Bathymetric patterns of genetic variation in a deep-sea protobranch bivalve, Deminucula atacellana

The origin of the deep-sea benthic fauna is poorly understood and represents an enormous gap in our understanding of basic evolutionary phenomena. One obstacle to studying evolutionary patterns in the deep sea has been the technical difficulty of measuring genetic variation in species that are typically minute, rare, and must be recovered from extreme depths. We used molecular genetic techniques to quantify variation in the 16S rRNA mitochondrial gene within and among populations of the common protobranch bivalve Deminucula atacellana (Schenck, 1939). We analyzed 89 individuals from nine samples collected in the 1960s along a depth gradient from 1100 to 3800 m in the western North Atlantic. Genetic variability within populations is much lower than between populations, and peak haplotype numbers occur near the center of its depth distribution. Continental slope (<2500 m) and rise (>2500 m) populations were genetically distinct despite the lack of any obvious topographic or oceanographic features that would impede gene flow. These findings indicate that the deep-sea macrofauna can have strong population structure over small (134 km) spatial scales, similar to that observed in shallow-water and terrestrial organisms. This surprisingly high biodiversity at the genetic level affords the potential for adaptation and evolutionary diversification, the ultimate historical causes of high species diversity in the deep-sea benthos. Received: 24 July 1997 / Accepted: 26 January 1998     

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     

Predation on meiofaunal and macrofaunal invertebrates by western sandpipers (Calidris mauri): evidence for dual foraging modes

Western sandpiper (Calidris mauri) predation was examined by concurrent experiments and direct observations of foraging behaviour on high intertidal mudflats of the Fraser River estuary, British Columbia. Western sandpipers foraged by either “pecking” on the surface (64% of observational time) or probing into sediment (29%). The first experiment (probe-mark method) consisted of collecting small-volume cores (21.2 cm³) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min feeding period. The second experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collection of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal and meiofaunal size fractions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control sediments revealed significant differences, attributed to shorebird predation, for both experiments. The probe-mark experiment detected the removal of large infaunal polychaetes (∼ 20 mm), while the exclosure experiment showed depletion of epifaunal harpacticoid copepods (0.063–0.5 mm). Predation on macrofaunal cumaceans was detected in both experiments. Invertebrates selected by western sandpipers neither fell within traditional infaunal size classifications (macro- vs. meiofauna; 500 μm delineation) nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western sandpipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal size ranges and (2) selective probing for larger infauna, such as polychaetes. These findings were facilitated by the combination of methodologies employed. Received: 29 December 1999 / Accepted: 11 September 2000     

A novel symbiosis between a cyanobacterium, Synechococcus sp., an aplastidic protist, Solenicola setigera, and a diatom, Leptocylindrus mediterraneus, in the open ocean

The abundance, biomass and distribution of Solenicola setigera, a colonial heterotrophic protist found only with the centric chain-forming diatom Leptocylindusmediterraneus, are reported for four major ocean basins. The distribution is cosmopolitan, and abundances and biomass are usually low (<500 colonies l⁻¹); however, in the summer of 1993, we observed a major biomass component (range = 5 to 31 μg C l⁻¹) in the surface waters of the North Atlantic attributable to S. setigera. These colonies of S. setigera were exceptionally large, and unusual in possessing high abundances of Synechococcus sp., a normally solitary cyanobacterium, embedded in the matrix covering the cells. We hypothesize that this relationship was mutually beneficial for both Solenicola setigera and Synechococcus sp. Received: 5 January 1998 / Accepted: 22 May 1998     

Quantitative characterization of shallow marine benthic assemblages at Ryder Bay,Adelaide Island,Antarctica

Non-destructive photographic techniques are valuable for surveying benthic epifauna in sensitive areas. Inherent in these techniques, however, are selectivity biases related to the scale at which surveys are conducted. Such biases have not been addressed for surveys of marine benthos. The present study surveyed epifaunal assemblages at two depths (8 m and 20 m) and three locations close to Rothera Point, Adelaide Island, Antarctica. Two scales of photographic sampling were employed in order to encompass a broad range of taxon sizes and distributions, and their parallel use allowed qualitative and quantitative comparisons to be made between results from each scale. An additional physical sampling element was incorporated to assess sessile cryptic assemblages. A total of 79 faunal taxa from 10 phyla, together with 6 algal taxa, were recorded in the survey. Echinoderms were the most speciose group at both photographic scales, whereas cryptic assemblages were dominated by bryozoans. There were clear resolution-related differences between scales but all three methods revealed pronounced differences in assemblages by depth and location. Assemblages at both depths were notable for consistently high densities (~100 individuals m^–2) of the regular echinoid Sterechinus neumayeri, and at 8 m for high densities (~100 individuals m^–2) of the limpet Nacella concinna. The results emphasise the general requirement for sampling methodologies to be matched to the size spectra of the organisms under study and, more specifically, highlight the need for studies to assess the relative importance of biotic and physical factors in structuring the Antarctic benthos.Communicated by J.P. Thorpe, Port Erin     

Sponge-inhabiting barnacles on Red Sea coral reefs

In this study eight different species of barnacles were found within nine species of sponges from the Red Sea. This brings to 11 the number of sponge-symbiotic barnacles reported from the Red Sea, two of these are new Acasta species (not described herein) and one (A. tzetlini Kolbasov) is a new record for this sea. This number is much higher than that of symbiotic barnacles found within sponges from either the N. Atlantic (2) or the Mediterranean (4). Two possible explanations for this are the presence of numerous predators in coral reefs and scarcity of available substrate for settlement. These factors can lead to high incidence of symbiotic relationships. Of the nine sponge species, only one (Suberites cf. clavatus) had previously been known to contain barnacles. Even at the family level, this is the first record of symbiotic barnacles in two out of the seven sponge families (Latrunculiidae, Theonellidae). Our present findings strengthen the apparent rule that the wider the openings in a barnacle shell, the fewer the host taxa with which it will associate, usually from one or two closely related families, and the more frequent it will associate with elastic sponges. Most Neoacasta laevigata found on Carteriospongia foliascens were located on the same side as the sponge's ostia, i.e. facing the incoming water. This adaptation allows the barnacles to catch more suspended particles from the water, provides them with more oxygen and prevents their exposure to discharged sponge waste. The highest density of barnacles observed on one face of a “leaf ” (with ostia) was 0.389 barnacles cm⁻² (one barnacle per 2.57 cm²) and on average 0.181 ± 0.68, while the average on the other side was only 0.068 ± 0.52 barnacles cm⁻². As indicated by the Morisita index, these barnacles most frequently (58%, n = 12) had a clumped spatial distribution (while the rest were randomly distributed), as is to be expected from such sessile organisms with internal fertilization via copulation. The presence of N. laevigata induced the growth of secondary perpendicular projections of its host C. foliascens. Of the N. laevigata examined, 17% brooded 324 ± 41 embryos each, of 286 ± 17 μm total length; only 5.7% (n = 123) were found to be dead. Size distribution analysis of skeletal elements from dead barnacles showed them to be significantly larger than the skeletal elements of the population of live barnacles ( p < 0.05). Received: 26 June 1998 / Accepted: 1 December 1998     

Purification and partial characterisation of phenoloxidase from the colonial ascidian Botryllus schlosseri

Phenoloxidase (PO) from the colonial ascidian Botryllus schlosseri was purified using two different chromatographic strategies. A three-step purification was developed in order to maintain enzyme activity, whereas an easier purification procedure was adopted to obtain enough PO for the production of specific polyclonal antibodies. The enzyme showed optimal pH and temperature values of 7.0 to 7.5 and 35 °C, respectively, and a K _m value of 4.62 ± 0.76 mM was estimated using l-DOPA as substrate. A molecular weight of 160 kDa was determined after SDS-PAGE under non-reducing conditions. The addition of the reducing agent β-mercaptoethanol caused the disappearance of the 160 kDa band and the appearance of a new band at 80 kDa, suggesting that active PO is a dimer and the two subunits are linked by disulphide bridges. Received: 14 December 1998 / Accepted: 24 August 1999     

Short-term variation and long-term changes in the oceanographic environment and zooplankton community in the vicinity of a sub-Antarctic archipelago

 Mesozooplankton community structure in the vicinity of the Prince Edward Islands (PEIs) was investigated during six surveys conducted in late austral summer (April/May) from 1996 to 1999. Zooplankton samples were collected by oblique tows using a Bongo net fitted with 300-μm mesh. Surface temperature, average temperature and chlorophyll a were measured in conjunction with each net tow. The positions of the Sub-Antarctic Front (SAF) and the Antarctic Polar Front (APF), in relation to the islands, were determined by CTD and/or XBT transects to the west of the islands (upstream). Both fronts were characterized by a high degree of latitudinal variation. Changes in position of the fronts occurred rapidly, the SAF moving up to ∼120 km in a 2-week period. Consequently, the oceanographic environment in the vicinity of the PEIs was subject to a high degree of intra- and inter-survey variation. The positions of the SAF and APF appeared to have a significant impact on phytoplankton biomass in the vicinity of the PEIs, possibly through the alteration of local oceanographic flow dynamics. Water retention over the island shelf in 1996, associated with location of the SAF far to the north of the PEIs, corresponded to enhanced chlorophyll-a concentrations (∼1.54 mg m⁻³). Conversely, when the fronts were close to the islands, as in 1997 and 1999, higher current velocity limited water retention and chlorophyll-a concentrations in the inter-island region were relatively low (∼0.4 mg m⁻³). Cluster analyses showed that, in many instances, there was greater similarity among zooplankton communities from different surveys than among communities within surveys, indicating that short-term variability exceeded inter-annual variability. The population structure of the copepod Calanus simillimus indicated that there was inter-annual variation in the timing of the biological season. Differences in the population structure of species, and consequently their contribution to abundance and biomass, may therefore have been an important contributor to inter-annual variation in community structure. Evidence is provided of a long-term southward shift in the position of the SAF. It is postulated that this may affect the PEIs by increasing the proportion of allochthonous energy input, because the PEIs now lie in the path of the front, altering the tropho-dynamics of the island ecosystem. Lower mesozooplankton biomass associated with warmer sub-Antarctic water may have important negative consequences for higher trophic levels that depend on mesozooplankton for food. Received: 10 June 2000 / Accepted: 22 September 2000     

UV absorption by mycosporine-like amino acids in Phaeocystis antarctica Karsten induced by photosynthetically available radiation

Mycosporine-like amino acids (MAAs), which occur in diverse taxonomic groups, exhibit in vivo absorption maxima between 310 nm and 360 nm and may play a photoprotective role against ultraviolet (UV) exposure. Using cultures of colonial Phaeocystis antarctica, we examined the relationship between MAA concentration, in vivo UV absorption, photoprotective (carotenoid) and photosynthetic pigments, and photosynthetically available radiation (PAR, 350–700 nm). UV absorption was high; chlorophyll-specific absorption, a ^* _ph, at 330 nm ranged from 0.06 to 0.41 m²/mg chlorophyll a. Values of a ^* _ph (330) were 4–13 times greater than a ^* _ph (676). Mycosporine-glycine, shinorine, and mycosporine-glycine valine are responsible for the strong in vivo UV absorption. The sum of all MAAs increased with irradiance when normalized to chlorophyll a or carbon concentrations, whereas individual MAAs varied independently from each other. Mycosporine-glycine concentrations showed no statistically significant change over the range of light intensities, whereas mycosporine-glycine and shinorine concentrations increased at higher irradiances. The relative fluorescence yield for chlorophyll a was low in the UV region compared to the visible region, implying that absorbed UV radiation (<375 nm) is transferred inefficiently to chlorophyll a in the reaction center. Quantitative estimates of UV screening by MAAs are attributed to elevated MAA concentrations and increased diameter at high light. Received: 31 March 1999 / Accepted: 13 July 2000     

Population structure, growth and recruitment of the euryalinid brittle-star Astrobrachion constrictum (Echinodermata: Ophiuroidea) in Doubtful Sound, Fiordland, New Zealand

The euryalinid brittle-star (snake star) Astrobrachion constrictum (Farquhar) lives coiled around the branches of black coral (Antipathes fiordensis) colonies. Twenty-two vertical transects, 10 m wide by 30 m deep, were swum in Doubtful Sound over a 2.5 yr period from 1993 to 1995. Numbers, disc diameters and colour morphotypes of brittle-stars inhabiting coral colonies were recorded. 36.3% of the coral colonies >200 mm tall (n = 292) hosted ≥1 Astrobrachion constrictum (range 0 to 12). Overall, the population was patchily distributed on the available coral habitat. The dark red colour morphotype of A. constrictum was most common (87%, n = 279) followed by the yellow, striped, and then spotted varieties. The population was comprised mainly of large (≥10 mm disc diam) individuals, and juveniles were rarely encountered, indicating low rates of recruitment or a high mortality of recruits. Disc-diameter data gathered from this and previous studies indicated that growth in A. constrictum is initially rapid, with individuals reaching a disc diameter of 15 mm in ≃2.5 yr; growth decreases with age, as in other deep-sea ophiuroids. Growth rate within years, however, was not constant, with faster growth in the spring/summer. Maximum size for A. constrictum is reached in ≃8 yr at ˜23 mm disc diam. Anecdotal evidence indicates that A. constrictum may not be confined solely to black coral colonies. Received: 25 September 1996 / Accepted: 16 October 1996     

The ecology of cirratulid mudballs on the Oman margin, northwest Arabian Sea

Mudball-building cirratulid polychaetes have been described previously only from the southern California margin. During a study of oxygen minimum-zone benthos in fall 1994, we observed dense aggregations of agglutinated mudballs at 840 to 875 m on the Oman margin in the northwest Arabian Sea. These were inhabited, and probably constructed, by a cirratulid polychaete species in the genus Monticellina. The mudballs were cigar-shaped, 4.5 to 25 mm long, and positioned vertically so as to protrude several millimeters above the sediment–water interface. Total mudball densities were ∼16 000 m⁻². Occupied mudballs occurred at densities of 2 112 m⁻²; 89% were in the uppermost 2 cm of sediment, and no occupied mudballs were found below 10 cm. Organisms other than the cirratulid were present on 1.7% of the mudballs examined, and included epizoic polychaetes, agglutinated and calcareous Foraminifera. Various polychaetes, a nemertean and nematodes were found inside tests. Mudball abundance exhibited positive associations with densities of several paraonid polychaete species, and with densities of burrowing and subsurface-deposit-feeding polychaetes. Negative associations were observed between mudballs and three tube-building taxa (two polychaetes and an amphipod). Mudball-inhabiting cirratulids are abundant in at least two low-oxygen, margin settings. We expect further sampling of bathyal environments to yield additional systems in which cirratulid mudballs are common. Such observations are valuable because mudballs appear to represent a significant source of heterogeneity that can influence macrofaunal community structure in deep-sea sediments. Received: 15 January 1997 / Accepted: 30 January 1997     

Genetic variation and clonal structure in the scleractinian coral Pocillopora damicornis in the Ryukyu Archipelago, southern Japan

Samples of the scleractinian coral Pocillopora damicornis were collected from six sites located around four islands in the Ryukyu Archipelago, southern Japan, and subjected to allozyme electrophoresis. Seven polymorphic loci were examined for their allelic patterns. The ratio of observed to expected genotypic diversity (0.30 < G _o :G _e  < 0.64), the ratio of the observed number of genotypes to the number of individuals (0.47 < N _g :N _i  < 0.75), and deviations from Hardy–Weinberg equilibrium indicated that asexual reproduction plays a major role in the maintenance of established populations. However, populations were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples. The high frequency of typhoons in the region suggests that, in P. damicornis, fragmentation caused through occasional exposure to powerful waves is a major mode of asexual reproduction, but asexual production of planulae may also be contributing to the maintenance of populations. A significant genetic differentiation (F ST) was found between the six populations examined (0.027 < F ST < 0.092, average F ST = 0.056). The moderate gene flow is discussed according to characteristics of the larval stage of the species, and to circulation patterns in the region. Received: 7 August 1998 / Accepted: 18 May 1999     

Distribution of encrusting bryozoans and other epifauna on the subtidal bivalveChlamys opercularis

The shells of living queen scallops,Chlamys opercularis (L.), are often completely colonised with epifauna. In this study, the distribution of the major epifaunal groups across the surfaces of left (upper) and right (lower) valves is examined using samples from two commercial queen scallop beds off the Isle of Man. The composition of the epifauna differs across and between the valves and also varies with the age of the scallop. YoungerC. opercularis are commonly encrusted by sponges, but on older shells other epifauna colonise the surfaces. The encrusting bryozoan assemblage found on livingC. opercularis is not as abundant or speciose as on some other substrata such as stones and dead shell. This may be a result of intense competition from other phyla. The distribution of different bryozoan species between the surfaces is examined. Most species are more abundant on the lower valves.     

To what extent does upright sessile epifauna affect benthic biodiversity and community composition?

Small-scale habitat complexity, including that caused by biological structures, is an important factor in structuring benthic communities and also sometimes in increasing biodiversity. The aim of this study was to determine if hydroid colonies have an effect on the composition of benthic communities in the Irish Sea, and if so, which components of the fauna are affected. Forty-six seabed core samples were taken by divers from two sites off Port Erin, Isle of Man, Irish Sea. Half of these were centred on hydroid colonies, half were not. All taxa retained by a 63-m sieve from the cores were identified and counted. Community composition and diversity were compared between hydroid and non-hydroid cores using multivariate and univariate methods. Benthic communities were significantly different between the two sample groups. This was almost entirely due to the presence of sessile and mobile epifaunal taxa in the hydroid cores. The tube-building amphipod, Ericthonius punctatus, was particularly abundant attached to the hydroid stems. Infauna was not significantly different between the two groups. Upright sessile epifauna may play a particularly important role in the Irish Sea as a settlement substrate for juvenile scallops (Pecten maximus and Aequipecten opercularis), which are an important fishery resource in this area. The 11-year closure of an area to dredging has not only enhanced scallop stocks but has had the added benefit of enhancing habitat complexity and biodiversity.Communicated by J.P. Thorpe, Port Erin     

Restricted gene flow and evolutionary divergence between geographically separated populations of the Antarctic octopus Pareledone turqueti

Samples of the Antarctic octopus Pareledone turqueti were taken from three locations on the Scotia Ridge in the Southern Ocean. The genetic homogeneity of these populations was investigated using isozyme electrophoresis. Whilst panmixia appeared to be maintained around South Georgia (F _ST = 0) gene flow between this island and Shag Rocks, an island only 150 km away but separated by great depths, was extremely limited (F _ST = 0.74). These results are examined with respect to the discontinuous distribution of P. turqueti throughout Antarctica. An estimate of effective population size was also calculated (N _e = 3600). Received: 7 March 1997 / Accepted: 27 March 1997     

Temporal and spatial occurrence of UV-absorbing mycosporine-like amino acids in tissues of the antarctic sea urchin Sterechinusneumayeri during springtime ozone-depletion

From September to November 1991, UV-absorbing mycosporine-like amino acids (MAAs) were monitored in a natural population of the sea urchin Sterechinusneumayeri from a coastal area of Anvers Island (Antarctic Peninsula). MAA concentrations were determined for specific tissues (gonad, digestive tract and body wall) from adults collected at four depths (intertidal, 8, 15 and 24 m). Four MAAs were identified: mycosporine-glycine, shinorine, porphyra-334 and paly-thine. Concentrations of MAAs among replicate individuals varied considerably. Ovaries had high concentrations of MAAs (84 to 1389 μg g⁻¹ dry wt), while testes had non-detectable levels. The relative abundance of specific MAAs in ovaries appeared to be related to the spawning cycle. Digestive-tract samples had MAA concentrations as high as 3000 μg g⁻¹ dry wt, but the mean MAA content in intertidal individuals decreased by 70% over 3 mo during spring. The body walls of sea urchins had very low amounts of MAAs (≤ 0.08 μg g⁻¹ dry wt). There were significant depth differences in the␣total MAA content of the ovary ( p <0.001), ( p <0.015), digestive tract ( p <0.001), and body wall with organisms from the intertidal and 8 m depth having the highest concentrations of MAAs. Biological dosimetry indicated that UV-B (280 to 320 nm) wavelengths penetrated 3 to 7 m below the sea ice during the study period. The total MAA content in ovaries decreased with depth on all sample dates; however, the MAA content of the digestive tract and body wall did not exhibit a consistent pattern of change with depth. The MAA content of tissues did not change significantly with the temporal gradient of light exposure that was established by both ozone depletion and increasing photoperiod, except in the digestive tract sampled from intertidal specimens. Adult urchins are probably well-protected from UV exposure by the water column and a calcareous test; however, the results of this study suggest that, even under ice cover, depth of habitation is a determinant of MAA content in S. neumayeri. Large daily and seasonal fluctuations in the light regime, which are characteristic of Antarctic coastal environments, apparently do not provide reliable cues to elicit a detectable, temporal, biochemical response. Received: 19 February 1997 / Accepted: 26 March 1997     

Species composition, biomass and vertical distribution of micronekton over the mid-slope region off southern Tasmania, Australia

Seasonal sampling was carried out based on day/night, vertically stratified tows (100 or 125 m strata) in the upper 900 m of the water column over the mid-slope commercial fishing grounds south of Tasmania. A large midwater trawl (105 m² mouth area) was used with an opening/closing cod-end. Subtropical convergence and subtropical species dominated the fauna, but many less abundant, more widely-distributed species were also present. Fishes, which contributed 89% of micronekton biomass and 135 of 178 species, were dominated by the Myctophidae (48% biomass and 48 species). Twenty micronekton species made up 80% of the total biomass. Overall, the micronekton fish biomass in this region was 2.2 g m⁻² wet weight. A pronounced day/night shift in the distribution of biomass was attributable to diel migratory species. During the day, <0.2% of the total micronekton biomass was found in 0 to 300 m; most biomass was below 400 m, with peaks at 400 to 525 m and 775 to 900 m. At night, 53% of the biomass was found in 0 to 300 m, with progressively less in each deeper stratum. The vertical ranges of individual species typically exceeded 400 to 500 m during the day and night and were non-coincident, although nyctoepipelagic migrators were concentrated in the surface 200 m at night. Distinct epipelagic, lower and upper mesopelagic assemblages were identified, and patterns of epipelagic migration, limited migration and non-migration were categorised for species from each of the lower and upper mesopelagic assemblages. The vertical distribution of these assemblages was coincident with the primary water masses: subantarctic mode water (∼250 to 600 m) and antarctic intermediate water (below ∼700 m). The flux of migrating micronekton, estimated at 0.94 to 3.36 g C m⁻² yr⁻¹ to the lower mesopelagic and 1.14 to 4.06 g C m⁻² yr⁻¹ to the upper mesopelagic, appeared to be considerably outweighed by the consumption needs of aggregated mid-slope benthopelagic predators. We suggest that advection of mesopelagic prey in antarctic intermediate water may sustain aggregated populations of orange roughy (Hoplostethus atlanticus) and other predators on the micronekton in mid-slope depths at this site. Received: 2 April 1997 / Accepted: 21 August 1997     

Distribution and abundance of ctenophores, and their zooplankton food in the Black Sea. I. Pleurobrachia pileus

The distribution of Pleurobrachia pileus Müller, 1776 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum of 200 m) in the winter, spring, and summer of 1991 to 1995. The summer samples were collected in 1991 to 1993 (for a previous) and are included in this paper for comparative purposes. High concentrations of P. pileus were found at the northern edges of anticyclonic eddies along the southern coastal regions. The biomass and abundance of P. pileus increased from winter through spring to a peak in summer. The highest mean wet weight during a sampling period was 250 g m⁻², while the maximum wet weight was 1429 g m⁻². P. pileus was mostly found in a layer extending from the lower parts of the thermocline down to the anoxic zone, where the temperature was <8 °C. The vertical distribution of P. pileus biomass had two clear maxima at night: an upper maximum at 20 to 40 m was less pronounced than the lower maximum at 90 to 120 m depth. Mean body length of P. pileus did not exceed 12 mm. Smaller individuals (9 to 10 mm length) occurred in winter. P. pileus had two length classes in early spring (March 1995) and late summer (August 1993), indicating the presence of both newly hatched and larger individuals. Overall, the stomach contents of P. pileus consisted mainly of Copepoda (90%), Cladocera (1%), Mollusca (1%), fish eggs and larvae (1%), and other taxa (7%). The preferred food of P. pileus (frequency of occurrence) was: Calanus euxinus (39%), Pseudocalanus elongatus (30%), Acartia clausi (28%), Oithona similis (2%), and Paracalanus parvus (1%). The endoparasite Hysterothylacium aduncum was commonly found in P. pileus. Abundances of Mnemiopsis leidyi and P. pileus were either negatively correlated (r = −0.5 to −0.7) or positively correlated at a low significance level (r = 0.25 to 0.3) with abundance of A. clausi in different months of the year. Aurelia aurita abundance was correlated mainly with the abundance of C. euxinus from June 1991 to March/April 1995. Over the same period the abundance of P. pileus was significantly correlated with the abundance of P. elongatus, an important prey species. Received: 1 November 1997 / Accepted: 30 August 1999     

Morphological and fluorescence analysis of the Montastraea annularis species complex in Florida

The taxonomic status of the Montastraea annularis species complex is unclear. Much evidence has been accumulated to support the separation into 3 species, but the presence of intermediate morphotypes and the apparent lack of effective reproductive barriers in some areas are yet unexplained. Several authors have made a call for the introduction of new traits that can be used to resolve differences among closely related coral species. We collected skeletal and tissue samples from corals within the M. annularis species complex (15 each of M. annularis, M. faveolata, and M. franksi) and 10 morphological intermediates from several reefs in the Florida Keys. Multivariate analysis of corallite skeletal measurements supported the separation of the species complex into three taxa. We detected two main fluorescence emission peaks at 480 nm (turquoise) and 515 nm (green) that were not distributed equally among the three species. Every M. annularis colony had a major turquoise fluorescence peak. Some had a weak green secondary fluorescence peak. Colonies of M. faveolata and M. franksi had either the green or turquoise fluorescence peak, but at significantly different frequencies. The intermediate morphotypes proved to be highly heterogeneous with respect to both micromorphology and fluorescence, and their nature could not be fully explained. We were not able to separate the three species using fluorescence characters alone, however this new trait does increase our understanding of the taxonomic structure within the M. annularis species complex. Received: 21 January 2000 / Accepted: 1 September 2000     

Estuarine epifauna recruit despite periodic hypoxia stress

In some estuaries, the recruitment of epifaunal benthic invertebrates coincides with a significant environmental stress, low water-column dissolved oxygen, termed hypoxia (̀ mg O₂ l^-1). Recruitment of epifaunal species was measured in the lower York River, a subestuary of the Chesapeake Bay, USA, which experiences predictable, periodic hypoxia associated with neap/spring tidal cycling during summer. Recruitment substrata were exposed during 48-h deployments in two areas with differing levels of hypoxia, and epifauna were allowed to recruit during periods of low oxygen (neap tides) and high oxygen (spring tides) in 1996 and 1997. Recruitment was often high during neap tides, even when severe oxygen depletion (<0.5 mg O₂ l^-1, <0.71 ml O₂ l^-1) occurred during deployments; indeed, peak recruitment episodes of several dominant epifaunal taxa, and of total epifauna, coincided with hypoxic events during both summers. Increased recruitment during neap tides suggests that factors besides hypoxia influenced recruitment in the York River; these factors may have included changes in larval availability and lower current speeds. This study illustrates how the relationship between recruitment and large-scale stresses, such as hypoxia, may be difficult to predict, since large-scale stresses are often correlated with numerous other factors. Short-term hypoxia appears to have little effect on recruitment in the field for many epifaunal species in this ecosystem, which may explain, in part, why substrata exposed for longer durations (1 month) in this system showed few effects of hypoxia on community composition or diversity. High larval tolerance of hypoxic stress may allow communities to persist even though the summer hypoxia season coincides with the recruitment of many epifaunal species. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00227-002-0930-6. On that page (frame on the left side), a link takes you directly to the supplementary material.