Cross-shelf distribution of copepods and fish larvae across the central Great Barrier Reef

The distribution of total dry weight of zooplankton, copepod numbers and ichthyoplankton across the outer continental shelf in the central Great Barrier Reef was examined at bi-weekly intervals for three months over summer of 1983. Copepods were sampled (236 m net) within 10 m of the surface and within 10 m of the bottom. Mean densities in surface waters decreased markedly from the mid-shelf to outer shelf and the Coral Sea, but no cross-shelf gradient occurred in the bottom-water. Densities of copepods on the mid-shelf (surface and bottom waters) and in bottom-waters of the outer shelf were typically ca. 400 m^–3. Significantly lower densities (ca. 100 m^–3) occurred in surface waters of the outer shelf, except during outbursts of Acartia australis, when densities in these waters differed little from those elsewhere on the shelf. In oceanic waters, 10 km from the outer shelf station, copepod densities in surface waters were ca. 40 m^–3. Four of the five most abundant copepod taxa in surface waters, Paracalanus spp., Eucalanus crassus, Acrocalanus gracilis and Canthocalanus pauper, tended to be most abundant at the mid-shelf end of the transect. Acartia australis was sporadically very abundant in surface waters of the outer shelf, as was Paracalanus spp. in bottom-water of the outer shelf. An assemblage of Coral Sea species of copepod occurred in bottom-water of the outer shelf during two major intrusions, but not at other times. Densities of all common species varied considerably between cruises. Maximum densities of all common species except A. australis tended to be associated with diatom blooms linked to intrusions but a bloom did not necessarily mean all common species were abundant. Fish larvae included both reef and non-reef taxa, with reef taxa predominating on the outer shelf (approx 2:1 in density of individuals) and non-reef taxa dominating in nearshore samples (approx 2:1). Nine of the ten most abundant taxa analysed showed highly significant variation in numbers among stations and all but one of these also exhibited significant station x cruise interactions. Interactions generally reflected changes in the rank importance of adjacent stations from one cruise to the next or lack of any significant cross-shelf variation on some cruises where overall abundance of the taxa was low.     

Ecological investigations of the late-stage phyllosoma and puerulus larvae of the western rock lobster Panulirus longipes cygnus

The distribution and abundance of the late-stage phyllosoma larvae of Panulirus longipes cygnus George and the distribution and densities of the final larval stage, the puerulus, both in the plankton and at settlement along the coast, were investigated. A total of 3,617 late-stage phyllosoma (Stages VI to IX) and 301 puerulus larvae were caught at 187 plankton stations during the July to November periods 1974, 1975 and 1976 off the west coast of Australia between 29°00 to 32°30S and 113°30 to 115°00E. The depth range sampled was 0 to 35 m on the continental shelf and 0 to 90 m off the shelf. During onshore/offshore cruises with similar sampling effort on and off the shelf, 1,169 late-stage phyllosoma larvae were taken, of which only 9 were caught on the shelf, and these near the outer edge. A series of cruises sampling two areas beyond the shelf near 29°30 and 32°00S yielded 2448 late-stage phyllosoma, with greater densities of larvae in the northern location. The settlement of puerulus-stage larvae along the coast in the same geographical range was also greater in the north than in the south. The data from the onshore/offshore cruises showed a definite effect of moon phase on numbers of puerulus larvae caught on the shelf, with higher catches near new moon. The low numbers of puerulus larvae (usually 0, 1 or 2 individuals) caught at all stations showed that the puerulus stage is sparsely distributed in the plankton. Fewer puerulus larvae were present at the surface than at lower depths, but it was not possible to determine a depth preference for the puerulus between 10 m and the lowest depths sampled because of the low catch numbers. No relationships were found between puerulus larvae density and surface-water temperature, salinity, or plankton biomass at each station. Data on the larval distributions indicate that, near the end of their planktonic existence, the majority of the late-stage phyllosoma larvae of P. longipes cygnus are not carried onto the shelf, where mixing of oceanic and continental shelf waters occurs only on the outer third, but are transported southward by oceanic circulation beyond the shelf. The puerulus moults from the last phyllosoma stage beyond the shelf and completes the larval cycle by swimming across the shelf and settling in the shallow reef areas.     

Are local extinctions and recolonizations continuing at the colder limits of marine fish distributions? Halobatrachus didactylus (Bloch & Schneider, 1801), a possible candidate

In this paper, we compare the populations of the Lusitanian toadfish Halobatrachus didactylus along the Portuguese shore, using a putative fragment of the mitochondrial control region and the first intron of the S7 ribosomal protein gene. This demersal species ranges from the tropical West African coast to the Iberian Peninsula and the Mediterranean, having its northern limit in the Tagus River. For the putative control region, a single haplotype occurs in all fish from Tagus and Sado (the location immediately to the south) with a clear increase in diversity to the South, peaking at Algarve (south Portugal). The data seem to point to a very recent Holocenic colonization of Tagus and Sado from the South (possibly Algarve). We suggest that even small oscillations in sea surface temperature (SST) may cause local extinctions and subsequent recolonizations in populations of thermophilic fish that are at the cold limit of their distributions.     

Modelling the spatial distribution of red grouper (Epinephelus morio) at Campeche Bank, México, with respect substrate

The red grouper (Epinephelus morio) has historically been one of the most important fisheries of the Campeche Bank area in the Gulf of México, where two fleets from México and one from Cuba participate. Current stocks have been estimated to be a third or less as abundant as in the 1970s; hence, the fishery has been declared to be over-exploited. The federal government of México has expressed the need to investigate the potential use of protected areas as a management tool for recovery of this species. Based on artificial neural network (ANN) modelling, we analysed the spatial distribution of red grouper with respect substrate, considering the demographic structure of the stock. We found a significant relationship between the type of substrate and the three stages of development (juveniles, pre-adults and adults). Juveniles are distributed in shallow waters close to the coast on coral substrates; pre-adults are also associated with coral substrates, but at an intermediate depth; adults are found in deeper waters on sandy substrates. We also identified seasonal reproductive aggregation patterns, always in relation to substrate types. Reproductive aggregation during the winter and early spring in the north-eastern continental shelf and the almost permanent (exception in autumn) concentration of juveniles in coastal waters around the central and eastern coast of Yucatan are of particular importance for management. These patterns represent seasons and areas of high fish vulnerability, which is the basic criterion for further analysis of protected areas.     

Analysis of the structure of decapod crustacean assemblages off the Catalan coast (North-West Mediterranean)

We sampled the communities of decapod crustaceans inhabiting the depth zone between 3 and 871 m off the Catalan coast (North-West Mediterranean) from June 1981 to June 1983. The 185 samples comprised 90 species differing widely in their depth distributions. Multivariate analysis revealed four distinct faunistic assemblages, (1) littoral communities over sandy bottoms, (2) shelf communities over terrigenous muds, (3) upper-slope communitics, and (4) lower-slope or bathyal communities. The brachyuran crab Liocarcinus depurator is the most abundant species of the shelf assemblage, although L. vernalis dominates over the shallow sandy bottoms of the shelf. The dominant species of the upper-slope assemblage are nektobenthic species (Solenocera membranacea, Plesionika heterocarpus, Processa canaliculata), pelagic species (Pasiphaea sivado, Sergestes arcticus), and benthic species (Macropipus tuberculatus, Munida intermedia, Nephrops norvegicus). Aristeus antennatus comprise most of the biomass of the lower-slope community, which supports a greater diversity than the other assemblages. The main assemblages appear to be related to different hydrological characteristics, the extent of seasonal fluctuations, and to the changes in sediment structure associated with changes in the steepness of the bottom.     

Spatio-temporal variations in megabenthos abundance in three different habitats of the Catalan deep-sea (Western Mediterranean)

The spatio-temporal variations in the megafauna (fishes, crustaceans, and other invertebrates) in three different habitats on the slope in the Western Mediterranean (Catalan coast off Barcelona) have been investigated. Samples were collected at two fixed stations during 1991 and 1992, one inside and one outside a submarine canyon (at depths between 450 and 600 m) and additional samples were collected at another station on the lower slope (at a depth of 1200 m) in each season of the year. Three replicates of each sample were taken at each station. Differences observed between habitats, seasons, and zoological groupings were verified statistically by factorial ANOVA. The habitat appeared to be the main factor responsible for the differences observed in biomass and abundance values for the zoological groupings considered. Decapod crustacean biomass was higher in the submarine canyon than in neighbouring zones, and smaller species and juveniles were more abundant inside the canyon in the case of both fishes and crustaceans. Fish biomass was most abundant in the lower slope. Seasonality was also a factor, although to a lesser extent. Seasonal variations in biomass, especially fish biomass, were recorded in the deepest zone (1200 m). Crustaceans displayed lower sensitivity to seasonal factors, whereas other invertebrates did not, on the whole, exhibit seasonal variations.     

Distribution and abundance of ophiuroids on the continental shelf and slope of the East Sea (southwestern Sea of Japan), Korea

The present study was carried out to describe the distribution of ophiuroids on the continental shelf and slope of the East Sea, off the Korean Peninsula. Three cruises were made from 1985 to 1987 in the area between 35 to 38°N and 129 to 130°E. A total of ten species was identified from the samples collected at 136 stations. Among them four species (Amphiodia craterodometa, Amphioplus macraspis, Ophiura leptoctenia and Ophiura sarsi) constituted more than 98% of abundance by number. The continental shelf off Gampo, near 35°40N, could be considered to be the southern limit of cold-water species. The abundance of dominant ophiuroids changed according to depth. Three faunal zones, namely, the inner shelf, the shelf edge and the slope zones, coincided with the topographic divisions of continental shelf and slope. The major inhabitant of the inner shelf was A. craterodometa and that of the continental slope was O. leptoctenia. The shelf edge area revealed a transitional zone where all four species occurred, contributing to the total number of individuals with almost equal proportions. The temperature of bottom waters was considered to be a factor determining zonal distribution. The sediment characteristics were not closely related to ophiuroid distribution.     

The role of diatom resting stages in the onset of the spring bloom in the East China Sea

The abundance and species composition of diatoms were investigated along the PN line from the Okinawa Islands to the inner continental shelf in the East China Sea in the early spring of 1996. Viable diatom resting stages in sediments on the shelf were also enumerated by the extinction dilution method (most probable number method). Clear differentiation in the water masses was observed, with less saline, cold water (shelf water) on the shelf region, and warm, saline water (Kuroshio water) prevalent off the shelf and on the shelf edge. In the Kuroshio water, the abundance of diatoms was generally low but species diversity of diatoms was relatively high. In contrast, the spring bloom of diatoms was clearly observed in the shelf water where the water column was weakly stratified. The bloom was dominated by Chaetoceros debilis, contributing occasionally >80% of the diatom community. Resting stages of this species were also the most abundant taxon in the sediments on the shelf. Resuspension of the sediment during winter mixing of the water column should have enabled the resting stages to germinate at the surface. Subsequent vegetative growth after germination led to the formation of an early spring bloom of C. debilis when the water column was stratified and light availability had increased. Intermittent resuspension of sediment on the shelf, driven by strong winds and tidal currents, probably provides opportunities for diatoms with a resting stage to exploit favorable conditions for their germination and subsequent vegetative growth. It is further suggested that complex hydrographic conditions in the East China Sea result in a dynamic bloom with contributions by both autochthonous and allochthonous species.Communicated by T. Ikeda, Hakodate     

Depth zonation and metabolic adaptation in Dover sole,Microstomus pacificus,and other deep-living flatfishes: factors that affect the sole

Flatfishes of Monterey Bay, central California, undergo species replacements with increasing depth along a transect from 100 m on the continental shelf down to a depth of 1400 m on the continental slope. The Dover sole, Microstomus pacificus, differs from the other local flatfish species by undergoing an extensive ontogenetic vertical migration, occupying all depth zones at different life stages, and having its maximum spawning biomass in the oxygen minimum zone between 600 and 1000 m. Size-activity relationships and depth-activity relationships for the glycolytic enzyme lactate dehydrogenase (LDH) and for two enzymes associated with aerobic metabolism, malate dehydrogenase and citrate synthase (CS), were examined in white-muscle tissue of shallow-living, deep-living and ontogenetically-migrating species. Scaling coefficients (b) for weight-specific enzyme activity (log activity)=a+b (log wet weight), varied in sign as well as magnitude for fishes living at different depths. In the shallow-living California halibut Paralichthys californicus, LDH scaled positively (0.39) and CS scaled negatively (-0.15) with size, a pattern observed previously for most shallow-water fish species. The permanently deep-living species, the deepsea sole Embassichthys bathybius, differed in that both LDH and CS scaled strongly negative (-2.0 and-1.5, respectively). For the ontogenetically migrating Dover sole Microstomus pacificus, there was a shelf-slope transition. For the shelf specimens (200 m), LDH scaled positive (0.11) and CS negative (-0.29) and for the slope specimens (400 m), LDH scaled negative (-0.65) and CS strongly negative (-0.63). Rex sole, Glyptocephalus zachirus, showed a similar shelf-slope transition. Intraspecific depth-enzyme activity differences were not incremental, but changed abruptly between the continental shelf stations (100 to 200 m) and the continental slope (400 to 1400 m). Based on comparisons with laboratory-maintained individuals, the decline in the metabolic capacity of the white muscle of Dover sole is a phenotypic response to the low food and oxygen conditions of the continental slope. Contrary to expectation, anaerobic capacity (LDH activity) decreased in response to low oxygen conditions, suggesting that in a permanently hypoxic environment such as the oxygen minimum zone the metabolic strategy may be to not incur an oxygen debt that would be difficult to pay back.     

Deep-water decapod crustacean communities in the Northwestern Mediterranean: influence of submarine canyons and season

The specific composition and abundance of bathyal decapods in the Catalan Sea were investigated. A total of 109 bottom trawls were effected at depths ranging from 141 to 730 m on the continental slope in the Catalan Sea (northwestern Mediterranean) during two sampling cruises in spring and autumn 1991. Multivariate analysis of the samples revealed four groups of the decapod crustacean communities: (1) A shelf-slope transition-zone group at depths between 146 and 296 m, primarily characterized by the presence of Plesionika heterocarpus; (2) an upper-slope community between 245 and 485 m, characterized by the presence of the mesopelagic species Pasiphaea sivado and Sergestes arcticus, with Processa nouveli, Solenocera membranacea and Nephrops norvegicus as secondary species; (3) a middle-slope community below 514 m, with Aristeus antennatus and Calocaris macandreae as the most abundant species; (4) a group at 430 to 515 m, comprising all samples collected exclusively within or in the vicinity of submarine canyons. Mesopelagic decapods were predominant on the slope, while benthopelagic fishes (Merluccius merluccius, Micromesistius poutassou, Gadiculus argenteus) replaced mesopelagic decapods on the shelf. There were seasonal variations, with higher densities of mesopelagic species in spring, which were probably related, among other factors, to variations in the photoperiod. Our surveys also revealed higher species richness in the canyons together with seasonal changes in the megafaunal biomass. Generally, the upper and middle-slope communities both displayed seasonal changes in the composition and abundance of megabenthos.     

Population structure and reproductive patterns of the NW Mediterranean deep-sea macrourid Trachyrincus scabrus (Rafinesque, 1810)

Macrourid fish are one of the most abundant marine species on continental margins worldwide. Although they play an important role in the ecosystem, little is known about their overall biology. We report here a large data set of the most abundant macrourid in Mediterranean waters, Trachyrincus scabrus, showing the main population and reproductive characteristics. The study was based on 3,239 specimens collected between 300 and 1,500 m depth on the northwestern Mediterranean in 2003–2004 and 2008–2009. The population showed a depth-related structure with the largest individuals at 1,100 m depth and the smallest (i.e., immature) at shallower depths. Macroscopic and microscopic analyses of the gonads showed that T. scabrus has a highly seasonal reproductive pattern. Spawning females were found during winter when the organic matter fluxes were highest on the continental slope. T. scabrus is a batch spawner with group-synchronous oocyte development and present low average fecundity of 14,191 oocytes.     

Corallum morphology and composition of crustacean cryptofauna of the hermatypic coralMadracis mirabilis

The motile crustacean cryptofauna of the hermatypic coralMadracis mirabilis (Duchassaing and Michelotti) was examined from three sites on reefs along the west coast of Barbados in 1985. In addition to site differences in degree of eutrophication, two distinct with-in-corallum habitats were recognized, based on differences in the growth form of the coral host. One form grows in isolated, hemispherical heads, with short, robust, widely separated branches covered by 50 to 80% live tissue. The second form occurs within large continuous beds of long, thin, tightly spaced branches tipped by 20 to 30% live tissue. Coral habitats consisting of small isolated heads support a crustacean fauna of high decapod and amphipod diversity, with numerous rare and site endemic species and an abundance of relatively large decapods and amphipods. The continuous beds favour a community of lower decapod and amphipod diversity, fewer rare and site endemic species and with an abundance of smaller sized isopods and copepods. It is suggested that spatial separation of branches and colonies as well as food availability associated with corallum morphology are factors influencing the variation in species abundance and faunal composition of the two habitats.     

Enzymatic and morphological criteria for distinguishing between Cardium edule and C. glaucum of the Portuguese coast

Cardium edule L. and C. glaucum Bruguière are two sibling species of cockles commonly found in littoral systems along the Portuguese coast, where they sometimes occur as mixed populations. The problem of morphological separation of these two cockle species has not been completely solved. Indeed, the distinctive morphological criterion proposed by Brock after identifying the specimens by means of electrophoretic patterns of hepatopancreatic malate dehydrogenase was not effective when applied to Portuguese Cardium spp. The object of the present study was to establish morphological discrimination criteria for distinguishing Portuguese C. edule and C. glaucum on the basis of initial identification of individuals through the enzymatic criterion established by Brock. Cockles were collected from 1988 to 1991 at 16 estuarine and lagoonal systems along the Portuguese coast. Hepatopancreatic extracts were submitted to horizontal starch gel electrophoresis following Brock's procedure. The malate dehydrogenase patterns obtained were identical to those found by Brock. Clear interspecific differences in the leucine aminopeptidase and esterase isozyme systems were also detected. The morphological study of the shells involved 7 morphometric and 9 qualitative and semiquantitative characters. Canonical variate analysis of the morphometric characters showed that these could not be used for identification purposes. However, a clear separation between the two species was obtained by a principal-component analysis of qualitative and semiquantitative characters. The characters mainly useful for distinguishing between the two species were valve profile, shape of ventral valve junction, type of calcareous scales and shell colour.     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks     

Population and reproductive biology of two sibling amphipod species from ascidians and sponges

 The population dynamics and reproductive biology of an ascidian- and a sponge-dwelling amphipod were examined. The two undescribed amphipod species, Leucothoe“ascidicola” and L.“spongicola”, are closely related to each other, and occur in ascidians and sponges, respectively, along the Florida Atlantic coast. L. “ascidicola” was abundant in solitary ascidians during fall 1997, disappeared during spring/early summer, and became abundant again in September 1998. During the time when L. “ascidicola” were absent from their hosts, a copepod became a frequent inhabitant of the ascidians but disappeared again when L.“ascidicola” returned to the ascidians in September 1998. The numbers of L.“spongicola” in sponges increased substantially during spring, when high reproductive activity was observed. Following this reproductive peak, both adult and juvenile amphipods apparently left the sponges, and during the summer amphipod numbers in the sponges were very low. Another small amphipod species, which often co-occurred with L.“spongicola”, showed less seasonal variation and was found in sponges throughout the whole study period. The percentage of ovigerous females per host unit was usually lower in the ascidian-dwelling than in the sponge-dwelling amphipods. In solitary ascidians, L.“ascidicola” amphipods usually occurred in groups of several adults, yet there never was more than one ovigerous female per ascidian. In contrast, several ovigerous L.“spongicola” females were found to cohabit in the same spongocoel. This suggests that intrasexual aggression may be stronger among reproductive amphipod females in the ascidians than in sponges. The size distributions of juvenile cohorts indicate that juvenile L.“ascidicola” remain for relatively long time periods in the parental ascidian, where they may reach sexual maturity. In contrast, in L.“spongicola”, only cohorts of very small juveniles could be identified, indicating that juveniles disperse shortly after emerging from the female's brood pouch. It is concluded that extended parental care is of very short duration or does not occur in the sponge-dwelling amphipod L. “spongicola”, possibly because fast-growing sponges with a highly branched spongocoel system do not allow long-lasting coexistence of parent-offspring groups. In contrast, the discrete character of the solitary ascidians may enhance the potential for exclusion of other species, resource monopolization by reproductive females, and furthermore for long-lasting extended parental care in the ascidian-dwelling amphipod. Groups of single parents together with cohorts of large juveniles are reported in the literature for amphipods and isopods from brachiopods, bivalves and ascidians, suggesting that these discrete biotic microhabitats may favor the evolution of extended parental care in peracarid crustaceans. Received: 30 July 1999 / Accepted: 8 May 2000     

Spatial patterns of ichthyoplankton distribution in relation to hydrographic features in the Northern Benguela region

The spatial distribution of the most abundant eggs and larvae of teleost fish species on the continental shelf and slope off the northern Benguela region was studied in April 1986. The horizontal and vertical distribution of eggs and larvae were analysed together with environmental data, in order to determine patterns of ichthyoplankton distribution. Both species composition and relative egg and larval abundance levels exhibited important latitudinal differences during a period of quiescent upwelling with an intense intrusion of Angolan water into the system. Larval diversity was higher in the northern part of the study area, where, because of the intrusion of the warmer Angolan water, the water column was more stratified than in the southern part, where the affect of upwelling of South Atlantic Central Water was continuous and only a few species spawned. The frontal zone appeared to be a nursery ground for the most important pelagic species of the region:Trachurus trachurus capensis, Engraulis capensis, andSardinops ocellatus. Vertical egg and larval distributions showed evidence of stratification, with highest concentrations located in the uppermost 50 m. In comparison, during periods of intense upwelling, longitudinal gradients were responsible for the horizontal distribution of ichthyoplankton, and the vertical distribution of eggs and larvae were much more extensive because of the greater mixing of the water column.     

Spatial structure of communities on dead pen shells (<Emphasis Type="Italic">Atrina rigida</Emphasis>) in sea grass beds

Delimiting communities in marine habitats is difficult because co-occurring species often have different life histories and the life stages experience the environment at different spatial scales. The habitat of a particular community is embedded within a larger habitat or ecosystem with many species shared between the focal community and the larger system. Pen shells (Atrina rigida) are large bivalves that, once the mollusk dies, provide shelter for motile species and hard substrate for settling larval invertebrates and egg-laying fishes. In St. Joseph’s Bay, Florida (29°45′N, 85°15′W), pen shells are the most abundant source of hard substrate, especially inside sea grass (Thalassia testudinum) beds, where they reach densities of 0.1–4.0 m⁻². This study, which was conducted from May to August 2005, measured the overlap in species densities between dead pen shells and the surrounding sea grass communities at eight sites to determine the discreteness of the pen shell communities. Of the 70-epibenthic taxa recorded, 66% were found on the pen shells but not in the surrounding sea grass habitat. Community structure, which varied among shells within sites and among the eight sites, could be related to sea grass characteristics such as blade density and length either directly (e.g., inhabitants of pen shells directly benefit from the surrounding sea grass) or indirectly (e.g., pen shells and sea grass both benefit from similar factors such as current and nutrients). Pen shells were randomly distributed at several spatial scales within the 15 × 15 m sites as were many motile species. Two exceptions were the shrimp, Palaemon floridanus and the amphipod, Dulichella appendiculata, whose distributions were clumped. Most of the sessile species had clumped distributions, tending to be very abundant when they were present. These pen shell communities provide an opportunity for experimental studies of factors affecting species diversity on small, discrete, naturally occurring habitats. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

A critical habitat for Mediterranean fish resources: shelf-break areas with<Emphasis Type="Italic"> Leptometra phalangium</Emphasis> (Echinodermata: Crinoidea)

This paper considers the potential role of the crinoid Leptometra phalangium as an indicator of highly productive areas along the shelf break that can sustain large biomasses of benthopelagic fish and recruits. The structure of fish assemblages in the central Mediterranean Sea (central-western coast of Italy), analysed on the basis of surveys carried out in summer and autumn from 1997 to 2001, revealed the presence of a well-defined group of species on the shelf break. This area, occurring at a depth of between 120 and 170 m, is characterised by detritic organogenic sediments colonised by the crinoid L. phalangium, a suspension-feeding macro-epibenthic species confined in the Mediterranean to the shelf-break area. Its abundance in the studied area can reach 12–15 ind. m^–2. A total of 121 species belonging to 66 families of demersal organisms (crustacean decapods and stomatopods, cephalopods, selaceens and teleosteens) were caught at shelf-break stations from September to October. The species which typified the assemblage were the fishes Trisopterus minutus capelanus, Merluccius merluccius, Glossanodon leioglossus, Argentina sphyraena, Capros aper, Macroramphos scolopax and Lepidotrigla cavillone, the crustacean decapod Parapenaeus longirostris and the cephalopods Illex coindetii and Todaropsis eblanae. Detritic shelf-break stations showed a higher abundance of demersal organisms than stations distributed on muddy bottoms in the same depth range (100–200 m). Such differences appeared to be significant in September–October, when a clear increase in benthopelagic zooplanktivorus species, such as Glossanodon leioglossus, Trachurus trachurus, Trachurus picturatus, was found. The length structure of species occurring on the shelf break showed that for some of them the selection of this area is related to specific phases of their life cycle. Significant highest abundance of recruits and juveniles was observed for Merluccius merluccius, Helicolenus dactylopterus, Phycis blennoides, Parapenaeus longirostris and Capros aper in at least one of the two seasons. Similarly, an increased abundance of spawners of red mullet Mullus barbatus and four-spotted megrim Lepidorhombus boscii was observed on the shelf break. Results of this study may have important consequences for management of fish stocks and assemblages in the central Mediterranean. The co-occurrence of high densities of L. phalangium and benthopelagic fish, occurring mainly with juveniles and spawners, strongly indicates a potential role of L. phalangium as an indicator of highly productive areas around the shelf break. Such areas appear to play a major role in the production of some of the most abundant and commercially important fish species, such as the Mediterranean hake and red mullet.Communicated by R. Cattaneo-Vietti, Genova     

Does the composition of the demersal fish assemblages in temperate coastal waters change with depth and undergo consistent seasonal changes?

The aim of this study was to determine whether the composition of the demersal fish fauna in coastal marine waters in temperate Australia changes markedly with increasing water depth and distance from the shore and whether the composition of the fish fauna in water depths of 5 to 35 m undergoes cyclic, seasonal changes. Samples of demersal fishes were therefore collected by trawling over the predominantly sandy substrate at nine sites located in water depths of 5 to 15 m or 20 to 35 m and within 20 km of the shore in four regions along ∼200 km on the lower west coast of Australia. The sampling regime involved trawling for fishes at each site at night in seven consecutive seasons between the summer of 1990/1991 and winter of 1992. A total of 72 435 fishes, representing 77 families, 143 genera and 172 species was caught. The compositions of the fish faunas in offshore waters with depths of 5 to 35 m were shown to differ markedly from those previously recorded for nearshore marine waters in the same regions. However, as some species, such as Sillago burrus, S. vittata, S. bassensis and Rhabdosargus sarba, increase in size, they move out from their nursery areas in nearshore waters into deeper and more offshore waters, where spawning occurs. Ordination showed that, in each of the four regions, the composition of the fish fauna in depths of 5 to 15 m differs from that in depths of 20 to 35 m. This difference is attributable to the fact that some species, such as  S. burrus, S. vittata and Upeneichthys lineatus, are far more abundant in depths of 5 to 15 m, whereas other species, such as S. robusta, U. stotti and Lepidotrigla modesta, occur predominantly in depths of 20 to 35 m. However, the samples collected from the single site that was inshore but in deeper water demonstrate that the composition of the fish fauna is influenced by distance from shore as well as by water depth. The compositions of the fish faunas differed with latitude, largely due to the fact that some subtropical species, such as Polyspina piosae, S. burrus and  S. robusta, did not extend down into the more southern regions. Ordination also showed that the composition of the fish faunas at all but one of the nine sites underwent pronounced and consistent cyclic, seasonal changes. This seasonal cyclicity at the different sites was attributable to sequential patterns of immigrations and emigrations by a number of fish species during the course of the year. These seasonal migrations involved, inter alia (1) movements of certain species from their nursery areas into these deeper waters, e.g.  S. bassensis and Scobinichthys granulatus; (2) migrations into and off the sandy areas of the inner continental shelf, e.g. Arnoglossus muelleri; (3) migrations to spawning areas, e.g. Sillago robusta; and (4) movements into areas where detached macrophytes accumulate in winter, e.g. Cnidoglanis macrocephalus and Apogon rueppellii. Received: 21 August 1998 / Accepted: 9 February 1999