Metamorphosis and developmental evolution in <Emphasis Type="Italic">Ophionereis</Emphasis> (Echinodermata: Ophiuroidea)

The brittle star Ophionereis schayeri has abbreviated non-feeding development through a reduced ophiopluteus and a vitellaria larva. The metamorphic changes involved with development from a bilateral larva to a radial juvenile were examined in detail. The reduced ophiopluteus has a continuous ciliated band that breaks up into discontinuous ciliary ridges at the vitellaria stage. As the vitellaria develops, the juvenile rudiment forms in the mid-ventral region. The rudiment then undergoes a morphogenetic movement to the left of the larval anterior/posterior axis. This results in a dramatic transformation from bilateral to radial symmetry and is accompanied by development of juvenile structures and settlement, 6–7 days after fertilisation. Ophiuroid development through one larval stage, the ophiopluteus, is termed Type I; whereas development through two larval stages, the ophiopluteus and vitellaria, is termed Type II. We examined the evolutionary changes in the expression of Type II development in Ophionereis by comparing the ontogeny of six species with a range of larval forms. O. fasciata has the ancestral-like planktotrophic ophiopluteus. Vestigial pluteal structures in the larvae of O. schayeri, provide a link between ophiopluteal and vitellarial forms during evolution of non-feeding development. The diversity of larval forms in Ophionereis indicates that evolution of non-feeding development through a vitellaria (Type II) may have involved an increase in egg size, reduction of pluteal structures and shortening of the time to metamorphosis. Assessment of the phylogenetic significance of Types I and II development awaits additional comparative data on the metamorphic stages of other ophiuroid genera.Communicated by G.F. Humphrey, Sydney     

Interaction and impacts of two introduced species on a soft-sediment marine assemblage in SE Tasmania

Introduced species are having major impacts in terrestrial, freshwater and marine ecosystems world-wide. It is increasingly recognised that effects of multiple species often cannot be predicted from the effect of each species alone, due to complex interactions, but most investigations of invasion impacts have examined only one non-native species at a time and have not addressed the interactive effects of multiple species. We conducted a field experiment to compare the individual and combined effects of two introduced marine predators, the northern Pacific seastar Asterias amurensis and the European green crab Carcinus maenas, on a soft-sediment invertebrate assemblage in Tasmania. Spatial overlap in the distribution of these invaders is just beginning in Tasmania, and appears imminent as their respective ranges expand, suggesting a strong overlap in food resources will result from the shared proclivity for bivalve prey. A. amurensis and C. maenas provide good models to test the interaction between multiple introduced predators, because they leave clear predator-specific traces of their predatory activity for a number of common prey taxa (bivalves and gastropods). Our experiments demonstrate that both predators had a major effect on the abundance of bivalves, reducing populations of the commercial bivalves Fulvia tenuicostata and Katelysia rhytiphora. The interaction between C. maenas and A. amurensis appears to be one of resource competition, resulting in partitioning of bivalves according to size between predators, with A. amurensis consuming the large and C. maenas the small bivalves. At a large spatial scale, we predict that the combined effect on bivalves may be greater than that due to each predator alone simply because their combined distribution is likely to cover a broader range of habitats. At a smaller scale, in the shallow subtidal, where spatial overlap is expected to be most extensive, our results indicate the individual effects of each predator are likely to be modified in the presence of the other as densities increase. These results further highlight the need to consider the interactive effects of introduced species, especially with continued increases in the number of established invasions.Communicated by M.S. Johnson, Crawley     

Soil quality evolution after land use change from paddy soil to vegetable land

A survey was done in 15 typical villages, 150 soil and 86 vegetable plant samples were taken in Jiaxin prefecture of the Taihu Lake region, northern Zhejian province. Results indicate that after 15-20 years land use changed from the paddy rice-wheat (or oilseed rape) double cropping system, to a continuous vegetable land has caused soil quality dramatic change. (1) Acidification: average soil pH was 5.4; about 61% of total samples were pH < 5.5. It was 0.9 units lower than 10 years ago with same upland vegetable cultivation and was 1.2 units lower than soil pH of paddy rice-wheat (or oilseed rape) rotation. (2) Fertilizer salt accumulation: the average salt content was 0.28%, among these about 36.2% of the total samples contained more than 0.3%. (3) Nitrate N and available phosphorus (P) over accumulation: on average it was 279 mg NO3-N/kg, and 45-115 mg P/kg. Nitrate N four times higher and available P 4-10 times more than it is in present paddy rice-wheat rotation soils respectively. This has caused wide concern because of possible groundwater and well drinking water pollution by leached nitrate N and the P losses to water by runoff from vegetable lands induce surface water eutrophication.     

Microhabitat utilisation patterns in cryptobenthic coral reef fish communities

The cryptobenthic reef fish communities from four microhabitats at Orpheus Island, central Great Barrier Reef are described. Eighty-four 0.4m² samples yielded a total of 368 individuals from 42 species in eight families, with a mean density of 11 individuals m^–2 (±1.7SE) and 2.9 species 0.4 m^–2 (±0.2SE). Caves contained the highest number of both individuals (120) and species (26), followed by sand/rubble, soft coral, and open reefs. Microhabitat associations included cave and soft coral specialists. Site fidelity in 71 tagged individuals of 4 species was high, with a mean recapture rate of 53% (±8.4SE) remaining within the ~0.4 m² sampling area after a 48-h period. Behavioural observations also reflect this limited movement, with the dominant mode of behaviour in 7 species being a motionless state (67.5% ±11.6SE), followed by feeding (21.8% ±8.7SE), hiding (6.3% ±1.6SE), and swimming (4.4% ±1.5SE). Two distinct behavioural groups are identified: (1) sedentary forms, characterised by long periods of immobility (5 species); and (2) winnowers, characterised by long feeding bouts (2 species). The fine-scale partitioning of microhabitats, restricted home ranges, and sedentary behaviour of many cryptobenthic reef fish species suggest that this reef fish community exhibits similar patterns of habitat utilisation to their larger reef-fish counterparts, but at a much finer scale.     

Effect of epibionts on the microdistribution of chlorophyll in corals and its detection by fluorescence spectral imaging

The spatial distribution of chlorophyll in three coral species carrying invertebrate symbionts was determined using spectral imaging techniques. From each pixel of the image, full fluorescence spectral data was obtained as well as comparative ratios of fluorescence. The multipixel fluorescence map and the relative-intensity fluorescence ratios demonstrated a high concentration of chlorophyll a next to the pits of Cryptochirus coralliodytes in Favites halicora. Spectral similarity maps of Goniastrea retiformis infested with Lithophaga lessepsiana and of Millepora dichotoma infested with Savignium milleporum revealed relatively higher chlorophyll concentrations in these two corals next to the symbionts. We hypothesize that the invertebrate symbionts fertilize their immediate surroundings with their excreta, enhancing algal growth. The spectral analysis system used in this study made it possible to distinguish such changes by spatial quantitation of the fluorescence emitted from small surface areas.     

Genetic heterogeneity among blacktip shark,<Emphasis Type="Italic"> Carcharhinus limbatus</Emphasis>, continental nurseries along the U.S. Atlantic and Gulf of Mexico

Genetic population structure of the blacktip shark, Carcharhinus limbatus, a commercially and recreationally important species in the southeast U.S. shark fishery, was investigated using mitochondrial DNA control region sequences. Neonate blacktip sharks were sampled from three nurseries, Pine Island Sound, Terra Ceia Bay, and Yankeetown, along the Gulf of Mexico coast of Florida (Gulf) and one nursery, Bulls Bay, on the Atlantic Ocean coast of South Carolina (Atlantic). Sequencing of the complete mitochondrial control region of 169 neonates revealed 10 polymorphic sites and 13 haplotypes. Overall haplotype diversity and percent nucleotide diversity were 0.710 and 0.106%, respectively. Haplotype frequencies were compared among nurseries to determine if the high mobility and seasonal migrations of adult blacktip sharks have maintained genetic homogeneity among nurseries in the Atlantic and Gulf. Chi-square analysis and AMOVA did not detect significant structuring of haplotypes among the three Gulf nurseries, P(²)=0.294, _ST=–0.005 to –0.002. All pairwise AMOVA between Gulf nurseries and the Atlantic nursery detected significant partitioning of haplotypes between the Gulf and Atlantic (_ST=0.087–0.129, P<0.008), as did comparison between grouped Florida Gulf nurseries and the Atlantic, _CT=0.090, P<0.001. Based upon the dispersal abilities and seasonal migrations of blacktip sharks, these results support the presence of philopatry for nursery areas among female blacktip sharks. Our data also support the treatment of Atlantic and Gulf blacktip shark nursery areas as separate management units.Communicated by P.W.Sammarco, Chauvin     

Genetic variability and level of differentiation in North Sea and Baltic Sea populations of the green alga<Emphasis Type="Italic"> Cladophora rupestris</Emphasis>

Cladophora rupestris is a perennial filamentous macroalga belonging to the Chlorophyta. It is widely distributed on both sides of the northern Atlantic Ocean and penetrates into the brackish Baltic Sea down to ca. 4 psu salinity. In this paper we present evidence for genetic differentiation of a Baltic form of this marine alga. We assessed genetic structure within and among 11 populations ranging along a salinity gradient from the Norwegian coast to the northern Baltic Sea proper. Samples of 328 individuals were studied using starch-gel protein electrophoresis to evaluate genetic variability and interpopulation differentiation based on allozymes. Of 11 loci examined, only one was polymorphic. For this locus, encoding superoxide dismutase (SOD-3), a total of seven alleles were distinguished. We found two genetically differentiated groups of populations of C. rupestris, one Baltic Sea group and one North Sea group, with a distinct border in the southern Kattegat near the entrance to the Baltic Sea. The genetic differentiation for SOD-3, expressed as pairwise F_ST values between the populations, was generally higher within the Baltic Sea group (0.10-0.43) than within the North Sea group (0.05-0.10); in the latter group also fewer pairs of populations differed significantly. Pairs of populations from different groups had the highest F_ST values (0.20-0.60). Hierarchical analysis of variance showed that 29.6% of the total variation in the SOD-3 locus was explained by variation between the two groups, while only 4.2% was explained by variation among the populations within the groups. The remaining variation (66.2%) was found within the populations.     

House formation and feeding behaviour of<Emphasis Type="Italic"> Fritillaria borealis</Emphasis> (Appendicularia: Tunicata)

Numerous marine animals use elaborate filtration mechanisms to feed on particles suspended in the sea. Video-microscopy reveals that the common appendicularian Fritillaria borealis do so in a unique way. They live inside acellular, mucous houses, where their tail undulations act like a peristaltic pump in a close-fitting tail chamber to force water through a complex set of filters. Suspended particles, between ~0.2 and 30 µm in diameter, are retained in this process and propagated towards the mouth of the organism to be ingested. Coarser particles are back-washed out of the house during brief intervals of tail arrest, when the elastic recoil of the house walls reduces its volume to one-eighth of its inflated state. At the same time, the bifurcated tip of the animal's tail slides into a sleeve-like extension of the house. This sleeve acts like a flipper to propel the animal and collapsed house several millimetres to a few centimetres through the sea as soon as muscular activity of the tail is resumed. After a second tail arrest, a special muscular twist brings the tail out of the sleeve, whereupon undulatory pumping movements are resumed to reinflate and process particle-laden water through the house. The complete behavioural cycle lasts ~30 s. The repetitive inflation and deflation cycles of the house are probably of importance for unclogging of the coarse "inlet" filters downstream to the tail pump, as well as for agglutination of the smallest food particles trapped in the house to ease their recapture in the endostylar pharyngeal filter during the ingestion process. The unique tail sleeve and flipper at the same time enable the animal to move away from the previously filtered water and rejected coarse particles, before the house is reinflated for resumed food filtration.     

Fluctuating asymmetry and nutritional condition of Baltic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) larvae

The level of fluctuating asymmetry (FA), defined as random deviations from perfect bilateral symmetry, is assumed to reflect the developmental instability (DI) of an organism. Because environmental and genetic stress may increase DI, FA has been used to assess the level of stress experienced by, for example, fish. In this study, left–right asymmetry of lapillar otoliths was related to nutritional condition as estimated from RNA/DNA ratios, in order to investigate the utility of FA to detect feeding-related stress in Baltic cod, Gadus morhua L., larvae. Cod larvae in intermediate and good nutritional condition showed similar values of FA, and these were more symmetric than for larvae in poor condition. As increased levels of FA were restricted to larvae in a condition comparable to that of larvae experiencing at least 3 days of starvation in laboratory experiments, it is suggested that FA is an insensitive indicator of short-term feeding success of larval cod. However, FA can be used to reveal severely starved larva populations and probably also populations that have been subjected to prolonged sub-optimal feeding conditions.     

Effect of temperature on growth,sexual maturity and reproduction of<Emphasis Type="Italic"> Acanthomysis robusta</Emphasis> (Crustacea: Mysidacea) reared in the laboratory

The growth, sexual maturity and reproduction of a shallow, temperate-water mysid, Acanthomysis robusta Murano, were investigated by rearing this species through a complete life cycle at 10°C, 15°C, 20°C and 25°C. The average daily growth rate, which ranged from 0.08 to 0.29 mm for immature mysids and from 0.04 to 0.15 mm for mature mysids, increased with increasing temperature. Water temperature had little effect on the molt increment (the increase in body length between successive molts), but clearly shortened the intermolt period (the interval between successive molts) with increasing temperature. Thus, the faster growth rate at higher temperatures is responsible for the shortened intermolt period. External sexual differentiation first became apparent at the 4th or 5th post-marsupial molt, when body length was 4.3–4.9 mm. Thereafter, males reached sexual maturity at the 9th or 10th post-marsupial molt (7.3–9.8 mm in body length), while females reached maturity at the 10th–13th molt (8.2–12.2 mm). In contrast to this small difference in molt number for sexual maturity, the post-marsupial age at first maturity, which ranged from 13 to 57 days for males and from 17 to 78 days for females, decreased markedly with increasing temperature. The incubation time of ovigerous females, which varied from 5 to 24 days, also decreased with increasing temperature. The relationships between water temperature and the three development times, the intermolt period, the age at first sexual maturity and the incubation time conformed to the effective day-degree concept. Although mature females maximally produced four broods in a lifetime, egg-bearing significantly lengthened the intermolt period and consequently functioned as a factor decreasing the growth rate.Communicated by T. Ikeda, Hakodate