Red Sea Coral Reefs Research Articles

Porifera, one new species Suberea purpureaflava n. sp. (Demospongiae, Verongida, Aplysinellidae) from northern Red Sea coral reefs, with short descriptions of Red Sea Verongida and known Suberea species

Porifera, one new species Suberea purpureaflava n. sp. (Demospongiae, Verongida, Aplysinellidae) from northern Red Sea coral reefs, with short descriptions of Red Sea Verongida and known Suberea species

Maternal-larval population genetic traits in Stylophora pistillata, a hermaphroditic brooding coral species

Aspects of maternal-planula larval genetics in the monoecious scleractinian coral Stylophora pistillata (Red Sea, Eilat) were studied by amplified fragment length polymorphism (AFLP) methodology in two successive reproductive seasons. In total, 293 planulae and 10 adult colonies were analyzed. In June 2006, 147 planulae were collected from 10 shallow water colonies. In March, April and June 2007, 146 additional planulae were sampled from five of the ten 2006 sampled colonies. All AFLP products showed unalike band profiles indicating a fully sexual production pattern. We used 181 and 210 putative AFLP loci, of which the overall level of polymorphism in 2006 was 92 and 99% in 2007 (respectively). Differences were also observed between 2006 and 2007 reproductive seasons in terms of total average gene diversity (0.191 vs. 0.247, respectively), suggesting fast turnover of sperm donor genotypes. In addition, increased numbers of potential sperm donor colonies in the vicinity of gravid females showed no impact on genetic differentiation levels in released larvae. UPGMA tree revealed clustering of maternal genotypes and their offspring, suggesting, as expected, high relatedness between planulae and their mothers. In addition, the average heterozygosity of each group of siblings was persistently lower than heterozygosity calculated for the respective maternal colony, suggesting the possibility of partial inbreeding. This trend of reduced genetic heterogeneity in Stylophora pistillata is an alarming sign for populations residing in the northern Red Sea coral reefs.

KAUST launches offshore research station to study Red Sea coral reefs

KAUST launches offshore research station to study Red Sea coral reefs

Porifera, one new species Suberea purpureaflava n. sp. (Demospongiae, Verongida, Aplysinellidae) from northern Red Sea coral reefs, with short descriptions of Red Sea Verongida and known Suberea species

A new verongid sponge (Suberea purpureaflava n. sp.) is described from Dahab, Gulf of Aqaba, in the northern Red Sea. It has a pronounced colour change (a dark red ectosome with whitish pore sieves and a light yellow choanosome in the living specimen changes to a uniform dark violet in the fixed state) and rather rare dendritic fibres with pith and bark and a diameter of about 110–165µm, with the pith occupying 80–90 % of the fibre. Especially the pore sieves were very striking in situ. The new species is compared to all verongid sponges so far recorded from the Red Sea and to all known and accepted Suberea species worldwide. An identification key to all known Suberea species is given. This new species record brings the number of Suberea species described to a total of 11.

Ammonia flux, physiological parameters, and Symbiodinium diversity in the anemonefish symbiosis on Red Sea coral reefs

Despite the ecological importance of anemonefish symbioses, little is known about how nutritional contributions from anemonefish interact with sea anemone physiology and Symbiodinium (endosymbiotic dinoflagellate) genetic identity under field conditions. On Red Sea coral reefs, we measured variation in ammonia concentrations near anemones, excretion rates of anemonefish, physiological parameters of anemones and Symbiodinium, and genetic identity of Symbiodinium within anemones. Ammonia concentrations among anemone tentacles were up to 49% above background levels, and anemonefish excreted ammonia significantly more rapidly after diurnal feeding than they did after nocturnal rest, similar to their excretion patterns under laboratory conditions. Levels of 4 physiological parameters (anemone protein content, and Symbiodinium abundance, chlorophyll a concentration, and division rate) were similar to those known for laboratory-cultured anemones, and in the field did not depend on the number of anemonefish per anemone or depth below sea surface. Symbiodinium abundance varied significantly with irradiance in the shaded reef microhabitats occupied by anemones. Most anemones at all depths harbored a novel Symbiodinium 18S rDNA variant within internal transcribed spacer region 2 (ITS2) type C1, while the rest hosted known ITS2 type C1. Association with Symbiodinium Clade C is consistent with the symbiotic pattern of these anemones on other Indo-Pacific reefs, but the C1 variant of Symbiodinium identified here has not been described previously. We conclude that in the field, anemonefish excrete ammonia at rapid rates that correlate with elevated concentrations among host anemone tentacles. Limited natural variation in anemonefish abundance may contribute to consistently high levels of physiological parameters in both anemones and Symbiodinium, in contrast to laboratory manipulations where removal of fish causes anemones to shrink and Symbiodinium to become less abundant.

Daytime broadcast spawning of Pocillopora verrucosa on coral reefs of the central Red Sea

[Extract] Pocillopora verrucosa (Ellis & Solander 1786) was originally described as a brooder although most subsequent work suggests that it is a broadcast spawner like all other Pocillopora described to date with the exception of P. damicornis (Baird et al. 2009). Although P. verrucosa colonies from Okinawa, Japan, have been observed to spawn in containers supplied with unfiltered water (Kinzie III 1993; Hirose et al. 2000) around the new moon in the early morning, the reproductive behaviour of the species has yet to be described in the field. During a survey to observe reproductive synchrony in scleractinian corals on the morning of the 31st May 2011, one day before the new moon, on the reefs of Thuwal, Central Red Sea (22°13.442'N, 38°58.169'E), over 20 colonies of P. verrucosa were observed to release gametes between 8:40am (3h after sunrise) and 9:15am.

Enhanced pore-water nutrient fluxes by the upside-down jellyfish Cassiopea sp. in a Red Sea coral reef

The common circum-tropical jellyfish Cassiopea sp., unlike other members of the Rhi- zostomidae (Scyphozoa), exhibits a primarily benthic life. The peculiar orientation of its exumbrella against the sediment is believed to be associated with its mainly autotrophic nutrition, i.e. exposing its zooxanthellae-bearing photosynthetic oral appendages to the sunlight. Here we show that the jel- lyfish also acts as a nutrient pump, drawing nutrient-rich pore waters from the permeable sediments. Depletion of pore-water ammonium in situ, light-enhanced ammonium uptake, and high rates of photosynthesis determined via oxygen flux measurements and underwater fluorometer analysis (rapid light curves) show that Cassiopea sp. effectively harnesses pore-water nutrients. At high den- sities Cassiopea sp. may facilitate benthic-pelagic coupling and primary production in oligotrophic coral reefs.

Organic matter release by Red Sea coral reef organisms—potential effects on microbial activity and in situ O2 availability

This study presents a comprehensive dataset (223 reef organisms that were separately in- cubated during 44 independent experiments during 4 seasonal expeditions) of dissolved and particulate organic matter (DOM and POM) release by dominant benthic organisms from the Northern Red Sea. Reef organisms studied were scleractinian and fire corals, the upside-down jellyfish and reef-associated algae. Subsequently, the effect of this organic matter (OM) release on microbial activity was determined. These studies were complemented by high resolution, in situ O2 concentration measurements within reef environments that were dominated by corals or algae. Dissolved organic carbon (DOC) release was 14.5 ± 2.3 mg m -2 surface area h -1 for all 9 investigated reef algae, which was significantly higher than DOC release by scleractinian corals during all seasons except winter. POM release (particulate organic carbon and nitrogen, POC and PON, respectively) was observed for all investigated reef organisms. Ben- thic reef algae released 5.1 ± 0.5 mg POC m -2 h -1 and 0.35 ± 0.03 mg PON m -2 h -1 , which are significantly higher than POM release rates by scleractinian corals in spring and autumn. Algae-derived OM, pre- sumably the DOC fraction, stimulated microbial activity in the adjacent water more significantly than OM released by the investigated scleractinian and fire corals. Consequently, the daily mean and min- imum in situ O2 concentrations in the water directly above the reef (≤10 cm) were significantly higher in coral dominated than in algae dominated sites, confirming the in situ relevance of results of previous lab- oratory studies. Findings also suggest that benthic reef algae decrease O2 availability in waters close to reef environments via the release of labile OM and its subsequent fast microbial degradation.

Oxygen consumption by a coral reef sponge

Oxygen consumption of the Red Sea coral reef sponge Negombata magnifica was measured using both incubation and steady-state methods. The latter method was found to be the more reliable because sponge activity remained stable over time. Oxygen consumption rate was measured during three levels of sponge activity: full activity, reduced activity and basal activity (starved). It was found that the active oxygen consumption rate of N. magnifica averaged 37.3+/-4.6 nmol O2 min(-1) g(-1) wet mass, which is within the upper range reported for other tropical marine sponges. Fully active N. magnifica individuals consumed an average of 41.8+/-3.2 nmol O2 min(-1) g(-1) wet mass. The mean basal respiration rate was 20.2+/-1.2 nmol O2 min(-1) g(-1) wet mass, which is 51.6+/-2.5% of the active respiration rate. Therefore, the oxygen used for water pumping was calculated to be at most 10.6+/-1.8 nmol O2 min(-1) g(-1) wet mass, which is 25.1+/-3.6% of the total respiration. Combined oxygen used for maintenance and water pumping activity was calculated to be 30.8 nmol O2 min(-1) g(-1) wet mass, which is approximately 74% of the sponge's total oxygen requirement. The remaining oxygen is directed to other physiological activities, mainly the energy requirement of growth. These findings suggest that only a relatively minor amount of energy is potentially available for growth, and thus might be a factor in controlling the growth rate of N. magnifica in oligotrophic coral reefs.

Assessment of Committed Effective Dose due to consumption of Red Sea coral reef fishes collected from the local market (Sudan)

An assessment of Committed Effective Dose (CED) due to consumption of Red Sea fish containing 210Po and 137Cs was performed for 23 different marine fish samples collected from the local market at Port Sudan. The fish were classified according to their feeding habits into three categories: carnivores, herbivores, and omnivores. Measured activity concentrations of 210Po were found in the ranges 0.25–6.42 (carnivores), 0.7–5 (omnivores) and 1.5–3.8 (herbivores) Bq/kg fresh weight. In the same study, activity concentrations of Cs-137 were determined to be in the ranges 0.1–0.46 (carnivores), 0.09–0.35 (omnivores) and 0.09–0.32 (herbivores) Bq/kg fresh weight, which were several times lower than those of 210Po. Appropriate conversion factors were used to derive the CED, which was found to be 0.012, 0.01 and 0.01 (µSv/yr) in carnivores, omnivores and herbivores, respectively, for 137Cs. This contributes about 0.4% of the total dose exclusively by ingestion of fish. For 210Po, it was found to be 3.47, 4.81 and 4.14 (µSv/yr) in carnivores, omnivores and herbivores, respectively, which represents 99.6% of the total dose (exclusively by ingestion of fish). The results of CED calculations suggest that the dose received by the Sudanese population from the consumption of marine fish is rather small and that the contribution of 137Cs is negligible compared to 210Po.

Reef walking on Red Sea reef flats – Quantifying impacts and identifying motives

Reef walking on the shallow reef flats of fringing reefs is an activity commonly observed on Red Sea coral reefs. Of the 137 in-water observations of reef walkers made at Ras Um Sidd, Sharm El Sheikh, South Sinai, 12.4% caused coral damage. Eighteen incidents of coral damage were recorded, 44.4% being breakages of branching corals, 39.8% abrasions of various growth forms and 16.7% detachments of soft coral colonies. Sediment was stirred-up on 31.4% of observations. Tramplers broke 0.1 branches per minute and raised 1.5 sediment clouds, thereby causing more damage than reported from divers. Damage caused depended on coral community structure and past trampling history, with communities dominated by fragile branching corals being most susceptible to mechanical damage and communities heavily trampled in the past being less susceptible. Given that main motives for reef walking included crossing the reef flat, adjusting gear or communicating, management interventions, such as the construction of a walkway combined with a public awareness campaign, could reduce the numbers of reef walkers by up to 39%.

Possible effects of downwelling on the recruitment of coral reef fishes to the Eilat (Red Sea) coral reefs

We compared water current measurements with data on the recruitment of reef fishes to a coral reef in Eilat, Red Sea for two consecutive recruitment seasons. There was a clear correlation between the daily number of recruits and the magnitude of the downwelling flow, suggesting that an increase in offshore (near bottom) flow induces higher recruitment. The higher recruitment may be the result of larval swimming against the flow. Although the exact mechanism explaining this correlation is unclear, it may involve odor transport from the reef by the downwelling currents, which reaches competent larvae in deeper water and directs them to potential settlement sites.

How to influence environmental decision makers? The case of Eilat (Red Sea) coral reefs

In a recent overview of the literature, Rinkevich (2005b), hereafter ‘Rinkevich’, aims to critically examine some of the paradigms concerning the degradation of Eilat's coral reef. He selects 5 of the major challenging outcomes/points he identifies, to motivate critical evaluation of the published literature and future research activities. He further states that his synopsis may also serve as an important application tool for managers and policy makers (p.185). In view of the widely publicized and intense ongoing (7–8 years) scientific and public debate on the possible connection between intensive fish farms' activity in Eilat and the state of the reef (see Erez et al., 2000; Genin, 2000; Gordin, 2000; Lazar et al., 2000; Atkinson et al., 2001; Genin et al., 2001; Bongiorni et al., 2003; Loya and Kramarsky-Winter, 2003; Rinkevich et al., 2003; Wielgus et al., 2003, 2004; Loya, 2004a; Atkinson et al., 2004; Loya et al., 2004, 2005; Abelson et al., 2005; Rinkevich, 2005a; Israel Ministry of Environmental Protection, 2006), Rinkevich's synopsis might prove to be critical for the possibility of legal measures that may (or may not) be undertaken, in the near future, with respect to the fish farm industry. Thus, the fate of the fish

Endoscopic exploration of Red Sea coral reefs reveals dense populations of cavity-dwelling sponges.

Framework cavities are the largest but least explored coral reef habitat. Previous dive studies of caverns, spaces below plate corals, rubble and artificial cavities suggest that cavity-dwelling (coelobite) filter-feeders are important in the trophodynamics of reefs. Quantitative community data are lacking, however, as the bulk of the narrow crevices interlacing the reef framework are inaccessible to conventional analysis methods. Here we have developed endoscopic techniques to explore Red Sea framework crevices up to 4 m into the carbonate rock, revealing a large internal surface (2.5-7.4 m2 per projected m2 reef) dominated by encrusting filter-feeders. Sponges alone provided up to 60% of coelobite cover, outweighing epi-reefal filter-feeder biomass by two orders of magnitude. Coelobite community filtration removed more than 60% of the phytoplankton in the course of its less than 5-minute passage through the crevices, corresponding to an uptake of roughly 0.9 g carbon m-2 d-1. Mineralization of the largely allochthonous organic material is a principal source of nutrients supporting coral and algal growth. The supply of new material by coelobites may provide a key to understanding the 'coral reef paradox'-a rich ecosystem thriving in nutrient-poor water.

REDESCRIPTION OF PHYLLIDIOPSIS SINAIENSIS (YONOW, 1988) (NUDIBRANCHIA: DORIDOIDEA: PHYLLIDIIDAE), WITH A REVIEW OF THE RED SEA PHYLLIDIIDAE

Phyllidiopsis sinaiensis (Yonow, 1988), of which only one single preserved specimen had been known, was rediscovered in the northern Red Sea. The dorsal colour-pattern of living individuals is presented for the first time, and the anatomy of P. sinaiensis is described in detail. Major features of the holotype previously used to characterise the species are shown to be artifacts. Principal distinguishing characters of P. sinaiensis include the central tubercle ridge, large and multicompound, broad based, pink tubercles, bicoloured black and pink rhinophores, the very elongate muscular oesophageal segment, the distinct stomach and the extremely long, convoluted prostate that is bound together by connective tissue. In some specimens, the bases of the tubercles may fuse leading to a predominantely pink dorsum with only a few black lines. Phyllidiopsis sinaiensis appears to be among the most common nudibranchs in the Gulf of Aqaba. The fact that it has not been reported since its original collection in 1980 can be explained by being confused with the externally similar, well-known species Phyllidiella pustulosa (Cuvier, 1804). A review of the phyllidiid fauna of the Red Sea reveals eleven valid species that can all be identified with the key presented herein. With four out of eleven species, phyllidiid endemism (36%) is high in the Red Sea. This is in contrast to recent palaeobiological hypotheses of a post-glacial origin of the Red Sea coral reef fauna.

Marginal tentacles of the corallimorpharian Rhodactis rhodostoma . 2. Induced development and long-term effects on coral competitors

Polyps of the corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) form aggregations that monopolise patches of space on the shallow reef flats of some Red Sea coral reefs. Some of these polyps bear specialised bulbous marginal tentacles (BMTs) where they contact cnidarian competitors. BMTs differ from the normally filiform marginal tentacles (FMTs) of R. rhodostoma, and appear to develop from them. However, their morphogenesis and long-term impacts on spatial competition with reef corals are unknown. We experimentally induced contacts between R. rhodostoma polyps and colonies of the branching stony coral Acropora eurystoma on a shallow coral reef at Eilat, northern Red Sea. During the first 24 d of contact, the A. eurystoma colonies extruded mesenterial filaments that damaged the tissues of the corallimorpharian polyps. After 18 d,>90% of R. rhodostoma individuals had developed BMTs, which resulted in a reversal in the direction of competitive damage. During the subsequent 1.5 years of observation, the corallimorpharians maintained well-developed BMTs, unilaterally damaged the tissues of A. eurystoma, and in some cases moved onto the stony coral skeletons and partially overgrew them. BMTs developed from FMTs in a series of four distinct stages, accompanied by significant changes in their morphology, cnidom, and density of nematocysts. Isolated control polyps did not develop BMTs or show any signs of damage. In contrast, corallimorpharian polyps transplanted into contact with colonies of the massive stony coral Platygyra daedalea began to develop sporadic BMTs, but were unilaterally and severely damaged by the corals, and started to disappear within 21 d, after the corals developed sweeper tentacles. We conclude that long-term outcomes of competition between R. rhodostoma and reef-building corals depend largely on the relative aggressive reach of the competitive mechanisms developed by each species. As a consequence, this corallimorpharian is an intermediate competitor in the aggressive hierarchy among Indo-Pacific reef corals. This study confirms that R. rhodostoma polyps may actively damage and overgrow some stony corals, leading to the formation of an almost continuous blanket of polyps in large patches of some shallow reef flats.

Sponge-inhabiting barnacles on Red Sea coral reefs

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

Endobionts of the Coral Reef Sponge Theonella swinhoei (Porifera, Demospongiae)

Endobionts associated with the coral reef sponge Theonella swinhoei (Lithistida, Demospongiae), from the Red Sea and Indian Ocean, were characterized through histological and ultrastructural investigations. Prokaryotes included unicellular heterotrophic bacteria (mean concentration, 20% of the sponge tissue), unicellular cyanobacteria (Aphanocapsa feldmanni, 15% of the sponge tissue), and non-photosynthetic filamentous forms, probably in the Beggiatoa-group (sulfur bacteria, family Beggiatoaceae, 40% of the sponge tissue). Evidence of ingestion of small pieces of filamentous bacteria by sponge cells was observed. The worm Haplosyllis spongicola (Syllidae, Polychaeta) inhabited the canals of the sponge aquiferous system (mean density, 73 worms/cm3). The abundance and distribution of the endobionts living in T. swinhoei from different localities are presented and discussed in comparison with those of other sponge species reported in literature. Additional key words: polychaetes, filamentous bacteria, cyanobacteria Sponges present an astonishing landscape of associations with various kinds of autotrophic and heterotrophic organisms (Pearse 1932; Pansini 1970; Ilan et al. 1994). The various parts of a sponge, especially the endosome, contain large populations of extracellular bacteria (Vacelet 1975; Vacelet & Donadey 1977), which may greatly differ from the bacterial population in the water surrounding the same sponge (Wilkinson 1978b). The presence of cyanobacteria or other photosynthetic organisms is less frequent. Even though the autotrophic organisms inhabit the more superficial layers in order to fulfill their metabolic needs, some peculiar localizations inside the skeletal fibers (Riitzler 1990) and along the siliceous spicule bundles (Gaino & Sara 1994) have been observed. In this last condition, siliceous spicules, acting as a light-conducting system (Cattaneo-Vietti et al. 1996), enhance photosynthesis. Since the first observations of Feldmann (1933), many ultrastructural studies have stressed the presence of cyanobacteria in sponges of both Mediterranean and tropical origin (Sara 1971; Vacelet 1971; Vacelet & Donadey 1977; Wilkinson 1978a,c; Rutzler 1990). In some cases symbionts are so tightly associated with the sponge tissues as to constitute a symbiocortex (Simpson 1984, pp. 114-123). Cyanobacteria protect a Author for correspondence. E-mail: zoologia@unige.it sponges from excessive solar radiation (Sara & Vacelet 1973; Wilkinson 1980) and variations in the photosynthetic symbiont concentration may effect changes in the color of the sponge surface (Gaino et al. 1977). Direct feeding on cyanobacteria has been frequently suggested but observed only for unicellular types (Sara 1966; Wilkinson 1980; Berthold et al. 1982; Rutzler 1990). The translocation of glycerol, fixed nitrogen, and other photosynthetically fixed nutrients represents a trophic advantage for the sponge host (Wilkinson 1979; Wilkinson & Fay 1979), mainly in coral reefs and other tropical environments where the low levels of available nutrients favor these associations (Wilkinson 1987). So far, only two species of unicellular cyanobacteria have been frequently described in association with sponges: Aphanocapsa feldmanni and A. raspaigellae (Sara 1966; Gaino et al. 1976; Wilkinson 1980; Rutzler 1990). A few reports treat filamentous multicellular cyanobacteria, differing in dimension and shape, most commonly Oscillatoria spongeliae (Sara 1966) and Phormidium spongeliae (Wilkinson 1980). Nonetheless, Bewley et al. (1996), in ultrastructural observations, reported the lack of a photosynthetic system in some prokaryotic filaments, previously attributed to the above mentioned cyanobacteria. In addition, sponges serve as hosts to a number of other heterotrophic organisms, providing food and shelter to species from many taxa. Polychaetes have This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:41:51 UTC All use subject to http://about.jstor.org/terms Magnino, Sara, Lancioni, & Gaino been commonly observed in sponges (Pearse 1950; Pawlik 1983; Koukouras et al. 1996; Martin 1996; Rutzler, 1997). In particular, the polychaete Haplosyllis spongicola (GRUBE 1855), family Syllidae, is an abundant inhabitant of sponges. Fauchald & Jumars (1979) proposed that this worm grazes on the sponge surface. A more intimate relationship with its host has been directly observed by Tsurumi & Reiswig (1997), who reported that H. spongicola feeds on the tissues surrounding the sponge water canals, within which it lives. It is often difficult to establish the correct relationship between sponges and their endobionts. The sponge Theonella swinhoei GRAY 1868 (order Lithistida) is a remarkable component of the sponge populations in coral reef communities. Heterospecific associations, representing many types of adaptive interactions, are very common in this peculiar environment (Wilkinson 1987; Castro 1988). Previous studies on T. swinhoei from the Red Sea showed the unicellular symbionts A. feldmanni associated with sponge tissues (Wilkinson 1979, 1980; Bewley et al. 1996). Here we document that specimens of T. swinhoei, from both Red Sea and Indian Ocean coral reefs, constitute a further example of how sponges may represent a suitable micro-environment for the development of various kinds of autotrophic and heterotrophic organisms, which inhabit different parts of the sponge body.

Echinoid Bioerosion as a Major Structuring Force of Red Sea Coral Reefs.

Echinoid bioerosion is ecologically important as a limiting factor for reef growth. Diadema setosum and Echinometra mathaei are the most abundant sea urchins in the Gulf of Eilat, Red Sea. Bioerosion by these urchins was estimated from field experiments with urchins placed in underwater chambers. A novel methodology was developed for measuring the CaCO3 content of fecal pellets and total gut contents. This method is based on measurement of the amount of gas formed by total acidification of a mixed sample. The population density of the urchins was determined for the reef flat (depth 0.6-1 m) and the reef slope (depth 6-8 m). D. setosum, which erodes 310 mg individual-1 d-1, is much less abundant on the reef flat (0.1 individuals m-2) than on the reef slope (6.4 individuals m-2). In contrast, E. mathaei, which erodes 120 mg individual-1 d-1, is more dominant on the reef flat (10.5 individuals m-2) than on the reef slope (3.7 individuals m--2). We estimate that echinoid bioerosion converts to carbonate sediments 7%-11% of the total reef flat calcification and 13%-22% of the total reef slope calcification. These findings emphasize the importance of echinoid bioerosion as a structuring force in the coral reef, and its potential importance to the dynamics of reef development. However, they imply that some previous studies may have overestimated the role of echinoid bioerosion, thus possibly underestimating internal bioerosion.

Cohabitation of a coral reef sponge and a colonial scyphozoan

Samples from populations of a common Red Sea coral reef sponge, Mycale fistulifera, contained 1.7 to 25% sponges associated with polyps of Nausithoe cf. punctata Kolliker, 1853 (Scyphozoa: order Coronata). In the northern Red Sea this colonial scyphozoan was found exclusively in symbiosis with sponges. The proportion of the symbiotic sponges within the population differed significantly between seasons. Polyps were found significantly more frequently on large (>30 cm2) sponges than on small ( 0.005), while Diadema setosum fed equally or less (P>0.05) on symbiotic compared with nonsymbiotic sponges. Echinotrix colamaris fed on both symbiotic and nonsymbiotic sponges. This mutual symbiosis seems to be facultative for the sponge and obligatory for the scyphozoan. Some of the demonstrated benefits of this symbiosis are: the sponge enhanced survival, size and relative protection; and the absence of free-living scyphozoans indicates the necessity of the association for the survival of N. cf. punctata.