Poecilogony in an estuarine opisthobranch: planktotrophy, lecithotrophy, and mixed clutches in a population of the ascoglossan Alderia modesta

A San Diego population of the opisthobranch mollusc Alderia modesta (Lovén, 1844) exhibits poecilogony, the presence of two development modes within a single species. In spring, half of the adults spawned masses containing ∼300 eggs with a mean diameter of 68 μm. After 3 d, these egg masses hatched planktotrophic veligers with a maximum shell dimension of 116 μm. The remaining adults spawned masses containing ∼30 eggs with a mean diameter of 105 μm. These egg masses hatched after 5 to 6 d, releasing lecithotrophic larvae with a maximum shell dimension of 186 μm. About 1% of field-collected adults produced mixed clutches containing a continuum of larval sizes, spanning the size extremes of planktotrophy and lecithotrophy and hatching larvae with a mean maximum shell dimension of 152 μm. Adults producing planktotrophic and lecithotrophic larvae were interfertile, and no hybrid breakdown was observed through the F3 generation. When starved, adults which previously produced only lecithotrophic larvae switched to producing planktotrophic larvae or mixed clutches with both planktotrophic and lecithotrophic larvae. Sequence-polymorphisms from a portion of the mitochondrial cytochrome c oxidase I gene support the conclusion that the two reproductive morphs represent a single species. Most of the lecithotrophic larvae and a small percentage of the larvae from mixed clutches were metamorphically competent within 3 d of hatching. A. modesta is the only molluscan species as yet known to have both planktotrophic and pelagic lecithotrophic development within a single natural population. Received: 14 August 1997 / Accepted: 11 April 1998     

Effects of physical disturbance on nematode communities in sand and mud: a microcosm experiment

A microcosm experiment was carried out to evaluate the effects of continuous and spasmodic physical disturbance of differing frequency on the structure of nematode communities of intertidal sand and mud. There was a marked, characteristic change in abundance and diversity for both sediment types. In the sand microcosms, the majority of univariate measures of community structure, including species diversity, were lowest in the sediments subjected to a high frequency of disturbance. For the mud microcosms, most univariate measures reached their highest values in the treatments with an intermediate frequency of disturbance and were lower in treatments subjected to both higher and lower frequencies. Multivariate ordinations for both nematode assemblages showed a clear separation of undisturbed controls and disturbed treatments, but only for the muddy sediment was there a graded change in community composition with increasing frequency of disturbance. These results confirmed our a priori expectation that nematode assemblages from mobile sandy sediments would be more resilient to physical disturbance than those from sheltered muds, and these observations are considered in the context of Connell's intermediate disturbance hypothesis. Received: 7 May 1997 / Accepted: 10 October 1997     

Anthropogenic disturbance of deep-sea megabenthic assemblages: a study with remotely operated vehicles in the Faroe-Shetland Channel,NE Atlantic

The effects of local-scale anthropogenic disturbance from active drilling platforms on epibenthic megafaunal abundance, diversity and assemblage pattern were examined in two west of Shetland hydrocarbon fields at 420 and 508 m water depth. These areas were selected to include a range of disturbance regimes and contrasting faunal assemblages associated with different temperature regimes. Remotely operated vehicle (ROV) video provided high-resolution megafaunal abundance and diversity data, which were related to the extent of visible disturbance from drilling spoil. These data, in conjunction with a study deeper in the Faroe-Shetland Channel, have allowed comparison of the effects of disturbance on megabenthos across a range of sites. Disturbance to megafaunal assemblages was found to be high within 50 m of the source of drill spoil and in areas where spoil was clearly visible on the seabed, with depressed abundances (Foinaven 1,900 individuals ha⁻¹; Schiehallion 2,178 individuals ha⁻¹) and diversity (H′ = 1.75 Foinaven; 1.12 Schiehallion) as a result of smothering effects. These effects extended to around 100 m from the source of disturbance, although this was variable, particularly with current regime and nature of drilling activity. Further from the source of disturbance, megafaunal assemblages became more typical of the background area with increased diversity (H′ = 2.02 Foinaven; 1.77 Schiehallion) and abundance (Foinaven 16,484 individuals ha⁻¹; Schiehallion 5,477 individuals ha⁻¹). Visible effects on megafaunal assemblages as a result of seabed drilling were limited in extent although assemblage responses were complex, being controlled by differing effects to individual species often based on their motility.     

Spawning and larval development of the black turban snail Tegula funebralis (Prosobranchia: Trochidae)

An understanding of spawning and larval development can be fundamental to interpreting the abundance, distribution, and population structure of marine invertebrate taxa. Tegula funebralis (A. Adams, 1855), the black turban snail, has been the focus of numerous ecological studies on the Pacific coast of North America. To date, there are only conflicting and anecdotal reports of spawning, and there is no information on larval or juvenile development for this conspicuous and abundant species. On 19 September 1995, two individuals of T. funebralis were observed free-spawning gametes into seawater in tanks at the Oregon Institute of Marine Biology. Embryos and larvae were subsequently reared to metamorphosis and beyond. Development was pelagic and similar to development described for other trochids, and larvae were observed not to feed at any stage. Larvae began to metamorphose at 5.7 to 6.7 d and settled at 260 μm shell length. Juveniles grew ≃ 10 μm in shell length per day and appeared to feed on detritus. Juveniles lacked some adult diagnostic shell characters, including two columellar nodes and a closed umbilicus. In the field, small (<3 mm) juveniles occurred in the adult habitat on all sampling dates between October and March. Small juveniles were found only under rocks and were most abundant under rocks partially buried in coarse sand, suggesting that juveniles may utilize a specific microhabitat within the adult T. funebralis habitat. Received: 7 October 1996 / Accepted: 17 October 1996     

Feeding ecology ofLeitoscoloplos fragilis

The present study examines the effects of density ofLeitoscoloplos fragilis and of fine sediment on benthic microalgal abundance and production in laboratory microcosms, and the effects of fine sediment on diffusive transport of ammonia. Microcosms having different densities ofLeitoscoloplos fragilis (Verrill, 1873) were determined in sediment collected from one of two field stations (each containing a different amount of fine particles <125 m) from Cape Henlopen, Delaware, USA, in August 1986. The worms were acclimated in a recirculating seawater system for two months prior to experiments. Chlorophylla concentrations were highest in sediments with less fine particles (<125 m). Benthic diatom production, total microbenthic metabolic activity, and concentrations of pore-water ammonia were higher in sediment microcosms containing high densities of worms.L. fragilis grew more in microcosms containing less fine particles and higher worm densities. The upward flux of ammonia across the sediment-water interface was higher in sediments with less fine particles. A greater abundance of fine particles in these sediments impedes the upward flux of ammonia to surface and nearsurface diatoms. The coupling between population density and diatom production, which can be altered by fine-particle abundance could control the distribution and stability of populations ofL. fragilis.     

Microplankton population structure in Southern California nearshore waters in late spring

Taxonomic composition, biomass as organic carbon, numerical abundance, and size distribution of the microplankton were determined at 6 Southern California nearshore locations in late May–early June, 1970. Samples were taken at approximately 5 m (10 m at one station) intervals through the upper 40 to 50 m to reveal some of the small-scale differences and levels of variability in the populations. Total microplankton biomass over all euphotic zone samples varied by more than two orders of magnitude (7.6 to 1,200 g C l^-1). Average biomass at comparable sites (n=5) ranged from 48 to 240 g C l^-1; biomass range within stations varied from about 5-fold to 120-fold. Total microplankton numbers varied approximately 22-fold (4.3×10⁵ to 9.5×10⁶ organisms l^-1) over all euphotic zone samples, but the range within stations was always less than an order of magnitude. At comparable stations, nanoplankton biomass had ranges extending from 3.7-fold to 12-fold; its average percentage contribution (±1 SD) to the total microplankton biomass varied from 39±5% to 54±13%. Netplankton biomass showed a similar minimal range, but its greatest range was more than two orders of magnitude. Ranges of abundance of major taxonomic groups within stations varied considerably from about 2-fold to more than three orders of magnitude. The small-scale variability of the populations probably affects the reliability of the microplankton as a food source for pelagic consumers.     

Feeding by larvae of two different developmental modes in Streblospio benedicti (Polychaeta: Spionidae)

Females of the spionid polychaete Streblospio benedicti (Webster) produce either small eggs (60–70 μm diameter) and planktotrophic larvae, or large eggs (100–200 μm) and lecithotrophic larvae that reportedly do not feed. This intraspecific polymorphism, a form of poecilogony, is potentially useful in studies of larval ecology and evolution, but necessary data on larval form and function are lacking. This study describes the morphology and nutritional biology of larvae obtained from Atlantic (South Carolina) and Pacific (California and Washington) populations from 2003 to 2005. The two types of larvae produced by Atlantic S. benedicti differed greatly in length (229±22 μm SD for planktotrophs vs. 638±40 μm for lecithotrophs) and chaetiger number (2–5 vs. 10–11) at release from the female’s brood pouch. Planktotrophic larvae bore long provisional chaetae on their first chaetiger; provisional chaetae were absent in lecithotrophic larvae. Larvae from Pacific populations were all of the lecithotrophic form, and were similar to their Atlantic counterparts in all respects. High-speed video microscopy revealed that both types of larvae used opposed bands of cilia to capture suspended particles and transport them to the mouth, where they were often ingested. Lecithotrophic larvae reared with suspended phytoplankton (Rhodomonas sp., 10⁴ cells ml⁻¹) for 2 days grew significantly faster than sibling larvae reared without added food, indicating that these larvae can digest and assimilate ingested food. Larvae of S. benedicti that develop from large eggs are thus facultative planktotrophs instead of obligately non-feeding lecithotrophs, a result that affects the interpretation of comparative studies of the ecology and evolution of larvae in S. benedicti and certain other marine invertebrates.     

Sediment processing in Capitella spp. (Polychaeta: Capitellidae): strain-specific differences and effects of the organic toxicant fluoranthene

The effects of the widespread polycyclic aromatic hydrocarbon (PAH) fluoranthene on sediment-processing rates in the infaunal polychaete Capitella spp. were investigated by comparing five populations of this deposit-feeding species complex: Capitella sp. I from New York, USA; Capitella sp. M from Milos, Greece; Capitella sp. S from Sylt, Germany; and two unidentified Capitella populations from salmon farm sediments –Capitella population K from Kilmelford, Scotland and Capitella population C from Cranford, Ireland. Replicate worms from each strain were exposed to 0, 10, and 95 μg (g dry wt sediment)⁻¹ fluoranthene (=μg/g fluoranthene) for a period of 16 days. Initial and final wet and dry weights (mg) of worms and worm-specific growth rates (WSGRs) were calculated. Sediment processing was measured as the sum of the total dry weight of pellets produced during the experiment, and we estimated size-specific processing rates (SSPRs) as a measure of sediment processed per mg worm dry weight per day. The five populations of Capitella spp. differed significantly in body size, WSGR, and sediment-processing rates. Capitella sp. I grew faster than all of the other populations. Capitella population C and Capitella population K from fish farm sediments, with the largest body lengths (up to 52.0 ± 27.2 mm), had the highest processing rates, whereas the small Capitella sp. S (up to 17.1 ± 5.6 mm) had the lowest. There were also significant differences in SSPR among populations with Capitella sp. I having a higher SSPR (about 12 × body wt/day) than Capitella population C (about 5 × body wt/day) and Capitella population K (3 × body wt/day). The fluoranthene concentrations used in the present study, while representing moderately to highly contaminated conditions, had only marginal effects on sediment-processing and growth rates of all of the Capitella populations examined. Processing of contaminated sediment by Capitella spp. may be important in the remediation of PAH-contaminated sediment. Received: 16 January 2000 / Accepted: 28 August 2000     

Selective feeding of littoral harpacticoids on diatom algae: hungry gourmands?

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm², so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.     

Field distribution and sulphide tolerance of Capitella capitata (Annelida: Polychaeta) around shallow water hydrothermal vents off Milos (Aegean Sea). A new sibling species?

The cosmopolitan polychaete Capitella capitata, known as a complex of opportunistic sibling species, usually dominates the macrobenthos of polluted or unpredictable environments. A population of C. capitata, termed Capitella sp. M, was found in a shallow water hydrothermal vent area south of Milos (Greece). Here, this population occurs close to vent outlets (termed the “transition zone”), an environment with steep gradients of temperature, salinity and pH and increased sulphide concentrations of up to 710 μM. The field distribution of C. capitata in relation to sulphide concentrations around the vent outlets was investigated and sulphide tolerance experiments were conducted on laboratory-cultured worms to elucidate possible adaptations of Capitella sp. M to these extreme environmental conditions. In order to investigate whether the population from the Milos hydrothermal vent area can be considered a distinct sibling species within the C. capitata complex, crossbreeding experiments and analysis of general protein patterns were conducted with Capitella sp. M and three other C. capitata populations of different ecological ranges. Capitella sp. M showed high resistance (median survival time: 107 ± 38 h) to anoxia plus high sulphide concentrations of 740 μM. It seems that the ability to survive high-sulphide conditions in combination with reduced interspecific competition enables the polychaete to maintain a continuous population in this rigorous habitat. From the extremely high tolerance to anoxia and sulphide, shown in both the crossbreeding experiments and the analysis of total proteins, it can be concluded that Capitella sp. M from the Milos hydrothermal vent area represents a separate sibling species within the C. capitata complex. Received: 3 March 1997 / Accepted: 12 September 1997     

The role of marine aggregates in the ingestion of picoplankton-size particles by suspension-feeding molluscs

Suspension-feeding molluscs are important members of coastal communities and a large body of literature focuses on their feeding processes, including the efficiency of particle capture. Some molluscs, such as bivalves, capture individual picoplankton cells (0.2–2.0 μm) with a retention efficiency of less than 50%, leading to the assumption that such particles are not an important food resource. Picoplankton, however, are often concentrated in particle aggregates of much larger size. This study investigates the ability of suspension feeders to ingest picoplankton-size particles (0.2–2.0 μm) bound in marine aggregates. We fed clams (Mercenaria mercenaria), mussels (Mytilus edulis), oysters (Crassostrea virginica), scallops (Argopecten irradians) and slipper snails (Crepidula fornicata) 1.0- and 0.5-μm fluorescent particles (either polystyrene beads or bacteria) that were (1) dispersed in seawater, or (2) embedded within laboratory-made aggregates. Dispersed 10-μm beads were also delivered so that feeding activity could be determined. Ingested fluorescent particles were recovered in feces or isolated digestive glands and quantified. Results indicate that aggregates significantly enhance the ingestion of 1.0- and 0.5-μm beads by all species of bivalves, and enhance the ingestion of bacteria (greatest cell dimension ca. 0.6 μm) by all suspension feeders examined. Differences among species in their ability to ingest aggregates and picoplankton-size particles, however, were evident. Compared to mussels and clams, scallops and oysters ingested fewer aggregates with 1.0-μm beads or bacteria, and slipper snails ingested the most dispersed beads and bacteria. These differences may be a consequence of variations in gill structure and mechanisms of particle processing. Our data demonstrate that suspension feeders can ingest picoplankton-size particles that are embedded within aggregates, and suggest that such constituent particles may be an important food resource.     

Spatial distribution patterns of the gorgonians <Emphasis Type="Italic">Eunicella singularis</Emphasis>, <Emphasis Type="Italic">Paramuricea clavata</Emphasis>, and <Emphasis Type="Italic">Leptogorgia sarmentosa</Emphasis> (Cape of Creus,Northwestern Mediterranean Sea)

Most of the current knowledge on Mediterranean gorgonians is restricted to investigations of those populations found within shallow sublittoral waters, and only limited data are available for populations located below scuba depth. To overcome this lack of information, the occurrence and abundance of the gorgonians Eunicella singularis, Paramuricea clavata, and Leptogorgia sarmentosa were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m deep) extent using a remotely operated vehicle (ROV). The greatest occurrence and abundances of E. singularis and P. clavata were concentrated in areas that are directly exposed to strong near-bottom currents. E. singularis was the most common and abundant species and displayed great plasticity and amplitude in its environmental preferences. Conversely, P. clavata showed a very patchy distribution that was associated with vertical rocky walls. Only isolated colonies of L. sarmentosa were observed in the study area. Hot spots of abundance of E. singularis and P. clavata were identified below a depth of 40 m, which demonstrates the importance of studying the distribution of benthic species over a wide geographical and bathymetrical extent.     

Differences of two polychaete species reflected in enzyme activities

Polychaetes constitute most of the benthic macroinvertebrates in estuarine and coastal environments. We investigated the utilization of organic matter in two polychaete species, Capitella sp. I and Perinereis nuntia brevicirris, living in different coastal habitats. The protease activity of Capitella sp. I (89.7 μg mg⁻¹) was about 10 times that of P. nuntia brevicirris (8.0 μg mg⁻¹). High cellulase (endo-β-1,4-glucanase) activity was detected in P. nuntia brevicirris (3.2 μg mg⁻¹), whereas scarcely any was detected in Capitella sp. I. We isolated cDNA clones of protease mRNA from Capitella sp. I and of cellulase mRNA from P. nuntia brevicirris. The high protease activity of Capitella sp. I enabled it to survive in the sediment under a fish farm, where it degrades organic matter. In contrast, the high cellulase activity of the estuary-dwelling P. nuntia brevicirris allowed it to degrade organic matter originating from terrestrial areas.     

Trophic status and feeding selectivity of blennies (Blenniidae: Salariini)

An evaluation of the dietary resources available to, and selected by, blennies of the tribe Salariini was used to assess their trophic status on the Great Barrier Reef. Gut-content analysis of nine species of blennies found detrital aggregates to be the dominant item ingested. Samples of the dietary resources available to one of these species, Salarias patzneri, were compared biochemically. Of the two main dietary categories (detrital aggregates and filamentous algae), detrital aggregates were the major resource available, accounting for 53 ± 4.6% (SE) of the organic matter present. The mean C:N values for detrital aggregates (17.2 ± 0.8) and filamentous algae (20.0 ± 1.8) were very similar, as were protein concentrations (1.8 ± 0.1 and 2.1 ± 0.1 μg mg⁻¹, respectively). However, mean carbohydrate concentrations were much lower in the aggregates than the filamentous algae (19 ± 2 and 76 ± 13 μg mg⁻¹, respectively). Comparison of the inorganic particles ingested to those available indicated that S. patzneri selectively fed on particles <125 μm, (predominantly detrital aggregates) and avoided particles >250 μm, (predominantly algal filaments). The patterns of resource availability, nutritional quality and selectivity suggest that salariin blennies utilise detrital aggregates as their primary dietary resource. Received: 24 May 1999 / Accepted: 29 November 1999     

Population biology of the annual grassTriplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40 – 90 m). Plants showed the greatest growth and reproduction at close distances to shore (30 – 40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Population biology of the annual grass Triplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40–90 m). Plants showed the greatest growth and reproduction at close distances to shore (30–40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Change in the concentrations of iron in different size fractions during growth of the oceanic diatom Chaetoceros sp.: importance of small colloidal iron

 Laboratory culture experiments were performed to study the changes in size-fractionated Fe concentrations during the growth of the oceanic diatom Chaetoceros sp. Fe concentration was estimated for three size fractions: large labile particles (>0.2 μm), small colloidal particles (0.2 μm to 200 kDa) and soluble species (<200 kDa). The size-fractionated Fe concentration in the nutrient-enriched filtered seawater medium without diatom cells became stable within 4 d after the spike of FeCl₃ solution. Light irradiation by white fluorescent tubes with a 14 h light:10 h dark cycle did not significantly alter concentrations of the size-fractionated Fe. For the phytoplankton culture experiment, Fe-starved diatom cells were inoculated into the nutrient-enriched media aged for 19 d after the addition of FeCl₃. With the growth of diatom cells, total acid-labile Fe concentrations decreased from 0.60 to 0.46 nM during 7 d of incubation. However, only the concentration of the small colloidal particles showed a significant decrease; the concentration of the other size fractions remained relatively constant. Although the media still contained sufficient amounts Fe as large labile particles and soluble species, diatom cells appeared to be Fe-limited once Fe as small colloidal particles had been used up. These results suggest that Fe in the small colloidal particle fraction was the most dynamic size fraction during the growth of the diatom Chaetoceros sp. In addition, to better understand Fe dynamics in the ocean, we must consider the influence of phytoplankton growth on small colloidal Fe [which is typically included in the dissolved Fe fraction (<0.2 μm)]. Received: 9 December 1999 / Accepted: 5 May 2000     

Corophium multisetosum (Amphipoda: Corophiidae) in Canal de Mira,Portugal: Some factors that affect its distribution

The population ofCorophium multisetosum (L.) from Canal de Mira, Ria de Aveiro, Portugal, was sampled seasonally from December 1985 to September 1986, as part of a larger survey of the benthic invertebrate macrofauna. Its distribution along the channel exposes the species to a range of salinities from freshwater to above 30. A principal components analysis, using the physical and chemical parameters of the sediment as variables and sampling stations as operational taxonomic units, indicated that abundance is negatively correlated with salinity, depth, and the occurrence of sediments rich in particles below 125µm and rich in organic matter. Abundance is positively correlated with temperature. The distribution of the species does not seem to be affected by the occurrence of sediment grades between 125 and 1 000µm. Paired-choice salinity experiments indicated thatC. multisetosum prefers salinities within the range 2.5 to 10. In multi-choice experiments concerning sediment grade, amphipods did not show any significant preference within the 125 to 500µm range, although the 125µm grade was chosen less frequently. The influence of temperature on the overall distribution ofC. multisetosum is discussed.     

Size-dependent reproduction in the Caribbean gorgonian Pseudoplexaura porosa

Reproductive success among clonal taxa is often portrayed as a simple function of clone size, but reproduction in Pseudoplexaura porosa (Houtuyn), a common Caribbean gorgonian, reflects a more complex relationship between size and gamete production. Tagged colonies were sampled at two reefs in the San Blas Islands, Panama during the 1995 spawning season to determine the colony size at first reproduction, size-dependent polyp fecundities, and whole colony gonad production, and to follow the gametogenic cycle. Additional data were obtained during non-spawning months in 1984 and 1985. Of 120 colonies ranging in height from 20 to 250 cm, only colonies taller than 50 cm were reproductive. An average of 4.33 oocytes polyp⁻¹, 560 to 800 μm in diameter, were released during monthly synchronous spawning events from June to September. Oocytes released each month developed from a group of intermediate size oocytes present at the end of the preceding month. Almost all oocytes >560 μm were released during each spawning event. Spermatogenesis was a shorter process than oogenesis. During each spawning month, mature spermaries developed from a group of <125 μm spermaries. Virtually all spermaries >190 μm were released each month, and most of the spermary volume in mature male polyps was generated anew each month of spawning. Among reproductive colonies, polyp fecundity increased with colony height from 50 to 200 cm. Per capita fecundity of >200 cm tall female colonies was lower than for 150 to 200 cm colonies, but whole colony fecundity of large colonies was greater due to the exponential increase in the number of polyps that occurs with increasing colony height. In male colonies, both polyp and colony reproductive output increased with colony height. The large amounts of sperm produced by large male colonies probably contributes to the high rates of in situ fertilization observed in P. porosa. Differences in fecundity as a function of colony size and sex suggest differences in some combination of the cost of reproduction and/or allocation of resources to reproduction. Received: 9 September 1998 / Accepted: 7 June 1999     

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