Reproduction in Anthelia glauca (Octocorallia: Xeniidae). II. Transmission of algal symbionts during planular brooding

The soft coral Anthelia glauca Lamarck, 1816, of the family Xeniidae, is found on the reefs of KwaZulu-Natal, South Africa. Its gastrodermal cells contain numerous endosymbiotic unicellular algae (zooxanthellae). A. glauca is a gonochoric species that simultaneously broods its planulae within the pharyngeal cavity of the polyps. Symbiotic algae appear with zygote formation within the pharynx, embedded in amorphous material. The algal cells adhere to the ciliated ectodermal surface of immature planulae and are most probably endocytosed by them. Zooxanthellae are translocated towards the basal part of the ectoderm. Gaps are subsequently opened in the mesoglea into which symbionts surrounded by ectodermally derived material, including plasma membrane, pass. The basal membrane of endodermal cells disintegrates, and the algae bulge into spaces formed in the underlying endoderm. Throughout the process, each zooxanthella resides within a vacuolar membrane in the detached ectodermal cytoplasm. The acquisition process is essentially one in which zooxanthellae are translocated from the pharyngeal cavity into the ectoderm and then through the mesoglea into the endoderm, culminating in the final symbiotic state. The direct transmission of symbiotic algae to the eggs or larvae probably provides the most efficient means whereby zooxanthellae are acquired by the host progeny. Received: 15 July 1997 / Accepted: 25 February 1998     

The role of chemosensory behavior of Symbiodinium microadriaticum,intermediate hosts,and host behavior in the infection of coelenterates and molluscs with zooxanthellae

Symbiotic dinoflagellates, Symbiodinium microadriaticum (=zooxanthellae), may gain access to aposymbiotic hosts (i.e., those lacking zooxanthellae) by chemosensory attraction of the motile algae by the potential host or via an intermediate host. Laboratory experiments showed that motile zooxanthellae were attracted to intact aposymbiotic host animals, but not to starved symbiotic hosts. Fed symbiotic hosts and brine shrimp (Artemia sp.) nauplii also attracted motile zooxanthellae. The attraction of these zooxanthellae was directly correlated with nitrogen levels in the seawater surrounding the hosts; thus ammonia and possibly nitrate could be atractants. Brine shrimp nauplii, acting as intermediate hosts actively ingested both motile and non-motile zooxanthellae. the ingested zooxanthellae tended to remain morphologically unaltered during and after passage through the gut of the brine shrimp. Capture and ingestion of brine shrimp containing zooxanthellae by aposymbiotic scyphistomae of the jellyfish Cassiopeia xamachana led to infection of the scyphistomae with zooxanthellae. Zooxanthellae isolated from 17 different species of coelenterates and molluscs could be transferred via brine shrimp to the endodermal cells of the scyphistomae. However only 10 of these isolates persisted to establish a permanent association with C. xamachana. Scyphistomae in suspensions of motile zooxanthellae responded by a classical coelenterate feeding response, which may facilitate ingestion of the potential symbionts and establishment of a symbiosis.     

Characteristics of feeding on dinoflagellates by newly hatched larval crabs

The zoeal larvae of brachyuran crabs must feed soon after hatching on a diet that includes large micro- and mesozooplankton in order to satisfy nutritional requirements. However, newly hatched larvae have been shown to ingest a variety of dinoflagellates, perhaps using microbial carbon sources to sustain them until they encounter more favored prey. Ingestion of dinoflagellates by larval crabs has been documented previously under conditions in which the larvae were exposed to algae provided in monoculture or in defined mixtures of cells. We report here on experiments conducted on the hatching stage of five crab species to determine if ingestion of dinoflagellates occurred when they were provided in combination with Artemia sp. nauplii or after a period of feeding on mesozooplankton. Quantitative measurements of chl a in the larval guts provided evidence of ingestion of algal cells. Active ingestion of the dinoflagellate Prorocentrum micans at specified intervals during an extended feeding period was determined on larvae of two crab species using fluorescently labeled cells provided for brief periods at prescribed time intervals. Stage 1 larvae of four of the five crab species ingested dinoflagellates when they were provided in combination with nauplii and larvae of all five species ingested cells after feeding solely on nauplii for 24 h. Ingestion of algal cells was first evident in the larval guts after 6 h of feeding at both low (200 cell ml⁻¹) and high (1,000 cells ml⁻¹) prey densities. Higher prey densities resulted in higher gut chl a. Larvae continuously exposed to dinoflagellates actively ingested cells at every 3 h interval tested over a 36 h period. Results confirm previous studies that larvae will ingest dinoflagellates even when they are encountered in a mixed prey field or when having previously fed. Ingestion of cells may occur on a continual basis over time.     

Effects of symbiotic status,flow speed,and prey type on prey capture by the facultatively symbiotic temperate coral <Emphasis Type="Italic">Oculina</Emphasis><Emphasis Type="Italic">arbuscula</Emphasis>

Symbiotic temperate corals can supplement prey capture by the coelenterate host with autotrophic carbon production by endosymbiotic zooxanthellae. To test the relationship between heterotrophic consumption and photosynthetic energy, prey capture by symbiotic and aposymbiotic specimens of the temperate scleractinian coral Oculina arbuscula (Verrill) was measured in January-April 2001. Corals were tested in a laboratory flume at five flow speeds, using Artemia franciscana cysts and nauplii as prey. Per-polyp capture rate and feeding efficiency were independent of symbiotic condition. Capture rate increased with flow speed, while capture efficiency declined. The location of capture shifted from the upstream to downstream side of the coral as flow speed increased. Differences in capture rate, location, and feeding efficiency for cysts and live brine shrimp nauplii were likely due to prey size rather than swimming ability.     

Ultrastructure of the digestive gland of larval and adult stages of the sacoglossan Elysia patagonica

Chloroplast retention in the sacoglossan gastropod Elysia patagonica Muniain and Ortea, 1997 was investigated. Intact chloroplasts of the green algae Bryopsis plumosa (Chlorophyta: Bryopsidacea) were observed by transmission electron microscopy in vacuoles of the digestive cells of benthic adults starved for 20 days. Intact chloroplasts from the microalgal Nannochloropsis sp. (Heterokontophyta: Eustigmatophycea) were found in the digestive cells of veliger larvae feeding for 8 days. Similar digestive cells were observed in adults and planktonic larvae of E. patagonica. The retention ability is confirmed in both stages.     

Effects on larval crabs of exposure to algal toxin via ingestion of heterotrophic prey

We tested whether ingesting toxic algae by heterotrophic prey affected their nutritional value to crab larval predators, using toxic algal strains that are either ingested directly by larval crabs or rejected by them. Ingestion of toxic strains of the dinoflagellates Alexandrium andersoni and A. fundyense by the rotifer Brachionus plicatilis was confirmed. Rotifers having ingested either algal type for five days were fed to freshly hatched larvae of three crab species, with larval survival and stage durations determined. For both algal/rotifer treatments in all three crab species, larvae fed algae directly died during the first zoeal stage, while those fed rotifers that had been fed either algal strain survived to the experiment’s end (zoeal stage 3). Survival was lower, and stage duration longer, for larvae fed rotifers cultured on toxic algae when compared to those fed non-toxic algae. The role of toxic algae in the planktonic food web may be influenced by its direct or indirect ingestion by larval crabs.     

Development of the escape response in larval walleye pollock (Theragra chalcogramma)

The development of the escape response of walleye pollock (Theragra chalcogramma) larvae from attacks by macrozooplanktonic and small-fish predators was quantified in laboratory experiments. Behavior was recorded using video cameras with silhouette illumination from infrared-emitting diodes and by visual observation. Laboratory-reared larvae of 1, 3, 8, 10, 12, 18, 22, 27, 42 days post-hatching, ranging in size from 4 mm to 10 mm total length, were used in the experiments. Even the youngest larvae were observed to exhibit a fast startle response. The percentage of successful larval escapes from the different predators increased as the larvae developed. Euphausiids (Thysanoessa raschii) and amphipods (Calliopiella pratti) often touched larvae but the larvae were usually able to escape and no successful captures of larvae over 22 days old were observed. Although successful escape from initial attacks by three-spine sticklebacks (Gasterosteus aculeatus) increased ontogenetically, sticklebacks were able to consume most larvae, even of the oldest age group, by repeated attacks. Day-old larvae had the lowest percent of escapes after encounters with jellyfish (Sarsia sp.), but the percentage of escapes increased dramatically for 3-day-old larvae. Escape speeds after an attack also increased with age, and tended to be higher after stickleback attacks and lower after jellyfish attacks. This study revealed that the escape response of larval pollock to attack by predators improves rapidly with development during the early larval stage.     

Alternate coral–bryozoan competitive superiority during coral bleaching

Bleaching of corals results from the loss of their symbiotic algae (zooxanthellae) and/or pigments. The supply of photoassimilates provided by the zooxanthellae to the coral declines during bleaching and reduces the ability to activate energy-costly processes such as maintenance, growth and reproduction. In the present study we compared the competitive outcomes, expressed as overgrowth and changes in colony sizes of Oculina patagonica (an encrusting Mediterranean stony coral) and the bryozoan Watersipora sp., growing in contact with each other, during and between bleaching events. Year-round observations of tagged colonies showed alternating competitive outcomes: O. patagonica wins over Watersipora sp. between bleaching events, but loses during bleaching events. Using the ¹⁴C-point-labeling technique on coral tissue, we examined intra-colonial translocation of photosynthetic products from the point-tissue labeling towards interaction zones. In non-bleached O. patagonica, competition resulted in preferentially oriented translocation of ¹⁴C products to the interaction zone located up to 8 cm away from the tissue-labeling site. Sites opposite the interaction zone received significantly less labeled photoassimilates compared to the interaction zone. In bleached colonies (40-85% bleached surface area), such translocation did not occur, probably explaining the failure to compete with the encrusting neighbor Watersipora sp. Our findings demonstrate the importance of colonial integration and resource orientation for the competitive superiority of O. patagonica.     

Larval survivorship, competency periods and settlement of two brooding corals, Heliopora coerulea and Pocillopora damicornis

Larval dispersal and recruitment are important in determining adult coral distribution; however, few studies have been made of coral larval dispersal. This study examined the larval behavior, survivorship competency periods and settlement of two brooding corals, Heliopora coerulea and Pocillopora damicornis, in relation to different potential larval dispersal patterns. We also examined the lipid content of H. coerulea as a means of flotation and a source of energy. Planulae of H. coerulea were on average 3.7 mm in length, lacked zooxanthellae, and were mostly benthic, probably because of restricted movement and low lipid content (54% by dry weight). Planulae of P. damicornis were on average 1.0 mm in length, had zooxanthellae and swam actively. The competency period of H. coerulea was shorter (30 days) than that of P. damicornis (100 days). Forty percent of H. coerulea planulae crawled onto the substrata within 1 h of release, and 47% settled within 6 h. By contrast, fewer than 10% of P. damicornis planulae crawled onto the substrata within the first hour and 25% settled within 6 h of release. The planulae of H. coerulea may have a narrower dispersal range than those of P. damicornis, settling and recruiting near parent colonies. Thus, brooding corals exhibit variations in larval dispersal patterns, which are characterized by their position in the water column and competency periods.     

Foraging behaviour of larval cod (Gadus morhua) influenced by prey density and hunger

Fish larvae meet diverse environmental conditions at sea, and larval growth and chance of survival depend on a flexible response to environmental variability. The present study focuses on the flexibility of the foraging behaviour of larval cod in a series of laboratory experiments on larval search activity, prey selectivity, and hunger in a variable prey environment. Gadus morhua eggs were collected in March 1992 and 1993 from the Kattegat area, Denmark, fertilised and incubated in the laboratory. After hatching, the larvae were transferred to rearing tanks of 172 litres. The behaviour of larvae (6 to 7 mm long) was observed visually, and prey attacks, swimming activity and gut contents were registered across a range of 1 to 120 copepod nauplii l^-1. When prey density decreased, larvae increased their swimming activity, increased their responsiveness to prey (distance of reaction) and decreased their prey size selectivity. Behavioural response was to a large degree determined by the level of hunger, represented by the number of newly ingested prey in the gut. The findings show that cod larvae have a flexible response to changes in feeding conditions and imply that larvae can grow and survive even in the lower range of (mean) prey densities measured at sea.     

Effects of exposure to toxic and non-toxic dinoflagellates on oxygen consumption and locomotion in stage 1 larvae of the crabs Cancer oregonensis and C. magister

Sublethal effects on larval crabs upon exposure to toxic dinoflagellates were examined in the laboratory in early 1999. Specifically, oxygen consumption rates and geotaxis responses were determined for stage 1 larvae of the crabs Cancer oregonensis (Dana) and C. magister Dana that were exposed to non-toxic (Alexandrium tamarense, strain 115) or toxic (A. fundyense, strain 1719) dinoflagellates or to freshly hatched nauplii of the brine shrimp Artemia sp. In C. oregonensis, larvae exposed to the toxic dinoflagellate showed reduced rates of oxygen consumption compared to those exposed to non-toxic dinoflagellates or brine shrimp nauplii. Larvae exposed to a filtrate of the non-toxic dinoflagellate showed no change in oxygen consumption, but a reduced rate when exposed to filtrate from the toxic alga at densities >5᎒² cells ml^-1. In C. magister, larvae exposed to the non-toxic A. tamarense or the toxic A. fundyense had reduced oxygen consumption rates. Larvae exposed to filtrates of non-toxic and toxic dinoflagellates had no change in oxygen consumption. In geotaxis tests, C. oregonensis larvae exposed for 1 day to the toxic A. fundyense reduced their level of locomotion compared to those exposed to non-toxic A. tamarense or to brine shrimp nauplii. C. magister larvae showed no change in activity after a 1-day exposure to the toxic A. fundyense. After a 4-day exposure to A. fundyense, C. magister larvae had much reduced locomotion. Reduced locomotory activity in larvae exposed to toxic algae is consistent with the changes in oxygen consumption rates. Responding to exposure to toxic algae by reducing locomotion may affect vertical migration in these negatively buoyant crab larvae, resulting in sinking below a toxic alga bloom, at least temporarily.     

A “CO2 supply” mechanism in zooxanthellate cnidarians: role of carbonic anhydrase

Carbonic anhydrase (CA, EC 4.2.1.1) activity was detected in 22 species of tropical cnidarians which contain endosymbiotic dinoflagellates (=zooxanthellae). CA activity was 2 to 3 times higher in animal tissue than in algae and ca. 29 times higher in zooxanthellate than azooxanthellate species. It was also higher in the zooxanthellate tentacle tissue than in the azooxanthellate column tissue of the anemone Condylactis gigantea. CA was therefore significantly related to the presence of endosymbiotic algae. Further results indicated that CA functions in the photosynthetic carbon metabolism of zooxanthellate cnidarians as evidenced by (1) low CA activity in shade-adapted and deep water colonies compared to the more productive shallow water, light-adapted colonies of the coral Stylophora pistillata, and (2) the 56 to 85% reduction in photosynthetic carbon assimilation by zooxanthellae in situ in the presence of Diamox, an inhibitor of CA. Although CA has been proposed to function in calcification, its association with zooxanthellae and photosynthetic activity in both calcifying and non-calcifying associations suggests a role in photosynthetic metabolism of algal/cnidarian symbioses. It is proposed that CA acts as a CO₂ supply mechanism by releasing CO₂ from bicarbonate, and enabling zooxanthellae to maintain high rates of photosynthesis in their intracellular environment.     

Alternative feeding mechanisms in megalopae of the blue crab Callinectes sapidus

The apparent mismatch between the energy requirements for planktotrophic growth and prey availability has long been paradoxical. One hypothesis to explain this paradox is that planktotrophic larvae display plasticity in feeding mechanisms in response to variable prey types and concentrations. This hypothesis was tested by videotaping megalopae of the brachyuran crab Callinectes sapidus Rathbun feeding on various-sized prey. Frame-by-frame analysis of the videotapes indicated that C. sapidus megalopae used both raptorial and suspension feeding to capture prey while in the water column. Raptorial feeding was used to capture macro-zooplankton, including copepods. The swimming form of suspension feeding was based on a modified fling-and-clap mechanism using the chelipeds. Suspension feeding while at rest utilized a weak current generated by the mouthparts to direct prey to the mouth. Both suspension-feeding mechanisms resulted in the efficient capture of rotifer-sized particles. The energy/handling time ratios for all three feeding mechanisms are very similar (E/H range 0.016-0.019 µg C s^-1) for the natural prey tested. These results support the hypothesis that feeding in brachyuran larvae is plastic and includes mechanisms of both raptorial and suspension feeding. The ability to suspension feed at rest is adaptive, since megalopae use selective tidal transport to re-invade an estuary and may spend up to 18 h day^-1 clinging to a benthic substrate. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0781-1.     

Stimulatory effect of ingested protein and/or free amino acids on the secretion of the gastro-endocrine hormone cholecystokinin and on tryptic activity, in early-feeding herring larvae, Clupea harengus

In the larvae of many marine teleosts, the stomach is absent until they approach or attain metamorphosis. Consequently, the formation of chyme containing specific free amino acids from the gastric digestion of protein, which are believed to be signals initiating the release of the digestive hormone cholecystokinin (CCK), is lacking. CCK, when secreted into the blood circulation from specialized intestinal cells, stimulates gallbladder motility and is a key factor causing the release of pancreatic digestive enzymes into the gut lumen. Using first-feeding Atlantic herring larvae (Clupea harengus) as a model, the aim of the present study was to determine if a CCK response together with tryptic activity could be elicited in larvae ingesting dietary protein and/or FAA. Larvae were tube fed single lamellar liposome vesicles (SLV) containing: (1) physiological saline (PS), (2) bovine serum albumin (BSA), (3) specific free amino acids (FAA), or (4) a ratio (1:1) of BSA and FAA. The CCK and trypsin levels were then assayed (radio-immunoassay) at 0, 15, 60 and 120 min after tube feeding. A marked CCK response was elicited in all treatments compared to the PS control at 15 and 30 min and was significant (p<0.05) at 120 min after tube feeding. Larvae tube fed the FAA treatment exhibited CCK levels that increased linearly from 1.6 to 5.6 fmol mg^-1 dry weight (DW) after 2 h of digestion, although this response was below the BSA and BSA:FAA treatments. The BSA and BSA:FAA treatments, after 15 min of digestion, showed a rapid CCK increase, over the PS and the FAA liposome treatments, to 8.1 and 5.4 fmol mg^-1 DW, respectively. At the end of the assay, BSA and BSA:FAA demonstrated similar levels (10.2 and 9.2 fmol mg^-1 DW, respectively). Larvae tube fed the PS control or the FAA liposome treatment did not demonstrate any appreciable increase in tryptic activity during the 2 h digestion period (0.03-0.071 and 0.03-0.048 mU mg^-1 DW, respectively). In contrast, the BSA:FAA treatment increased from 0.03-0.148 mU mg^-1 DW 1 h after feeding, which was significantly (p<0.05) higher than the PS and FAA liposomes, and then decreased markedly (0.085 mU mg^-1 DW) after 2 h of digestion. The larvae tube fed BSA liposomes, however, demonstrated steadily increasing tryptic activity throughout the sampling period, attaining 0.255 mU mg^-1 DW after 2 h, which was significantly (p<0.05) more than all the other treatments. The results showed that ingested liposomes containing FAA or the protein BSA or a combination of these two nutrients effectively stimulated CCK production in first-feeding herring larvae. In contrast, liposomes containing only physiological saline did not elicit a CCK response. In addition, liposomes containing BSA stimulated tryptic activity in herring larvae, which was not observed in fish fed liposomes that included only FAA or PS. This suggests that a suitable protein substrate is required to regulate protein digestion.     

Natural diets of vertically migrating zooplankton in the Sargasso Sea

The feeding preferences of three common diel vertically migrating zooplankton were investigated from December 1999 to October 2000 at the U.S. JGOFS Bermuda Atlantic Time-Series Study (BATS) station in the Sargasso Sea. Gut content analysis of the copepods Pleuromamma xiphias (Giesbrecht) and Euchirella messinensis (Claus) and of the euphausiid Thysanopoda aequalis (Hansen) indicated that all three species fed on a wide variety of phytoplankton, zooplankton, and detrital material. Diet changes generally reflected seasonal trends in phytoplankton community structure. However, species-specific feeding preferences and differences in feeding selectivity among the three species were evident, and in general agreement with feeding habits predicted from the analysis of mouthpart morphology. The euphausiid T. aequalis fed equally on more different food types compared to both copepod species. The copepod P. xiphias consumed a diverse assemblage of phytoplankton from late winter through the summer (78-93% of gut items, by number, were phytoplankton) and based its diet more strongly on carnivorous feeding in autumn and early winter (31% and 61% of gut items were phytoplankton, respectively). E. messinensis showed the greatest feeding specialization, with a strong preference for pennate diatoms in winter and spring and for coccolithophorids during late summer and fall (constituting 67-93% of gut items by number). All three species consumed diatoms more than other phytoplankton taxa, even though diatoms form only a small fraction of the phytoplankton biomass in the Sargasso Sea. Although the majority of gut items identified were phytoplankton cells, the relative biomass contribution of these small cells may be lower than that of zooplankton and detritus. Zooplankton on which the three species primarily preyed were protozoans and crustaceans, but also included other metazoans such as chaetognaths and cnidarians. Marine snow was also an important component of the diet in all three species, with typically >50% and rarely <20% of the gut content being olive-green debris. Marine snow from larvacean houses was found in the guts of all three species, while E. messinenis appeared to selectively consume marine snow aggregates enriched with bicapitate Nitzschia spp. Large cyanobacteria (>4 µm in diameter) found in guts were also likely consumed with marine snow. The species-specific differences in the diets of these three migrating species suggest that an individual species approach is important in determining how feeding habits affect the structure of pelagic food webs and carbon cycling in the sea. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0815-8.     

Establishment of the photosymbiosis in the early ontogeny of three giant clams

Distribution and morphology of zooxanthellae were investigated histologically and ultrastructurally in veligers and juveniles of three giant clam species, Tridacna crocea, T. derassa, and T. squamosa. No zooxanthellal cells were associated with gametes. In veliger larvae, zooxanthellae were ingested and digested in the stomach. Within several days after metamorphosis from veliger to a juvenile clam, the zooxanthellal tube, in which zooxanthellae were packed, elongated from the stomach toward the mantle. Zooxanthellae in the tube appeared in a line, and we designated the appearance of the lined zooxanthellae in the mantle of juvenile clams as the first sign of the establishment of symbiosis. The zooxanthellal tubular system developed as the clams grew, particularly in the mantle margin, and then hypertrophied siphonal tissue formed. In zooxanthellal tubes, zooxanthellae usually had intact ultrastructures suggesting that they were photosynthetically active, while the stomach always contained degraded zooxanthellae that were probably discharged from the zooxanthellal tube. Giant clams probably digest zooxanthellae directly, and ingest the secreted photosynthates from zooxanthellae. There may be a selection mechanism to discharge unhealthy zooxanthellae from the mantle into the stomach. In addition to zooxanthellae, digested diatoms and other unidentified digested materials in the stomach suggest that filter-feeding also contributes to giant clam nutrition.     

Temperature-induced ontogenetic plasticity in sea bass (Dicentrarchus labrax)

We studied the ontogeny of Dicentrarchus labrax comparatively under constant rearing temperatures of 13, 15 and 20°C. At hatching, yolk-sac larval morphometry differed significantly between the temperatures and especially between the two extremes, while at the end of the yolk-sac larval stage, it mainly differed between the two lower temperature regimes and that of 20°C. Compared with the two lower temperature conditions, at 20°C D. labrax presented a significant ontogenetic acceleration which was morphologically expressed either as a significantly smaller total length (TL) at feeding onset, notochord flexion and fin differentiation, or as shifts of the allometric inflection points of 8 out of the 15 morphometric characters studied. Additionally, temperature significantly affected the allometry coefficients, with a decreasing growth intensity as the temperature difference decreased. The rate of TL growth increased under elevated temperature conditions throughout the entire ontogenetic period, except during the early larval period (feeding onset to metamorphosis onset), at which time D. labrax presented equal growth rates at 15°C and 20°C. The results are discussed with respect to the ontogeny of the functional morphology and the meanings of temperature-induced ontogenetic plasticity for the survival of fish larvae.     

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     

Maristentor dinoferus n. gen., n. sp., a giant heterotrich ciliate (Spirotrichea: Heterotrichida) with zooxanthellae, from coral reefs on Guam, Mariana Islands

Maristentor dinoferus n. gen, n. sp., was discovered on coral reefs on Guam in 1996 and has since been found frequently, at depths of 3-20 m. It forms black clusters, visible to the naked eye, especially on Padina spp. (Phaeophyta) and other light-colored backgrounds. When fully extended, this sessile ciliate is trumpet-shaped, up to 1 mm tall and 300 µm wide across the cap. The ciliate is host to 500-800 symbiotic algae. The anterior cap, or peristomial area, is divided into two conspicuous lobes by a deep ventral indentation. There is a single globular macronucleus, many micronuclei and, on average, 101 somatic ciliary rows and 397 adoral membranelles. M. dinoferus may be closely related to limnetic Stentor spp., but differs in two conspicuous features: (1) the cilia on the peristomial bottom are scattered (ordered rows in Stentor spp.) and (2) the paroral membrane is very short and opposite the buccal portion of the adoral zone of membranelles (in Stentor spp., it accompanies the entire membranellar zone). The cells appear dark due to stripes of cortical granules; the granules are more concentrated in a "black band" below the cap. The cortical pigment(s) is red fluorescent with a broad absorption peak in the blue (ca. 420-480 nm), and sharp peaks in the yellow-green (ca. 550 nm) and red (600 nm). Ultrastructural and molecular data demonstrate that the symbiont is a dinoflagellate of the genus Symbiodinium, the first unequivocal report of zooxanthellae in a ciliate. Phylogenetic analysis of a portion of the large subunit ribosomal RNA gene (28S rDNA) showed that the symbionts belong to Symbiodinium sp. clade C, a lineage that also inhabits many corals on Guam. The ciliate changes shape at night, and the symbionts, which are spread out in the cap during the day, are mostly withdrawn into the stalk at night; these changes were apparently not simply a response to darkness.     

The trophic status of herbivorous fishes on coral reefs

Dietary analyses of 17 species of nominally herbivorous fishes on the northern Great Barrier Reef were carried out to resolve the questions to what extent diet in nominally herbivorous reef fishes is dominated by living plant material and whether short-chain fatty acid profiles in the alimentary tract predict diet. The fishes included members of the families Acanthuridae, Scaridae and Kyphosidae. The analyses revealed consumption of a wide range of food resources including macroscopic algae, turfing algae, pelagic and planktonic animal matter, organic detritus and sediments. In only 7 of the 17 species was living algae the dominant dietary item. Gastrointestinal short-chain fatty acids (SCFA) profiles reflecting the fermentative activities of gut symbionts were used as a framework for the initial analysis of dietary patterns. The feeding patterns implied by the SCFA profiles were validated by direct observation of food material through gut content analysis. We identified four main trophic groupings differentiated by fermentation profiles and diet. These were: (1) species with low levels of SCFA in gut samples (mean mM 9.2ǂ.8) but a high proportion of isovalerate (mean 15.4%dž.7): Acanthurus nigricauda, A. olivaceus, Ctenochaetus striatus, Chlorurus microrhinos, C.sordidus, Scarus schlegeli; (2) species that fed on turfing, filamentous and small thallate algae and displayed moderate levels of SCFA (mean mM 21.9DŽ.9): Acanthurus nigricans, A.lineatus, Naso tuberosus; (3) species that fed on large thallate (macroscopic) algae with high levels of SCFA (mean mM 41.2Dž.5) with very low proportions of isovalerate (mean 0.5%ǂ.1): Kyphosus cinerascens, K.vaigiensis, Naso unicornis, Zebrasoma scopas; and (4) species with diets dominated by planktonic animal material with moderate levels of SCFA (mean mM 21.3ǃ.5): Naso annulatus, N. brevirostris, N. hexacanthus, N. vlamingii. We conclude that a wide range of dietary items are taken by the 17 nominal herbivores, and that dietary groupings do not reflect taxonomic relationships. There was evidence of convergence in feeding modes and diet between phylogenetically distinct taxa and divergence within particular lineages.