinkage between mangroves and adjacent habitats, a research area which has received increasing attention in the last decade. Finally, we reviewed current insights into the degree to which mangrove litter fuels the mangrove food web, since this has been the subject of long-standing debate.     

The ecological role of the callitrichidae: a review

The marmosets and tamarins fill a unique ecological role among the anthropoid primates, one that has not been fully recognized. Many misconceptions--that they are primitive, monogamous, territorial, and squirrellike--pervade the literature. These misconceptions are largely the result of misinterpreting laboratory studies which have not been confirmed with identified animals in natural habitats. Recent field studies, reviewed here, indicate that marmosets and tamarins have a highly derived ecological role, are not monogamous, feed largely on insects and plant exudates, and have uniquely specialized positional behavior involving clinging to vertical tree trunks in order to feed on exudates. Accompanying these behavioral traits are a number clawlike nails on all digits except the hallux, and a three-cusped upper molar morphology. These form a suite of characteristics unique among the living primates, many of which are related to their ecological role. We believe that the marmosets and tamarins are members of a guild of exudate feeders in which plant exudates are an important component of the diet. It is within this framework of a primate foraging guild that we can best understand many of their morphological and behavioral adaptations.     

Chromosomal evolution in marine triclads and polyclads (Turbellaria)

From an analysis of chromosomal mutations in seven species among five genera of marine triclads and polyclads, three different types of mechanisms of such mutations are identified: 1) rearrangement involving the centromere such that its position is changed, 2) rearrangement of whole chromosome arms, and 3) Robertsonian mechanisms. These mechanisms are the same as those reported for freshwater turbellarians, but aneuploidy and polyploidy, which are common in freshwater species, were not detected in these marine turbellarians.     

Karyology of a marine population of Gyratrix hermaphroditus (Turbellaria,Rhabdocoela) and chromosomal evolution in this species complex

Karyology and reproductive biology of a marine population of the species complex Gyratrix hermaphroditus, from Roscoff (Brittany, France), have been investigated. A diploid complement of six chromosomes was determined from spermatogonial mitotic figures. One chromosome pair is metacentric, the second is intermediate between meta- and submetacentric, and the third is subtelocentric.In this population, regular meiosis occurs in both female and male germ lines, and the animals reproduce only by means of amphimictic eggs. Certain specimens of the population showed the elimination of one of the three bivalents during the first meiotic division in spermatogenesis. It seems that such animals produce normal and aneuploid sperm simultaneously; the aneuploid sperm are not capable of fertilization.The Roscoff population differs in its karyotype (2n = 6) from freshwater populations, which are either diploid (2n = 4) or polyploid (3n = 6, 4n = 8). These results suggest that aneuploidy played a role in the differentiation of freshwater populations from an originally marine species complex.     

Nitrogen inputs to a marine embayment: the importance of groundwater

We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods. Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m²/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove. Nitrogen inputs from groundwater were approximately 300 mmol-N/m³/y of Cove water. Road runoff contributed an additional 60 mmol/m³/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.     

Polar dinosaurs and the question of dinosaur extinction: a brief review

Although the physiology of dinosaurs is still a matter of controversy, there is no doubt that some of them were able to live in environments that were too cold for ectothermic reptiles, as shown by discoveries of Jurassic and Cretaceous polar vertebrate assemblages which contain dinosaurs but lack turtles and crocodiles. This adaptation of dinosaurs to cool climates invalidates hypotheses according to which dinosaur extinction at the end of the Cretaceous was a result of long-term climatic cooling. The pattern seen at the K/T boundary, with the disappearance of dinosaurs and the survival of ectothermic reptiles, is completely different from that seen in Arctic regions during the Late Cretaceous, where ectotherms disappeared, while dinosaurs subsisted, during cooler periods. The idea of an intense and enduring cold spell at the K/T boundary, caused by the Chicxulub impact, is extremely unlikely in view of the pattern of vertebrate extinction (survival of endotherms, extinction of dinosaurs). Models of environmental events following the impact must take this palaeontological constraint into consideration.     

The impact and control of biofouling in marine aquaculture: a review

Biofouling in marine aquaculture is a specific problem where both the target culture species and/or infrastructure are exposed to a diverse array of fouling organisms, with significant production impacts. In shellfish aquaculture the key impact is the direct fouling of stock causing physical damage, mechanical interference, biological competition and environmental modification, while infrastructure is also impacted. In contrast, the key impact in finfish aquaculture is the fouling of infrastructure which restricts water exchange, increases disease risk and causes deformation of cages and structures. Consequently, the economic costs associated with biofouling control are substantial. Conservative estimates are consistently between 5-10% of production costs (equivalent to US$ 1.5 to 3 billion yr(-1)), illustrating the need for effective mitigation methods and technologies. The control of biofouling in aquaculture is achieved through the avoidance of natural recruitment, physical removal and the use of antifoulants. However, the continued rise and expansion of the aquaculture industry and the increasingly stringent legislation for biocides in food production necessitates the development of innovative antifouling strategies. These must meet environmental, societal, and economic benchmarks while effectively preventing the settlement and growth of resilient multi-species consortia of biofouling organisms.     

The potential importance of podocysts to the formation of scyphozoan blooms: a review

Podocysts are cysts with stored reserves of organic compounds produced beneath the pedal discs of polyps of scyphozoans in the orders Rhizostomae (suborder Dactyliophorae) and Semaeostomae. They excyst small polyps that develop into fully active polyps (scyphistomae) capable of further podocyst production and of medusa production by strobilation. They contribute to increasing the number of polyps and also to survival through seasonal periods of reduced food availability or predation. These attributes may help support scyphozoan blooms, but as yet there are few quantitative data. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances