Prey, feeding rates, and asexual reproduction rates of the introduced oligohaline hydrozoan Moerisia lyonsi

Moerisia lyonsi Boulenger (Hydrozoa) medusae and benthic polyps were found at 0 to 5‰ salinity in the Choptank River subestuary of Chesapeake Bay, USA. This species was introduced to the bay at least 30 years before 1996. Medusae and polyps of M. lyonsi are very small and inconspicuous, and may occur widely, but unnoticed, in oligohaline waters of the Chesapeake Bay system and in other estuaries. Medusae consumed copepod nauplii and adults, but not barnacle nauplii, polychaete and ctenophore larvae or tintinnids, in laboratory experiments. Predation rates on copepods by medusae increased with increasing medusa diameter and prey densities. Feeding rates on copepod nauplii were higher than on adults and showed no saturation over the range of prey densities tested (1 to 64 prey l⁻¹). By contrast, predation on copepod adults was maximum (1 copepod medusa⁻¹ h⁻¹) at 32 and 64 copepods l⁻¹. Unexpectedly, M. lyonsi colonized mesocosms at the Horn Point Laboratory during the spring and summer in 4 years (1994 to 1997), and reached extremely high densities (up to 13.6 medusae l⁻¹). Densities of copepod adults and nauplii were low when medusa densities were high, and estimated predation effects suggested that M. lyonsi predation limited copepod populations in the mesocosms. Polyps of M. lyonsi asexually produced both polyp buds and medusae. Rates of asexual reproduction increased with increasing prey availability, from an average total during a 38 d experiment of 9.5 buds polyp⁻¹ when each polyp was fed 1 copepod d⁻¹, to an average total of 146.7 buds polyp⁻¹ when fed 8 copepods d⁻¹. The maximum daily production measured was 8 polyp buds and 22 medusae polyp⁻¹. The colonizing potential of this hydrozoan is great, given the high rates of asexual reproduction, fairly wide salinity tolerance, and existence of a cyst stage. Received: 29 October 1998 / Accepted: 3 March 1999     

The culturable microbial community of the Great Barrier Reef sponge Rhopaloeides odorabile is dominated by an α-Proteobacterium

The microbial community cultured from the marine sponge Rhopaloeides odorabile Thompson et al. is dominated by a single bacterium, designated strain NW001. Sequence analysis of 1212 bp of the16S rRNA gene of strain NW001 indicates that it is a member of the α-subgroup of the class Proteobacteria. The association between this bacterium and its host sponge was observed in healthy R. odorabile collected from six different reefs in the Great Barrier Reef representing a geographic distance of 460 km, and in four collections in different seasons in 1997–1998 at Davies Reef (18°49.6′S; 147°34.49′E). The proportion of colonies of strain NW001 in samples from R. odorabile, expressed as a percentage of the total heterotrophic bacterial colony count, showed no significant spatial (range: 81–98%) or temporal differences (range: 81–99%), although colony counts of strain NW001 varied by up to two orders of magnitude between reef sites and sampling periods. The location of strain NW001 within the sponge mesohyl was visualized by in situ hybridization, using fluorescently labeled probes based on the 16S rRNA gene sequence of this strain. Cells of strain NW001 surround the choanocyte chambers, suggesting that these bacteria may play a role in nutrient uptake by the sponge. The absence of strain NW001 from corresponding seawater samples indicates that it has a specific, intimate relationship with R. odorabile and is not being utilized as a food source. A unique cyanobacterium related to the genera Leptolyngbya and Plectonema was also isolated from R. odorabile and characterized by 16S rRNA gene sequencing. Received: 19 May 2000 / Accepted: 18 November 2000     

Evidence for a symbiosis between bacteria of the genus Rhodobacter and the marine sponge Halichondria panicea : harbor also for putatively toxic bacteria?

Halichondria panicea (Pallas) is a marine sponge, abundantly occurring in the Adriatic Sea, North Sea, and Baltic Sea. It was the aim of the present study to investigate if this sponge species harbors bacteria. Cross sections through H. panicea were taken and inspected by electron microscopy. The micrographs showed that this sponge species is colonized by bacteria in its mesohyl compartment. To identify the bacteria, polymerase chain reaction (PCR) analysis of the 16S rRNA gene segment, typical for bacteria, was performed. DNA was isolated from sponge material that had been collected near Rovinj (Adriatic Sea), Helgoland (North Sea), and Kiel (Baltic Sea) and was amplified with bacterial primers by PCR. The data gathered indicate that in all samples bacteria belonging to the genus Rhodobacter (Proteobacteria, subdivision α) are dominant, suggesting that these bacteria live in symbiotic relationship with the sponge. In addition, the results show that the different samples taken contain further bacterial species, some of them belonging to the same genus even though found in sponges from different locations. The possibility of the presence of toxic bacteria was supported by the finding that organic extracts prepared from sponge samples displayed toxicity, when analyzed in vitro using leukemia cells. Received: 7 March 1997 / Accepted: 2 October 1997     

Distribution of secondary metabolites in the sponge Oceanapia sp. and its ecological implications

The Micronesian sponge Oceanapia sp. has an unusual growth form that consists of an irregular turnip-shaped base, which is buried in the substrate. One to several fistules, which protrude through the sand, are attached to the base of the sponge. On top of each fistule is a small fragile capitum. We examined whether this conspicuous red-colored sponge was chemically defended and if intraspecimen variation existed in the distribution of secondary metabolites between different parts of the sponge. Furthermore we assessed the deterrent properties of the secondary metabolites to generalist and more specialized fish predators. We also wanted to see if the optimal defense theory holds in the case of a marine invertebrate. According to the theory, organisms evolve and allocate defenses in a way that maximizes individual fitness, assuming that defenses are costly to the fitness of the organisms. We were able to evaluate this hypothesis, since the different sponge parts in Oceanapia sp. were at different risk to damage by predators and had a different value in terms of fitness loss to the sponge (the capitum probably plays a role in asexual propagation). Concentrations of crude organic extract increased from the base to the capitum of the sponge. The major secondary metabolites kuanoniamine C and D also showed a sharp increase from the basal root to the capitum. There was no difference in structural material or ash content between the base and the fistule of the sponge, but fiber and protein content were significantly higher in the fistule. The methanol fraction was highly deterrent in field feeding assays towards generalist reef fish at base concentration. It also deterred feeding by the spongivorous angelfish Pomacanthus imperator in laboratory feeding experiments at the same concentration. The field feeding assays with pure compounds showed that kuanoniamine C and D deterred feeding by natural assemblages of reef fishes at fistule concentrations, confirming their role as defensive agents. The intraspecimen variation of secondary metabolites in Oceanapia sp. supports the optimal defense theory by showing the highest concentrations in those parts of the sponge that are most visible to predators and are likely to be most important for inclusive fitness. Received: 5 May 1999 / Accepted: 16 September 1999     

Silica uptake kinetics of Halichondria panicea in Kiel Bight

Besides diatoms Demospongiae are the most important consumers of dissolved silica in the sea. They can play an important role for the silica budget especially in the shallow water areas of the Baltic Sea. The dependence of the silica uptake rate on the silica concentration of the seawater was measured for the sponge Halichondria panicea (Pallas, 1766). The sponges were collected in Kiel Bight. The uptake conformed to Michaelis–Menten kinetics with a half-saturation constant of 46.41 μM and a saturated uptake rate of 19.33 μmol h⁻¹ g⁻¹ ( p < 0.01). In the red algae zone of Kiel Bight the sponges depend on silica supply from the surrounding waters and may be silica-limited rather than food-limited in growth. Because of the much faster uptake of silica by diatoms and their lower saturation point, as well as the difference in spatial distribution of the two main silica consumers, a competition for silica between sponges and diatoms seems unlikely. Received: 21 June 1997 / Accepted: 15 July 1997     

Assessment of natural selection in a hybrid population of mussels: evaluation of exogenous vs endogenous selection models

We examined natural selection within a population of marine mussels, sampled in southwestern England in June 1991, containing a high frequency of hybrids between Mytilus edulis L. and M. galloprovincialis Lmk. This system is particularly tractable for the assessment of natural selection because hybridization is common and individual mussels can be aged, allowing changes in the frequency of hybrid genotypes among age classes to be determined. We show that strong viability selection occurs among hybrid genotypes which results in the virtual elimination of M. edulis–like genotypes from the population over a period of 3 years. Recombinant hybrid genotypes are intermediate in fitness, with M. edulis–like genotypes having a lower survival rate and M. galloprovincialis–like genotypes having a higher survival rate than genotypes of mixed ancestry. Since intermediate fitness for hybrid genotypes is inconsistent with endogenous selection models we conclude that the structure and position of this hybrid zone is probably generated by exogenous selection. This pattern of selection is a recurring feature of this hybrid population and likely occurs elsewhere in the hybrid zone. Selection against M. edulis–like genotypes appears to be offset by extensive immigration of larvae dispersed from pure populations of M. edulis. Received: 14 July 1997 / Accepted: 24 February 1998     

Silica content and spicule size variations in Pellina semitubulosa (Porifera: Demospongiae)

 The variations in both silica content and spicular size were studied in two populations of the demosponge Pellina semitubulosa (Lieberkühn). Samples were collected over a period of 1 year (June 1994 to May 1995) in two Mediterranean coastal basins: Porto Cesareo (southwestern Apulia) and Marsala (northwestern Sicily). The values of spicule size (length and width) and sponge silica content were significantly higher in the population of Porto Cesareo, where the highest water silica concentration was recorded. In both Porto Cesareo and Marsala the sponge silica content showed a seasonal trend, positively correlated with water temperature values. In both populations, the smallest spicules were found in specimens collected from summer to late autumn, after sexual reproduction. Secretion of new spicules may be connected with the process of remodelling occurring in sponges after gamete and larval release. Received: 10 October 1999 / Accepted: 13 April 2000     

Genetic protein variation in red mullet (Mullus barbatus) and striped red mullet (M. surmuletus) populations from the Mediterranean Sea

Starch-gel electrophoresis of allozymes was used to differentiate the two red mullet species (Mullus barbatus L. and M. surmuletus L.) in the Mediterranean Sea and, further, to investigate the genetic stock structure of M. barbatus in the eastern Mediterranean area. Twenty putative enzyme-coding loci were examined in eight M. barbatus samples caught in the Aegean and Ionian Seas (Greece) and in the Gulf of Lion (France), and two M. surmuletus samples caught in the Aegean and Gulf of Lion. A high degree of genetic polymorphism was found in both species. Species-specific electrophoretic patterns were found in PGI* and PGM*. Estimates of variance of allele frequencies among samples (F _ST) and ² analyses both revealed significant differences (P < 0.05) among the M. barbatus samples. Most of the genetic variation was among samples regardless of region. The mean value of Nei's genetic distance between the two species was 0.329. Genetic distance among M. barbatus samples was low (maximum Nei's D = 0.012), with the sample from Platania differing most from other M. barbatus samples. This is probably be due to founder effects existing at this area. These results suggest that allozyme analysis may provide important information on the genetic structure of the red mullet to ensure sustainable management of this species. Received: 7 May 1997 / Accepted: 13 October 1997     

Reef coral reproduction in the Abrolhos Reef Complex, Brazil: the endemic genus Mussismilia

The current contribution deals with the reproductive biology of a genus endemic to Brazil –Mussismilia Ortmann, 1890 – including all three species of the genus: M. braziliensis (Verrill, 1868), M. hartti (Verrill, 1868), and M. hispida (Verrill, 1902), which occur sympatrically in the studied area, the Abrolhos Reef Complex, Brazil. Sexuality patterns, modes of reproduction, synchrony and spawning periods are reported, and were determined by histological examination of material. All three species started to develop female and male gametes over different periods in the same breeding season. The three species are probably broadcast spawners, since no embryos or planulae were observed in any species at any given time of the year. Each reproductive cycle lasted approximately 11 months. Oogenesis and spermatogenesis started in different periods, with spermaries appearing in approximately the eighth month of ovary development and lasting about 3 months. Reproductive cycles were annual. Spawning probably occurred in consecutive months in each species. In M. braziliensis, spawning presumably happened between March and the middle of May in 1996 and 1997. Evidence suggested that spawning events of M. hispida took place between the end of April and mid-June. M. hartti may have spawned between September and November. The data presented here suggested that all studied species have at least one exclusive spawning period, asynchronically with the others. A possible exception may be the simultaneous (or close) spawnings of M. braziliensis and M. hispida in May. It is suggested that asynchrony in spawning periods among species may reduce the chance of hybridization, gamete waste and the competition for settlement surfaces. The occurrence of extended spawning periods for each species may also reduce the risks of reproductive failure, due to temporary adverse conditions. Received: 8 December 1998 / Accepted: 15 July 1999     

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     

Anatomical and ultrastructural studies of chemical defence in the sponge Dysidea fragilis

The marine sponge Dysidea fragilis from El Mar Menor, a hypersaline coastal lagoon (Murcia, Spain), contains the furanosesquiterpenoid ent-furodysinin as the major secondary metabolite. D. fragilis emits a defensive white fluid when it is disturbed. Electron micrographs of this fluid revealed intact vesiculated cells together with other amorphous material. Dissociated cells are more rounded in shape but maintain the same ultrastructural features as cells observed in ultra-thin sections of the whole sponge. The defensive secretion is composed mainly of sponge cells with abundant light vesicles. Sometimes these light vesicles appear to open into the intercellular space; this correlates with surface blebs on these cells observed under scanning electron microscopy. The intracellular location of ent-furodysinin was confirmed by Erlich staining. In laboratory assays, we examined the role of ent-furodysinin as a feeding deterrent to generalist fish predators. It was isolated from D. fragilis and incorporated into a carrageenan-based artificial diet. The addition of ent-furodysinin to the artificial diet reduced feeding by the fish Thalassoma pavo. Similarly, fish did not feed on artificial diet above which defensive secretion of D. fragilis had been ejected with a small syringe. Received: 4 June 1997 / Accepted: 28 January 1998     

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     

Heterogeneous population growth, parental effects and genotype–environment interactions of a marine oligochaete

Cultures of asexually reproducing populations of the oligochaete Paranaislitoralis (Müller) collected from six different patches (3 to 50 m apart) on an intertidal mud flat in Flax Pond, New York, on two occasions, June and October 1993, showed significant differences among lines in life span, number of offspring produced, and in finite rate of increase (λ). Although growth rates were significantly lower in October than in June, they were always positive (λ > 1) in the laboratory cultures reared in field-collected sediment, while field data show that the densities of P. litoralis decreased sharply in summer and autumn from a seasonal high in early June. Cultures of worms reared at high densities without renewal of sediment crashed, and effects on individuals were irreversible: worms from late (declining) stages of population growth had a significantly higher mortality and lower reproduction than worms from earlier stages, also when transferred to high-quality food. Genetical analysis using RAPDs (random amplified polymorphic DNA) confirmed the existence of several clones of P. litoralis in our cultures. Experiments where parent and offspring were cultured in sediments of different qualities showed clone–environment interactions in the number of asexual offspring produced, but not in age at first reproduction. Clones also differed in that some showed significant parental effects of sediment quality on life-history characteristics while other clones did not. Our results indicate that P.litoralis populations in Flax Pond are not an example of a population subdivided into a set of permanent source and sink subpopulations, but rather an example of a continuously shifting mosaic of local growth conditions. Received: 21 April 1997 / Accepted: 3 September 1997     

Redwood of the reef: growth and age of the giant barrel sponge Xestospongia muta in the Florida Keys

The growth of animals in most taxa has long been well described, but the phylum Porifera has remained a notable exception. The giant barrel sponge Xestospongia muta dominates Caribbean coral reef communities, where it is an important spatial competitor, increases habitat complexity, and filters seawater. It has been called the ‘redwood of the reef’ because of its size (often >1 m height and diameter) and presumed long life, but very little is known about its demography. Since 1997, we have established and monitored 12 permanent 16 m diameter circular transects on the reef slope off Key Largo, Florida, to study this important species. Over a 4.5-year interval, we measured the volume of 104 tagged sponges using digital images to determine growth rates of X. muta. Five models were fit to the cubed root of initial and final volume estimates to determine which best described growth. Additional measurements of 33 sponges were taken over 6-month intervals to examine the relationship between the spongocoel, or inner-osculum space, and sponge size, and to examine short-term growth dynamics. Sponge volumes ranged from 24.05 to 80,281.67 cm³. Growth was variable, and specific growth rates decreased with increasing sponge size. The mean specific growth rate was 0.52 ± 0.65 year⁻¹, but sponges grew as fast or slow as 404 or 2% year⁻¹. Negative growth rates occurred over short temporal scales and growth varied seasonally, significantly faster during the summer. No differences in specific growth rate were found between transects at three different depths (15, 20, 30 m) or at two different reef sites. Spongocoel volume was positively allometric with increasing sponge size and scaling between the vertical and horizontal dimensions of the sponge indicated that morphology changes from a frustum of a cone to cylindrical as volume increases. Growth of X. muta was best described by the general von Bertalanffy and Tanaka growth curves. The largest sponge within our transects (1.23 × 0.98 m height × diameter) was estimated to be 127 years old. Although age extrapolations for very large sponges are subject to more error, the largest sponges on Caribbean reefs may be in excess of 2,300 years, placing X. muta among the longest-lived animals on earth.     

Fluctuations of the red tide flagellates Chattonella spp. (Raphidophyceae) and the algicidal bacterium Cytophaga sp. in the Seto Inland Sea, Japan

A marine algicidal gliding bacterium Cytophaga sp. strain J18/M01 was isolated in 1990 from a station in northern Harima-Nada, the Seto Inland Sea, Japan, using the harmful red tide alga Chattonella antiqua (Hada) Ono as a susceptible organism. The bacterium can prey upon various species of microalgae. Temporal fluctuations of this bacterium and Chattonella spp. [C. antiqua and C. marina (Subrahmanyan) Hara et Chihara] were investigated weekly at the above station in the summer of 1997 and 1998, using immunofluorescence assay employing highly specific polyclonal antibodies for the bacterium. In the summer of 1997, the cell density of Chattonella spp. showed a maximum value (70 cells ml⁻¹) on 8 July, and decreased thereafter. The bacterium Cytophaga sp. J18/M01 was commonly detected around a few hundreds of cells per milliliter or less. The number of Cytophaga sp. J18/M01 increased after the peak of Chattonella spp., and the maximum cell number of the bacterium was 1350 ml⁻¹. This algicidal bacterium also followed the changes of total amounts of microalgal biomass (chlorophyll a+pheophytin) when Chattonella spp. were absent. In the summer of 1998, Chattonella spp. were relatively less abundant (maximum 21 cells ml⁻¹), and the algicidal bacterium Cytophaga sp. J18/M01 showed a close relationship with the change of total microalgal biomass. The present study suggests that the algicidal bacterium Cytophaga sp. J18/M01 preyed upon, not only harmful red tide microalgae, but also other common microalgae such as diatoms, and the bacterium presumably plays an important role in regulating microalgal biomass in natural marine environments. Received: 20 April 2000 / Accepted: 1 December 2000     

Distribution and abundance of ctenophores and their zooplankton food in the Black Sea. II. Mnemiopsis leidyi

The distribution of Mnemiopsis leidyi Agassiz, 1865 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum depth 200 m) in the summer, winter, and spring from 1991 to 1995. Distribution was patchy. Average biomasses of 15 to 500 g m⁻² were measured, and abundances varied from 10 to 180 ind m⁻². Biomass and abundance peaked in winter, and there was a secondary peak in the summer. The distribution of M. leidyi was correlated with hydrographic features in the Black Sea with higher concentrations in anticyclonic gyres. The centers of the two main cyclonic gyres generally had a low biomass of M. leidyi. From July 1992 to March 1995, the populations were largely offshore. M. leidyi were confined to the upper part of the mixed layer both day and night. Some individuals displayed a negative taxis to daylight and were concentrated below the thermocline at night. Smaller M. leidyi (1.5 to 2 cm) were present in the winter, and individuals reached maximum size in the summer. Although reproduction was continuous throughout the year, there were two distinct peaks: the larger peak in the summer and the smaller peak in the winter. Microscopic analysis of stomach contents showed that copepods and molluscs form their main diet. Received: 1 November 1997 / Accepted: 30 August 1999     

Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea

The dietary habits of the pelagic mysid Mysismixta were studied during its growing season at an open sea location in the Gulf of Finland, the northern Baltic Sea. Stomach samples were taken twice a month from June to September 1997. The most abundant phytoplankton taxa in the stomachs were diatoms and dinoflagellates, and copepods and cladocerans were the most abundant zooplankton identified. A clear change was found in the diets during the study period. Small mysids (3 to 6 mm) fed on sedimented phytoplankton in the early summer (90% benthic particles in June) but shifted gradually to a more pelagic and carnivorous diet (>40% pelagic particles, consisting of ca. 60% zooplankton in September). Seasonal changes in mysid capture ability as well as food availability were suggested to affect the diet composition of mysids during their growth. The ratio of pelagic and benthic food particles could – irrespective of the season – be explained by mysid size, whereas the zooplankton:phytoplankton ratio was better explained by season. The stomach analysis suggests that the mysids needed to attain a threshold size of 8 to 11 mm to initiate feeding on the more evasive copepods. Mysids also started to grow faster at the same time as the proportion of copepods increased in the diet, which suggests that copepods are an important energy source for M. mixta in late summer. Finally, a comparison was made between the M. mixta diet and that of the less abundant M. relicta. The diets of the two pelagic mysid species overlapped by 75% (Schoener's index). The main difference was due to M. mixta eating more zooplankton and pelagic material than M. relicta. Received: 15 September 1999 / Accepted: 18 January 2000     

Larval bloom of the oviparous sponge Cliona viridis: coupling of larval abundance and adult distribution

 We performed an intensive year-round sampling with the aim of studying the abundance of sponge larvae in four Mediterranean benthic communities: photophilic algae, sciaphilous algae, semi-obscure (i.e. low light-intensity) caves and sandy bottoms. We record here for the first time, a larval bloom of Cliona viridis (Schmidt 1862), the most common excavating sponge in the Mediterranean, which took place simultaneously in several rocky communities of the Blanes sub-littoral (NE Spain), and discuss the role of restricted larval dispersal in the distribution of adult sponges. In the communities studied, C. viridis larvae bloomed synchronously once, in June. Spawning and consequent embryo development presumably occurred in May, when water temperature was 16 °C. The free larva is a small, evenly ciliated, weakly swimming parenchymella with low dispersal capabilities. The number of larvae m⁻³ and sponge abundance (as percent cover and biomass) were significantly higher in the community of sciaphilous algae than in the other communities studied. Because of limited larval dispersal, larval and adult abundance in the communities were positively correlated. Larvae developed into juvenile sponges 10 to 15 d after settlement. Settlers displayed distinctive features: a peripheral cuticle, vacuolar etching-like cells at the sponge base, absence of oscular chimneys, and the presence of zooxanthellae, which were presumably transmitted during oocyte maturation. Received: 24 January 2000 / Accepted: 4 July 2000     

Oxygen dynamics and transport in the Mediterranean sponge Aplysina aerophoba

The Mediterranean sponge Aplysina aerophoba kept in aquaria or cultivation tanks can stop pumping for several hours or even days. To investigate changes in the chemical microenvironments, we measured oxygen profiles over the surface and into the tissue of pumping and non-pumping A. aerophoba specimens with Clark-type oxygen microelectrodes (tip diameters 18–30 μm). Total oxygen consumption rates of whole sponges were measured in closed chambers. These rates were used to back-calculate the oxygen distribution in a finite-element model. Combining direct measurements with calculations of diffusive flux and modeling revealed that the tissue of non-pumping sponges turns anoxic within 15 min, with the exception of a 1 mm surface layer where oxygen intrudes due to molecular diffusion over the sponge surface. Molecular diffusion is the only transport mechanism for oxygen into non-pumping sponges, which allows total oxygen consumption rates of 6–12 μmol cm⁻³ sponge day⁻¹. Sponges of different sizes had similar diffusional uptake rates, which is explained by their similar surface/volume ratios. In pumping sponges, oxygen consumption rates were between 22 and 37 μmol cm⁻³ sponge day⁻¹, and the entire tissue was oxygenated. Combining different approaches of direct oxygen measurement in living sponges with a dynamic model, we can show that tissue anoxia is a direct function of the pumping behavior. The sponge-microbe system of A. aerophoba thus has the possibility to switch actively between aerobic and anaerobic metabolism by stopping the water flow for more than 15 min. These periods of anoxia will greatly influence physiological variety and activity of the sponge microbes. Detailed knowledge about the varying chemical microenvironments in sponges will help to develop protocols to cultivate sponge-associated microbial lineages and improve our understanding of the sponge-microbe-system.     

Selective feeding in shellfish: size-dependent rejection of large particles within pseudofaeces from Mytilus edulis, Ruditapes philippinarum and Tapes decussatus

Mytilus edulis L., Ruditapes philippinarum (Adams & Reeve) and Tapes decussatus L. were fed particles of the same shape (spherical), the same density (2.1 g cm⁻³) and the same chemical composition (SiO₂), but which varied in diameter from 5 to 37 μm. Findings obtained at different particle concentrations (mean ± SD) of 51 ± 2, 105 ± 18 and 171 ± 17 mg l⁻¹ invariably indicate that significant proportions of all particles with diameters larger than from between 7.5 and 22.5 μm were preferentially rejected as pseudofaeces. We define the preferential ingestion index (PII) as the ratio between average particle volume in pseudofaeces and average particle volume in food. Whatever the particle concentration or the species, this PII was always statistically higher than 1. Irrespective of particle concentration, PII values in M. edulis were lower than in T. decussatus (averages of 1.2 and 2, respectively). PII values in M. edulis were also lower than in R. philippinarum maintained at particle concentrations above 171 ± 17 mg l⁻¹. We suggest that preferential size-dependent rejection of larger particles could be of significant adaptive value in the natural environment, either if there are large inorganic particles, or if the average organic content of smaller particles is higher. Received: 11 January 1997 / Accepted: 8 March 1997