Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod<Emphasis Type="Italic"> Cratena peregrina</Emphasis>

The nudibranch gastropod Cratena peregrina (Opisthobranchia: Aeolidaceae), when feeding on polyps of the hydrozoan Eudendrium racemosum (Cnidaria: Anthoathecata), devours masses of small microbasic eurytele and holotrichous isorhiza nematocysts. Large proportions of these nematocysts were found undischarged in the alimentary tracts of the snails. Feeding experiments in this study tracked the fate of nematocysts as they passed through the alimentary canal to the digestive gland in the dorsal appendages, the cerata, to the cnidosacs, and finally in the faeces. In digestive cells, many structurally intact nematocysts were present in large phagosomes that remained unaffected even after 2 days fasting, and phagosomes containing nematocysts were found in the faeces. Thus, it is inferred that fusion of nematocyst-containing phagosomes with lysosomes and subsequent digestion of nematocysts is blocked. Masses of exposed, undigested and structurally intact nematocysts were discarded in the faeces. In the tips of the cerata, other nematocysts were phagocytosed by cnidophages and stored in the cnidosacs. After release, in contact with seawater, cnidosac nematocysts were able to discharge. When cnidophore tentacles of E. racemosum with only holotrichous isorhizas were fed to the snails, the isorhizas arrived in the cnidosacs about 2 h after feeding and mixed with existing small euryteles. Some cnidosacs also contained very large microbasic eurytele or large macrobasic eurytele nematocysts, possibly from Eudendrium ramosum and Eudendrium glomeratum, respectively. This indicates that the various types of nematocysts from food were all incorporated into the cnidosacs. Evidence for a selection process or digestion of a certain nematocyst type in the cnidophages was not obtained. It is concluded that a large proportion of the nematocysts ingested with the food are not digested, but are eliminated, structurally and functionally intact, via the alimentary canal and the tips of the cerata.     

Cnidosac morphology in dendronotacean and aeolidacean nudibranch molluscs: from expulsion of nematocysts to use in defense?

Nudibranchs (Mollusca, Gastropoda) feeding on tentacles and polyps of Cnidaria thereby ingest both latent and discharged nematocysts (NCs) along with the food mass. In eolid nudibranchs (Opisthobranchia, Aeolidacea), many of the undischarged NCs are transported to terminal cnidosacs in their body appendages (cerata) and incorporated as kleptocnidae for defense. In the present report, the occurrence and fate of NCs in the digestive tracts of eolids is compared with hydrozoan-feeding dendronotacean nudibranchs (Opisthobranchia, Dendronotacea), which may show more basic stages in the evolution of cnidosacs. Tomographic reconstructions of the distal tips of cerata were composed from series of semithin light microscopic sections, utilizing 3D-surface rendering software. Doto acuta (Dendronotacea, Dotidae) does not have cnidosacs; transmission electron micrographs show that the NCs are digested in lysosomes of digestive gland cells. In contrast, species of the genus Hancockia (Dendronotacea, Hancockiidae) have several small cnidosacs in each ceras; they accumulate NCs in the digestive cells, as well as in the cnidosacs. Many of the cnidosacs were found open to the exterior with NCs in the process of expulsion. These and other structural details suggest assigning a function of expelling the NCs to the Hancockia spp. cnidosacs. It is proposed that cnidosacs similar to those of Hancockia spp. provide a clue to understanding how the defensive function of eolid cnidosacs may have evolved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ingestion of a medusa (Aegina citrea) by the nematocyst-containing ctenophore Haeckelia rubra (formerly Euchlora rubra): phylogenetic implications

The rare ctenophore Haeckelia rubra (formerly Euchlora rubra) has long been known to have nematocysts rather than colloblasts in its tentacles. Five specimens were collected in the San Juan Archipelago, Washington State, USA in 1980 and 1981, and their feeding behavior was observed in the laboratory. We found that H. rubra readily eats the tentacles of a medusa, Aegina citrea, whose nematocysts (apotrichous isorhizas) are nearly identical in morphology to the nematocysts of the ctenophore. When H. rubra was offered 16 other species of hydromedusae and 1 siphonophore in the laboratory, the ctenophores showed little or no tendency to ingest these potential prey items. In addition to its routinely positive response to A. citrea, the ctenophore could be induced by manipulation and starvation to accept and ingest bits of the bodies of 4 additional species of hydromedusae and 1 siphonophore. These results, combined with the histological and rearing experiments of other investigators, leave little doubt that the nematocysts in H. rubra are not endogenous, but are kleptocnidae similar to those nematocysts retained and subsequently used by some species of nudibranchs that feed on Cnidaria. A close phylogenetic link between the Cnidaria and the Ctenophora is most unlikely.     

Discharge characteristics of nematocysts isolated from acontia ofCalliactis parasitica

Experiments were performed on specimens ofCalliactis parasitica collected near Plymouth in April 1987 and in the Mediterranean between September 1987 and April 1988. Undischarged nematocysts (basitrichous isorhizas) were separated from the acontia of by using 1M glycerol, 1M citrate, 0.5% Nonidet or 0.5% Triton X-100 as isolating agents, or by freezing the acontial tissue. The mechanisms of extrusion were not studied. The effectiveness of 50 mM thioglycolate in discharging nematocysts isolated by the above methods and suspended in Ca- and Mg-free artificial sea water (ASW) was investigated. Nematocysts extruded in glycerol were more responsive to thioglycolate than those extruded in citrate. Capsules isolated in non-ionic detergents, however, and those obtained by freezing were not discharged by thioglycolate. If capsules extruded in glycerol were suspended in ASW and this was replaced with distilled water, they did not respond to thioglycolate. The effect of distilled water was only partly reversible. It was observed that these nematocysts, unlike those ofPelagia noctiluca andAiptasia mutabilis previously investigated, did not release measurable amounts of free Ca²⁺ during discharge. It is suggested that if calcium is not involved in the mechanism of discharge of isolatedC. parasitica nematocysts then some other ionic species may stabilize the resting condition of these capsules.     

Protection of human skin against jellyfish (Cyanea capillata) stings

In tropical and subtropical regions of the world, jellyfish stings cause fatalities by means of venom injecting nematocysts. For nematocyst discharge an adequate combination of chemical and mechanical stimulation is required. In order to test whether skin care products can protect against nematocyst discharge, we tested two sunscreens and one lotion applied to pieces of live human skin and exposed them to Cyanea capillata tentacles. (Test specimens were collected in 1990 along the shore of Rømø, Denmark and in the Flensburger Förde.) The fine structure analysis of the cnidom of C. capillata showed a high grade of variation in shape and size. The basic distinctive characteristic for stomocnides and astomocnides, the terminal opening at the tubule tip, could not be found. The identification of spines at the basal tubule of atrichous isorhizas suggested that these should be characterized as basitrichous isorhizas. An association between nematocyst morphology and a special function such as penetration or entanglement was not observed. All nematocyst types penetrated unprotected skin. Parafilm (an inert material) and unprotected skin substrates served as controls. The discharged nematocysts on the skin and Parafilm surfaces were counted using scanning electron microscopy. The percentage of discharged nematocysts on test substance protected skin surfaces ranged from only 7.7 to 38.2%, compared to 100% on the unprotected control skin. In addition to this marked reduction in nematocyst discharge, the relatively few discharged nematocysts on protected skin showed malfunctions, and the injection of venom would have failed because the tubules of the nematocysts did not penetrate the skin. The results indicate a general possibility that human skin may be protected against nematocyst discharge of jellyfish with the application of sunscreen or lotion.     

Nematocyst complements of nudibranchs in the genus <Emphasis Type="Italic">Flabellina</Emphasis> in the Gulf of Maine and the effect of diet manipulations on the cnidom of <Emphasis Type="Italic">Flabellina verrucosa</Emphasis>

Aeolid nudibranchs maintain functional nematocysts, which are sequestered from the nudibranchs’ cnidarian prey and provide protection against predators. Some species exhibit extensive variation in incorporated nematocysts, while others maintain a limited number of types. This study examines the apparent diversity in uptake and patterns of nematocyst incorporation among related species. Nematocyst complements were described for four Gulf of Maine nudibranch species in the genus Flabellina exhibiting a variety of feeding strategies and prey specificities. Diet manipulations were performed to examine the response to changing nematocyst availability using a generalist consumer, Flabellina verrucosa, to assess nematocyst uptake based on diet. The flabellinid species examined exhibited significant differences in nematocyst incorporation, reflecting differences in their specificity as predators and nematocyst types available in their natural prey. The nematocyst complement of F. verrucosa was the most variable and differed among collection regions. When diet was manipulated, nematocyst uptake depended on the prey the nudibranchs consumed, but when offered a variety of prey, F. verrucosa selectively preferred nematocysts from scyphistomae. The observed variation in nematocyst uptake among species and regions probably relates to environmental disparities among populations.     

Mechanosensitivity in the model sea anemone <Emphasis Type="Italic">Nematostella vectensis</Emphasis>

Tentacles of the sea anemone, Nematostella vectensis, are covered with hair bundles. Hair bundles were deflected by water jets to test whether they are mechanoreceptors. Electrophysiological recordings confirm that deflections of hair bundles induce transients in membrane current. In a different species of anemone, hair bundle mechanoreceptors are known to change shape and responsiveness according to the activity of chemoreceptors that bind prey-derived compounds including N-acetylated sugars. In Nematostella, hair bundles significantly elongate upon exposure to NANA, an N-acetylated sugar. Based on a bioassay in which discharged nematocysts are counted in gelatin-coated test probes touched to tentacles, we find that NANA shifts vibration dependent discharge of basitrich nematocysts to lower frequencies overlapping those produced during swimming by known prey including planktonic crustaceans. Furthermore, we find for the first time that vibration detection extends at least 2.5 cm beyond the tentacle tips. Thus, Nematostella likely employs its hair bundles to detect swimming movements of nearby prey.     

The symbiotic relationship between Lychnorhiza lucerna (Scyphozoa, Rhizostomeae) and Libinia spinosa (Decapoda, Epialtidae) in the Río de la Plata (Argentina–Uruguay)

Herein, we characterize a symbiotic relationship between the scyphomedusa Lychnorhiza lucerna and the decapod crustacean Libinia spinosa in Río de la Plata, South America. Of 843 specimens of L. lucerna examined during the study, 69 (8.2 %) hosted L. spinosa within subgenital spaces. The broad spatial and temporal scale of the study, together with the large number of observations made, confirm an association between the two species. Medusae having crab associates were mature and larger than those lacking such symbionts. Adult crabs of both sexes, as well as juveniles and soft-shell individuals, were found as associates of medusae. Analysis of crab stomach contents revealed the presence of nematocysts and copepod remains. Our results suggest that medusae provide protection and possibly access to food for crabs. Benefits related to transportation were not clear and need further evaluation. Crabs of L. spinosa may acquire their scyphozoan symbionts either as larvae planktonic stages or as adult crabs attaching to jellyfish when aggregating close to bottom.     

Distribution of a population of annelid polychaetes in the “trottoir” of the midlittoral zone on the coast of North-East Spain,Western Mediterranean

The discharge and mode of action of the microbasic mastigophores (b-Rhabdoids) of Anemonia sulcata Pennant (Anthozoa) tentacles were examined using light and electron microscopical techniques. The statistical analysis of different morphological parameters of 100 nematocysts before and after their complete discharge revealed four different phases in the process of discharge. The first phase consists of the opening of the three opercular flaps. The second phase is characterized by the extrusion of the shaft. Osmotic pressure generated by the nematocyst propagates the extremely fast shaft discharge. The third phase, the evagination of the long tubule, is propagated by intrinsic mechanical forces stored in the tubule's twists and coils. During the process of evagination the tubule length increases by 95%, its circumference by 17%. Its forward motion is produced by the dilatation of its circumference and undoing of its twists. This motion is supported by the pressure which is generated by the hydration of the capsular matrix. The fourth phase consists in the extrusion of soluble capsular material at the tip of the evaginated tubule. During discharge the net volume of the nematocyst increases by 182%. This expansion is only possible if a high osmotic pressure is generated within the cyst. The extrusion of the shaft is the crucial phase in the discharge of the microbasic mastigophores, because without the puncture produced by the shaft the tubule is not able to overcome even surfaces of low viscosity media. The function of these tentacular nematocysts is the mechanical capture of prey organisms and, most probably, the introduction of unknown toxins into the latter.     

Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 1. Role in competition for space

The corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) forms aggregations that dominate patches on some coral reef flats in the Red Sea. The outcomes and mechanisms of competition for space between this corallimorpharian and other sessile organisms are poorly understood. Polyps of R. rhodostoma were observed to overgrow zoanthids, hydrozoan corals, sponges and encrusting macroalgae on a fringing reef at Eilat, northern Red Sea. R. rhodostoma polyps also damaged, and in some cases overgrew, reef-building corals in the families Poritidae, Acroporidae and Pocilloporidae, most of which form branching colonies with small polyps that are subordinate in coral competitive hierarchies. In contrast, most stony corals in the families Faviidae and Mussidae had standoff interactions with R. rhodostoma, in which they prevented the corallimorpharians from damaging them or approaching closer than 1 to 3 cm. The latter corals are ranked at the top of competitive hierarchies for Indo-Pacific corals, and they form massive colonies of large polyps which may develop aggressive organs termed sweeper tentacles. Some soft corals that exude allelopathic chemicals also avoided overgrowth by the corallimorpharians. Tentacles along the oral disk margin of R. rhodostoma polyps were swollen and bulbous during contacts with cnidarians. These bulbous marginal tentacles had significantly thicker ectoderm and a higher proportion of holotrichous nematocysts than did the normally filiform marginal tentacles of R. rhodostoma polyps. It is concluded that, on the reef flat at Eilat, this corallimorpharian damages and overgrows a variety of sessile competitors, including branching stony corals, via the application of specialised marginal tentacles filled with penetrating nematocysts. R. rhodostoma is an intermediate competitor in the aggressive hierarchy among Indo-Pacific Anthozoa, including the reef-building corals. Received: 1 July 1998 / Accepted: 24 March 1999     

Gulls and plovers: age-related differences in kleptoparasitism among black-headed gulls (Larus ridibundus)

Summary Kleptoparasitic attacks by adult and first winter (immature) black-headed gulls in flocks of lapwings (Vanellus vanellus) were compared. Attacks by immatures were less successful because they were more likely to be mistimed and detected by the target and immatures were apparently less selective in launching long distance attacks and choosing vantage points within flocks. As a result of their reduced success rate immature gulls tended to attack more often and were more likely to adopt other foraging techniques. There is some evidence that lapwings responded differentially to the arrival of adult and immature gulls. The success rate of attacks by immatures increased through the winter and immatures were correspondingly more likely to be encountered as kleptoparasites later in the season. Nevertheless success and intake rates among immatures did not increase to those among adults even by the end of the winter.     

Ultrastructural observations on prey capture and digestion in the scyphomedusa Aurelia aurita

Aurelia aurita medusae are able to catch their prey with their entire body surface. Catch efficiency in medusae caught in Kiel harbour in May 1985 was found to be highest at the tentacles and lowest at the subumbrella. Surface structures of the medusa as well as the cnidom are described by SEM observations. Microbasic heterotrichous euryteles and atrichous isorhizas were found. Discharged nematocysts on the prey's skin indicate different functions of the two types. The villi in the gastral cavity show a characteristic morphological differentiation that consists of a ciliated distal and a basal area covered by vesicles. Four types of glandular cells were identified by TEM observations. Mucous cell types preferably occur in densely ciliated areas. The presence of serous cells is restricted to the basal region of the gastral villi and gastral cavity where the extracellular predigestion takes place. The time of food passage in young medusae of A. aurita decreases from 19 h at 4°C to 4 h at 22°C.     

Nestmate recognition in three species of stenogastrine wasps (Hymenoptera,Vespidae)

The capacity to recognise a conspecific intruder was investigated in Parischnogaster jacobsoni, Liostenogaster flavolineata and L. vechti, three species of primitively social wasps of the subfamily Stenogastrinae. Results of behavioural experiments carried out in the field showed that females of all three species react pacifically if presented with female nestmates, but aggressively reject an intruder from a conspecific colony. As L. flavolineata and L. vechti both build large clusters of nests, often very close to each other, the recognition capacity among females from different nests, but in the same conspecific cluster, was also investigated. Females of both species were more aggressive towards females from a different colony in the same cluster than towards their female nestmates. Additional experiments on L. flavolineata showed that there was no difference in reaction towards females from colonies nearer or further from the tested colony but within the same cluster, nor towards females from a different cluster. The capacity to recognise an alien conspecific nest containing immature brood was investigated in P. jacobsoni. Adult females of this species, invited to land on an alien nest which had experimentally been exchanged for their own, accepted the new nest and partially destroyed the immature brood. The behaviour of the females when they land on an alien nest, however, suggests that they do recognise the nest as foreign. Acceptance of foreign nests coupled with low immature brood destruction is probably due to the high energetic costs of egg-deposition and larval rearing in stenogastrine wasps. These results suggest that nestmate recognition in these wasps is very efficient, even though they belong to the most primitive subfamily of social wasps. Received: 16 April 1996/Accepted after revision: 9 August 1996     

The effects of symbiotic state on heterotrophic feeding in the temperate sea anemone <Emphasis Type="Italic">Anthopleura elegantissima</Emphasis>

The temperate sea anemone Anthopleura elegantissima is facultatively symbiotic with unicellular algae. Symbiotic A. elegantissima can supplement heterotrophic feeding with excess photosynthate from their algal partners, while asymbiotic individuals must rely solely on heterotrophy. A. elegantissima individuals were collected from Swirl Rocks, Washington (48°25′6″ N, 122°50′58″ W) in July 2010, and prey capture and feeding characteristics were measured to determine whether asymbiotic individuals are more efficient predators. Feeding abilities were then measured again after a 3-week exposure to full sunlight or shaded conditions. Freshly collected asymbiotic anemones had larger nematocysts, but symbiotic individuals showed greater nematocyte sensitivity. Sunlight enhanced digestion and reduced cnida density in all anemones regardless of symbiotic state. Results suggest that the phototropic potential of A. elegantissima, as influenced by symbiotic condition, has little effect on heterotrophic capacity. The anemones appear to maximize heterotrophic energy input independent of the presence or identity of their algal symbionts.     

How does burrowing by the isopod <Emphasis Type="Italic">Limnoria agrostisa</Emphasis> (Crustacea: Limnoriidae) affect the leaf canopy of the southern Australian seagrass <Emphasis Type="Italic">Amphibolis griffithii</Emphasis>?

In south-western Australia, the isopod Limnoria agrostisa commonly burrows into leaf clusters and immature shoots of Amphibolis griffithii. The isopod also burrows into the sheath and rhizomes of Posidonia species. In A. griffithii, the isopod consumes new tissue within the sheath, damaging or destroying the meristem. This results in malformation of new leaves or destruction of whole leaf clusters with the potential to reduce the photosynthetic area of a shoot. The isopod has been found in all but one meadow of A. griffithii examined over 1,000 km of the Western Australian coastline. It was present throughout the year and showed little variation in abundance. Young were produced year round, but were more abundant in the summer months. Females, 3.5 mm in length or larger, produced 2–5 young that were brooded within the leaf cluster or base of an immature shoot. Within a meadow, 40–70% of shoots and 10–20% of leaf clusters were damaged by isopods. Seasonal trends were not consistent, but damage appeared to be higher in summer when isopod abundance was higher. Approximately 40% of clusters were destroyed by isopod damage. Isopods attack shoots of all ages, but damage was often located on apical clusters. There was no evidence that isopod damage initiated branching or leaf cluster formation. Estimations of clusters damaged or destroyed may be conservative, as only those clusters remaining on a shoot could be counted, and cluster loss could not be quantified. Examination of defoliated stems on upright shoots and horizontal rhizomes indicated that many were destroyed by isopods. The effect of L. agrostisa although substantial appears to be a feature of healthy seagrasses throughout southern Australia.     

Inter-colony movements,at-sea behaviour and foraging in an immature seabird: results from GPS-PPT tracking,radio-tracking and stable isotope analysis

Seabird populations contain large numbers of immatures––in some instances comprising >50% of the fully grown adults in the population. These birds are significant components of marine food webs and may contribute to compensatory recruitment and dispersal, but remain severely understudied. Here, we use GPS-PTTs, radio-tracking and analysis of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to investigate the movements and foraging ecology of immature seabirds. Our study focussed on immature northern gannets Morus bassanus aged 2–4 attending non-breeding aggregations alongside a large breeding colony. GPS-PTT tracking of five birds revealed that immatures have the ability to disperse widely during the breeding season, with some individuals potentially prospecting at other colonies. Overall, however, immatures were faithful to the colony of capture. During returns to the focal colony, immatures acted as central place foragers, conducted looping and commuting flights, and analysis of the variance in first-passage time revealed evidence of area-restricted search (ARS) behaviour. In addition, stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses indicate that immatures were isotopically segregated from breeders. Our findings provide insights into the foraging, prospecting and dispersal behaviour of immature seabirds, which may have important implications for understanding seabird ecology and conservation.     

Transient ontogenetic expression of hermaphroditic gonad morphology within the Gobiosoma group of the Neotropical seven-spined gobies (Teleostei: Gobiidae)

The Neotropical seven-spined gobies (tribe Gobiosomatini), including the Gobiosoma and Microgobius groups, constitute a speciose, monophyletic gobiid taxon. In particular, member species of the Gobiosoma group exhibit a combination of behavioral diversification and microhabitat specialization that may have played a major role in their collective rapid evolution and high rate of speciation. Functional hermaphroditism, which is common among gobiids, can promote exploitation of patchily distributed micro-niches by countering potential reductions in reproductive opportunities. However, the possible role of functional hermaphroditism in the exploitation of microhabitats and rapid speciation in the Gobiosoma group is unknown. One member species, Tigrigobius multifasciatus, is a functional hermaphrodite. Two other Gobiosoma group species are functionally gonochoric (i.e., constant-sexed), but exhibit transient hermaphroditic ovarian structure among immatures. In this study, ovarian morphology among immature and adult females of several Gobiosoma group species was examined to see if hermaphroditic gonadal features are present. Although no evidence of functional hermaphroditism in the form of precursive accessory gonadal structures (pAGS) associated with the adult ovary was found among newly examined species, all species exhibited the transient expression of hermaphroditic gonadal features associated with the immature ovary. In contrast, among six species of non-Gobiosomatini genera having no record of hermaphroditism, none exhibited similar transient hermaphroditic features associated with the immature ovary. These findings suggest that hermaphroditism may have been an ancestral trait which has been secondarily lost within some Gobiosoma group species. This study also shows that ontogenetic and morphogenic processes within the gobiid reproductive system may provide new insights into the evolution of life history traits and significantly further our understanding of the extraordinary evolutionary success of this group.     

Regulation of spirocyst discharge in the model sea anemone, Nematostella vectensis

Test probes were touched to tentacles to investigate whether discharge of spirocysts likely is regulated by hair bundle mechanoreceptors. Significantly more spirocysts discharge onto test probes in the presence of vibrations at 11–15 Hz as compared to 0 Hz. Adding N-acetylneuraminic acid, NANA, shifts maximal discharge of spirocysts upwards to 36–40 Hz, and possibly to 21–25 Hz. In contrast, NANA shifts maximal discharge of basitrichous isorhiza nematocysts downwards to 1–20 Hz. Thus, discharge of cnidae (‘stinging capsules’) is differentially regulated according to the type of cnida. Furthermore, it appears that chemodetection of N-acetylated sugars is not a prerequisite to capturing prey because, in seawater alone, maximal discharge of cnidae occurs at frequencies overlapping movements of calmly swimming prey. Nevertheless, chemodetection of N-acetylated sugars broadens the range of frequencies stimulating maximal discharge of cnidae and, therefore, likely enhances prey capture.     

Population structure and reproductive patterns of the NW Mediterranean deep-sea macrourid Trachyrincus scabrus (Rafinesque, 1810)

Macrourid fish are one of the most abundant marine species on continental margins worldwide. Although they play an important role in the ecosystem, little is known about their overall biology. We report here a large data set of the most abundant macrourid in Mediterranean waters, Trachyrincus scabrus, showing the main population and reproductive characteristics. The study was based on 3,239 specimens collected between 300 and 1,500 m depth on the northwestern Mediterranean in 2003–2004 and 2008–2009. The population showed a depth-related structure with the largest individuals at 1,100 m depth and the smallest (i.e., immature) at shallower depths. Macroscopic and microscopic analyses of the gonads showed that T. scabrus has a highly seasonal reproductive pattern. Spawning females were found during winter when the organic matter fluxes were highest on the continental slope. T. scabrus is a batch spawner with group-synchronous oocyte development and present low average fecundity of 14,191 oocytes.     

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. Received: 15 July 1998 / Accepted: 24 March 1999