An ultrastructural study of larval settlement in the sea anemone Urticina crassicornis (Cnidaria,Actiniaria)

Laboratory-reared larvae of the sea anemone Urticina (= Tealia) crassicornis have been examined by electron microscopy prior to and following settlement on algal substrata. At 18 days postfertilization, the free-swimming planula larva measures about 600 μm long. A stomodaeal invagination occurs at the narrow end of the larva and connects with a solid mass of endoderm in the core region. The endoderm possesses septa with well-developed myonemes and is situated subjacent to a thin sheet of mesoglea. The uniformly ciliated ectoderm that constitutes the outer layer of the larva contains: (1) spirocysts, (2) nematocysts, (3) mucus, (4) three types of membrane-bound granules, (5) a basiepithelial nerve plexus, and (6) a few nongranular cells that may represent sensory neurons. Within several minutes after the introduction of the algal substratum, the planula characteristically directs its broadened aboral end toward the alga and secretes a refractile sheet of material. As the aboral end attaches to the substratum, the larva becomes noticeably shorter along its oral-aboral axis, presumably owing to the contractions of myonemes that are located within the endodermal septa. All three types of granules and the ectodermal mucoid substances are exocytosed during settlement, but spirocysts and nematocysts characteristically remain undischarged. Ovoid, PAS⁺ granules are believed to be at least partly responsible for adhesion, since these granules are concentrated at the aboral end prior to settlement and are somewhat similar in ultrastructure to putative viscid granules produced by other species. Contrary to a previous report based on light microscopy, no discrete sensory organ is evident in serial sections of the aboral ectoderm. The ability of planulae to detect suitable substrata appears to depend instead on sparsely distributed sensory cells that occur throughout the larval ectoderm.     

Embryonic and larval development of the sea anemone Haliplanella lineata from Japan

The development of Haliplanella lineata, following fertilization in the laboratory, was studied by light and electron microscopy. Spawned ova were spherical, magenta in color and about 120–150 µm in diameter. Cleavage was holoblastic and radial. Gastrulation occurred by immigration and invagination. Eighteen hours after fertilization, the embryo became a swimming planula larva with an apical organ and ciliary tuft at the aboral end. In the laboratory, planulae lived for about 2 weeks in the swimming state but in no case was there any settlement by larvae in this study. The structural study of planulae concentrated on the development of the aboral ectoderm, because of the functional significance of its cellular organization in larval settlement.     

Localization of sensory mechanisms utilized by coral planulae to detect settlement cues

Coral planulae are induced to settle and metamorphose by contact with either crustose coralline algae or marine bacterial biofilms. Larvae of two coral species, Pocillopora damicornis and Montipora capitata, which respond to different metamorphic cues, were utilized to investigate the sensory mechanisms used to detect metamorphic cues. Because the aboral pole of the coral planula is the point of attachment to the substratum, we predicted that it is also the point of detection for cues. To determine where sensory cells for cues are localized along the body, individual larvae were transversely cut into oral and aboral portions at various levels along the oral–aboral axis, and exposed to settlement‐inducing substrata. Aboral ends of M. capitata metamorphosed, while oral ends continued to swim. However, in larvae of P. damicornis, ¾ oral ends (i.e., lacking the aboral pole) were also able to metamorphose, indicating that the cells that detect cues may be distributed along the sides of the body. These cells do not correspond to FMRFamide‐immunoreactive cells that are present throughout the body. Cesium ions induced both aboral and oral ends of larvae of both species to settle, suggesting that oral ends have not lost their capacity to metamorphose, despite lacking sensory cells to detect natural cues. To determine whether sensory cells in larvae of P. damicornis are restricted to one side of the body, swimming behavior over substrata was observed in larvae labeled with diI, a red fluorescent lipophilic membrane stain. The larvae were found to rotate around the oral–aboral axis, with their surface against the substratum, not favoring a particular side for detecting cues. While clarifying the regions of the larval body important for settlement and metamorphosis in coral planulae, we conclude that significant differences between coral species may be due to differences in the distribution of sensory structures in relation to different planular sizes.     

Embryonic development and metamorphosis of the scyphozoan <Emphasis Type="Italic">Aurelia</Emphasis>

We investigated the development of Aurelia (Cnidaria, Scyphozoa) during embryogenesis and metamorphosis into a polyp, using antibody markers combined with confocal and transmission electron microscopy. Early embryos form actively proliferating coeloblastulae. Invagination is observed during gastrulation. In the planula, (1) the ectoderm is pseudostratified with densely packed nuclei arranged in a superficial and a deep stratum, (2) the aboral pole consists of elongated ectodermal cells with basally located nuclei forming an apical organ, which is previously only known from anthozoan planulae, (3) endodermal cells are large and highly vacuolated, and (4) FMRFamide-immunoreactive nerve cells are found exclusively in the ectoderm of the aboral region. During metamorphosis into a polyp, cells in the planula endoderm, but not in the ectoderm, become strongly caspase 3 immunoreactive, suggesting that the planula endoderm, in part or in its entirety, undergoes apoptosis during metamorphosis. The polyp endoderm seems to be derived from the planula ectoderm in Aurelia, implicating the occurrence of “secondary” gastrulation during early metamorphosis.     

Ammonia induces metamorphosis of the oral half of buds into polyp heads in the scyphozoan Cassiopea

Summary The scyphozoan polyp Cassiopea forms vegetative free swimming buds that metamorphose into sessile polyps. In sterile sea water metamorphosis does not take place. Buds keep swimming for weeks. Application of millimolar quantities of NH ₄ ⁺ causes the buds to metamorphose within one day. The resulting animals bear hypostome and tentacles, however, only occasionally peduncle and foot. Almost all transform either completely into solitary polyp head or only the oral half of the bud developes into a head while the aboral half remains bud tissue which becomes constricted off. Under suited conditions this small bud is able to transform into a normal shaped polyp.     

The cnidome and ultrastructural morphology of late planulae in Lophelia pertusa (Linnaeus, 1758)—With implications for settling competency

The larval pre-competency period and competency window are important in delimiting the potential dispersal distance for pelagic larvae of sessile marine fauna. Here, we provide evidence for morphological changes in the late planulae of Lophelia pertusa that have implications for their dispersal potential. Three weeks after spawning, the planulae gain functional cnidocysts, indicating that they are competent to settle at this time. Cnidae have been shown to be used for primary anchoring during settling, and before this time point, the larvae most probably do not have the ability to attach to a substrate in high flow conditions. The appearance of functional cnidae coincides with larvae gaining a flexible mouth that can be opened to the full width of the larva. The larval isorhizas differ the most from the adult polyps isorhizas, while the p- and b-mastigophores bear more resemblance to the adult homologues of similar size. The external and internal morphology of late planulae is further described with demonstration of long apical cilia and its effect on swimming agility, morphological changes of the ciliated cells in the larval mouth region and an internal nerve plexus. This study also indicates that L. pertusa planulae seek out cryptic spaces for settling.     

Abundance,feeding behaviour and nematocysts of scyphopolyps (Cnidaria) and nematocysts in their predator,the nudibranch Coryphella verrucosa (Mollusca)

The abundance of Aurelia and Cyanea scyphopolyps on Laminaria saccharinawas higher in sheltered, shallow areas compared with more exposed or deepones. Liberated planulae probably were not transported far away fromstranded and trapped jellyfish and settled nearby, preferably in patches onthe downward side of the Laminaria thallus. The principal prey for thescyphopolyps seems to be small copepods and the cladocerans Podon sp. andEvadne sp., which are abundant in surface waters during the summer.Temporarily abundant planktonic organisms, e.g., Sagitta setosa,Pleurobrachia pileus and hydromedusae might, also be important prey.Harpacticids, halacarideans and Corophium sp., whose natural habitat is onL. saccharina, were not captured by the scyphopolyps. Scyphopolyps culturedin running sea water rich in detritus and phytoplankton fill their enteronwith organic substrates, particularly diatoms. A new category ofheterotrichous microbasic rhopaloid nematocysts was identified in thescyphopolyps. These rhopaloids were earlier included within the eurytelesand were not considered to be separate nematocysts. They are distinctivefrom the euryteles due to the two swellings on their discharged shaft. Theabsence or presence of the nudibranch Coryphella verrucosa on laminarianthalli possibly has an effect on the number of scyphopolyps, as thisnudibranch consumes numerous scyphopolyps. Isorhizas and the new category ofrhopaloid nematocysts, identical to those present in the Aurelia polyps,occurred in the cnidosacs of examined C. verrucosa. The proportion ofrhopaloid nematocysts compared with a-isorhizas was noticeably higher in C.verrucosa than in scyphopolyps. The nudibranch may selectively storerhopaloids.     

On the Morphology of Actiniarian Larvae

Planktonic larvae of the Athenaria correspond in a number of morphological characteristics to plankto-trophic thenarian planulae. In edwardsiid planulae there are often, during the later part of the planktonic phase, a well-developed aboral physa-"Anlage" and six triangular tentacles. Holotrichous haplonemes are not uncommon in actiniarian planulae. This nemato-cyst type is, however, limited to the larva stage in the edwardsiid and some thenarian species. The phylogenetical connection between the edwardsiid and the planktotrophic thenarian planulae is stressed The striking morphological resemblances between madreporarian and lecithotrophic planulae of the family Actiniidae indicate a close phylogenetic connection between the two planula types.     

Lefty acts as an essential modulator of Nodal activity during sea urchin oral-aboral axis formation

Nodal is a key player in the process regulating oral–aboral axis formation in the sea urchin embryo. Expressed early within an oral organizing centre, it is required to specify both the oral and aboral ectoderm territories by driving an oral–aboral gene regulatory network. A model for oral–aboral axis specification has been proposed relying on the self activation of Nodal and the diffusion of the long-range antagonist Lefty resulting in a sharp restriction of Nodal activity within the oral field. Here, we describe the expression pattern of lefty and analyse its function in the process of secondary axis formation. lefty expression starts at the 128-cell stage immediately after that of nodal, is rapidly restricted to the presumptive oral ectoderm then shifted toward the right side after gastrulation. Consistently with previous work, neither the oral nor the aboral ectoderm are specified in embryos in which Lefty is overexpressed. Conversely, when Lefty's function is blocked, most of the ectoderm is converted into oral ectoderm through ectopic expression of nodal. Reintroducing lefty mRNA in a restricted territory of Lefty depleted embryos caused a dose-dependent effect on nodal expression. Remarkably, injection of lefty mRNA into one blastomere at the 8-cell stage in Lefty depleted embryos blocked nodal expression in the whole ectoderm consistent with the highly diffusible character of Lefty in other models. Taken together, these results demonstrate that Lefty is essential for oral–aboral axis formation and suggest that Lefty acts as a long-range inhibitor of Nodal signalling in the sea urchin embryo.     

Notes on the morphology,ecology and life cycles of Fukaurahydra anthoformis and Hataia parva (Hydrozoa,Athecata)

Fukaurahydra anthoformis and Hataia parva are solitary athecate hydroids occurring in northern Japan. New information on the external morphology, nematocysts, ecology, and life cycles of these species is presented. It is noteworthy that H. parva bears stenoteles, which are generally not found among the families of Filifera. Neither species produces free medusae. The eggs are fertilized in the female gonophores, from which unciliated larvae are released. These larvae do not swim and soon attach to a substrate. After attachment the larvae become covered by a sheath to form cysts. The cysts rest on a substrate without any outer change for several months. As the water temperature drops in autumn to early winter the cysts begin to hatch, forming tiny polyps after the larva creeps out from the chitinous sheath. Cyst formation proves to be common also in other solitary hydroids, most of which are inhabitants of cool or cold waters.     

Microscopical Anatomy of the Cyrtocrinid Cyathidium meteorensis (sive foresti) (Echinodermata,Crinoidea)

Light microscopy and scanning electron microscopy of three specimens of Cyathidium meteorensis (order Cyrtocrinida) revealed some special morphological features. The brachial articulation is provided with a long tendon at the aboral side; the entire articulation surface, including the areas where ligaments attach is built up by labyrinthic stereom. The calycinal ossicle lacks any internal vestiges of a pentameric composition; vertical planes with changing stereom direction lie irrespectively of radial or interradial planes. Gut, ovary and testis are histologically inconspicuous, and the location of the gonads inside the calyx is quite unusual. Both sexes develop outer gonoducts which probably are functionally adapted madreporic canals. The coelomic system differs from that of other crinoids in that a chambered organ is completely lacking. Simultaneously, the aboral nervous subsystem has no aboral nerve centre and simply terminates aborally in the ring-shaped commissure. A glandular axial organ is absent, as are typical sacculi. The data are in accordance with two proposals made previously for Holopus rangii, viz., that the animals can feed raptorially, and that cyrtocrinids probably have evolved by loss of aboral calycinal ossicles of ancestors. In part, our observations differ from those in Cyathidium foresti, so we have chosen to use the species name meteorensis which has been considered a synonym of foresti.     

Scanning Electron Microscope Observations on Some Life History Stages of Carchesium polypinum (Ciliata,Peritrichia)1

SYNOPSIS. Sessile zooids, and mobile telotrochs and microgamonts of Carchesium polypinum (Protozoa, Ciliata, Peritrichia), were examined by scanning electron microscopy. The results were compared to earlier light and electron microscope studies in order to investigate structural changes concerned with adaptation and differentiation. Telotrochs and microgamonts always had a contracted peristome and usually had a long phalange of cilia. Striae around the contracted buccal apparatus in all 3 stages were convoluted and often had thickened margins; those in telotrochs and microgamonts had oral-aboral ectoplasmic cross-connections. Nonbuccal striae of telotrochs and microgamonts varied in structure and height differences between epiplasmic peaks and alveoli surface membranes. The number of striae were constant in all 3 stages. Pellicular pore structure did not vary in any of the stages examined and resembled parasomal sacs located near buccal structures. Fully relaxed sessile zooids had ectoplasmic ridges coursing from polykinety kinetosomes and cilia to an area in front of the ciliated portion of the haplokinety; these ridges were interpreted to be the interkinetal fibers. Telotroch bands of sessile zooids consisted of 2 or 3 parallel ectoplasmic ridges which circled the aboral region and contained structures resembling pores. Telotroch bands in telotrochs and microgamonts had 2 enlarged, parallel ectoplasmic ridges circling the aboral region; telotroch band cilia were found between these ridges. In addition, a fold-like, ectoplasmic structure extended beyond the 2 ridges and was located between the telotroch band cilia and the aboral ridge. The epiplasmic shelf surrounding the stalk in sessile zooids was enlarged in telotrochs, and cilia were seen in the scopula depression. No scopula organelle was seen in any microgamont.     

Reproductive Cycle and Developmental Processes During Embryogenesis of Clavularia hamra (Cnidaria,Octocorallia)

Clavularia hamra Gohar, 1948 is a common octocoral on the reefs of the Gulf of Eilat (northern Red Sea). Reproductive biology of C. hamra was studied for two years. Direct observations of spawning were conducted in situ and in aquaria. Cleavage of eggs and further embryo metamorphosis into mature planulae were examined by scanning electron microscopy and histological sections. Clavularia hamra is dioecious. Young oocytes appear annually in September, gradually grow in size and attain maturity within 11 months. The main spawning event of the population is highly synchronized, occurring on a single night each year between the last quarter and the new moon in mid summer. The released orange eggs are held together by mucus and remain attached amongst the polyps on the outside of the female colonies. Twelve hours after spawning, the young embryos have blastomeres of equal size with numerous microvilli on their external surface. Due to unequal cleavage, bizarre embryos are also formed. By 48 h a blastopore is visible, indicating that a gastrula is developing. Eight days after spawning mature planulae are observed. The external mode of embryo development on the surface of the parent colony reduces dispersal of the planulae. However, this reproductive feature enhances formation of locally dense populations of C. hamra, with distinct habitat preferences.     

Hydroid nematophores: morphological,structural, and behavioural variety from old knowledge and new data

There is a rich old literature on nematothecae, which have been described from many species of nine families of thecate hydroids, but detailed knowledge of nematophores is from a very few species. Using both old data and new observations, information was gathered on species belonging to the genera Hydrodendron, Zygophylax, Antennella, Plumularia, Monotheca, Macrorhynchia, Gymnangium, and Thecocarpus. Specimens belonging to five families collected on the coral reefs of Réunion Island (SW Indian Ocean) were observed alive and videotaped to gather information on nematophores. Nematophores are classified and named here as belonging to three types. An amoeboid nematophore of the four plumularid families Kirchenpaueriidae, Halopterididae, Plumulariidae, and Aglaopheniidae usually consists of two parts, which are more separate in aglaophenids than in the others. The cnidostyle (or nematostyle) is fixed, is formed of ectoderm and endoderm, and includes a cluster of large nematocysts at its top. It is inferred to serve in defence because nematocysts inside the nematotheca can be fired when stimulated. The sarcostyle is a mobile amoeboid layer of ectoderm capable of considerable extension; those that are bilobed on top can extend in two directions at a time. The sarcostyle can emit small pseudopods on its edges that phagocytose particles at the surface of the perisarc. Its hypothesized function of cleaning the colony and providing nutrition in the form of small detrital particles and bacteria taken up by phagocytosis remains to be confirmed by experimentation. The cnidostyle-like nematophore of the Campanulinidae, Linolariidae, and Clathrozoidae is a fixed structure formed of a bilayered pedicel ending with a cluster of nematocysts able to fire while still inside the nematotheca, like the cnidostyle part of the amoeboid nematophore. It functions in defence as well. The tentacle-like nematophore of the Haleciidae and Lafoeidae, described here for the first time, is a single very extensible process provided with nematocysts that issue from the nematotheca and formed by a file of endodermal chordal cells surrounded by ectoderm. In halecids, nematocysts are not always gathered on the very top, forming what looks like a strongly capitate tentacle, as was previously thought, but can be more scattered, giving the structure a filiform appearance. This type has active movements, being able to twist and bend in all directions. Its prey capture function has not been demonstrated; a sensory function is more probable. Additional data have to be collected for more species to compare the types with more confidence, but they appear to have different origins for their considerably different morphologies, structures, and behaviour.     

Reproductive biology, development, and planula behavior in the Caribbean gorgonian Pseudopterogorgia elisabethae

Abstract. The reproductive biology, development, and planula behavior of the gorgonian Pseudopterogorgia elisabethae were studied at 2 sites in the Bahamas between 1996 and 2001. Colonies were gonochoric, and females brooded planulae on the colony surface. Gonads were observed only in colonies 18 cm high or larger. Spawning was asynchronous within and between sites but was concentrated 2–10 days after the new moons from late November through early January. Fertilized eggs developed into planulae over 1–2 days and the planulae remained attached to the surface of the female colony for an additional 2–4 days. Planulae were negatively buoyant and field observations suggest that larvae may settle within tens of meters of the maternal colony. P. elisabethae is harvested for natural products, and information on the reproduction of this commercially important species is crucial to the understanding of its population biology and to the development of management plans for the conservation of the species.     

A New Method for the Separation of Different Types of Nematocysts from Scyphozoa and Investigation of Proteinaceous Toxins Utilizing Laser Catapulting and Subsequent Mass Spectrometry

Jellyfish have an increasing impact on marine ecology. Cnidocysts bearing stinging cells afford, amongst others, prey capture and defence. Several different types of stinging capsules are found in one species and they are supposed to have specific functions, e.g. paralysing prey or adhering to it. Due to these assumed different roles of the capsules, it is suggested that toxins, which are contained in the capsules, differ in composition. Analysis of distinct types of nematocysts requires an appropriate method for the separation of the different types. Mixtures of types of nematocysts were obtained of two species of jellyfish, Aurelia aurita and Cyanea lamarckii, by maceration of the tissue. These mixtures were treated with a method called laser microdissection and pressure catapulting (LMPC). Optimized maceration methods, which were firstly introduced as a method for this purpose, in conjunction with optimized LMPC parameters lead to sufficient amounts of separated capsules of individual types for subsequent mass-spectrometric analyses. In case of A. aurita, the resulting mass spectra had some constituents in common, whereas in the overall pattern, the two distinct nematocyst types differed.     

Rediscovery and redescription of the marine peritrichous ciliate Epicarchesium abrae (Precht, 1935) nov. comb. (Protozoa, Ciliophora, Peritrichia)

The morphology, infraciliature and silverline system of the marine peritrichous ciliate, Epicarchesium abrae (Precht, 1935) nov. comb., isolated from an abalone-farming pond off the coast of Qingdao, China, are investigated. E. abrae is characterized by: size of zooid in vivo 68 μm×48 μm on average; macronucleus usually J-shaped; one dorsally-located contractile-vacuole; colony regularly dichotomously branched with 4–16 zooids; total number of transverse silverlines 62–72, from peristome to aboral ciliary wreath 41–47, from aboral ciliary wreath to scopula 21–25; outer kinety of peniculus 3 prolonged and converges with peniculus 1.     

Substratum preferences and planulae settling of two red sea alcyonaceans: Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl)

The settling behaviour and substratum preferences of the planulae of the Red Sea soft corals Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl) were examined in the laboratory. The planulae of the two species have a short pelagic phase and they tend to settle immediately upon leaving the parent colonies. Mucous secretion is used by the larvae for crawling and adhering to the substratum. They exhibit an aggregated pattern of settlement. The developing polyps are found in depressions or pits of the substratum. The planulae preferentially settle on rough substrata and avoid smooth surfaces. They search for substrata covered with an organic coating, composed of turf or crustose coralline algae. Such substrata create better conditions for larval settlement and metamorphosis. The planulae of P. f. fulvum exhibit a striking preference for upside-down attachment on undersides of the substrata, while Xenia macrospiculata utilizes both substratum faces for settlement. Light intensity seems insignificant in determining attachment sites. The findings of the experiments correspond well with the distributional patterns of juveniles of the two species as found in the natural environment. The specific requirements for settling of both species increase their chances of successful development and thus enhance their survival.     

Phellinus setifer sp. nov. andP. acontextus, two noteworthy polypores from temperate areas of Japan, with notes on their allies

Type examination ofTrametes gilvoides revealed that it is distinct from the fungus hitherto known asPhellinus gilvoides in Japan.Phellinus setifer sp. nov. is described for the Japanese materials. This species is characterized by effusedreflexed basidiocarps with strigose pileus surface, often dentate dissepiments, subulate setae, and cylindrical basidiospores.Phellinus acontextus, known only from the type material collected in Nepal, is newly reported from Japan. This species is characterized by sessile and often pendent basidiocarps with multisulcate pileus surface, very thin context, lack of setae, and dark-colored basidiospores. Cultural characters of the two species are also described.Phellinus acifer comb. nov. is proposed.Phellinus contiguus andP. ferreus are newly reported from Japan.