Cnidae in the sea anemone Sagartiogeton viduatus (Muller, 1776) (Cnidaria,Anthozoa); A comparison to cnidae in the sea anemone Metridium senile (Linnaeus, 1761) (Cnidaria,Anthozoa)

The cnidom of the sea anemone Sagartiogeton viduatus (Muller, 1776) is described from interference‐contrast light micrographs (LMs) and scanning electron micrographs (SEMs). Special attention is given to nematocyst maturation, including the differentiation of the shaft into proximal and main regions as helical folding of the shaft wall proceeds. Comparisons are made with Metridium senile (Linnaeus, 1761), whose cnidom, with a few exceptions, is closely similar to that of S. viduatus. The two anemones possess b‐ and p‐mastigophores, p‐amastigophores, isorhizas and spirocysts. Although the majority of cnidae in S. viduatus is smaller than corresponding ones in M. senile, they are grouped into the same size classes as those of M. senile, namely small, medium and large. The main differences from M. senile cnidae are the followings: (1) Large acontia p‐amastigophores are the largest nematocysts in S. viduatus. (2) They are noticeably larger than the large acontia b‐mastigophores, and (3) they are separated from the p‐amastigophores of M. senile by the sinusoid pattern of their U‐shaped capsular matrix. (4) The large acontia b‐mastigophores are microbasic and not mesobasic as in M. Senile, and (5) they do not produce darts. (6) Another difference from M. senile is the absence of catch‐tentacle isorhizas.     

New family of sea anemones (Actiniaria,Acontiaria) from deep polar seas

We describe and illustrate two new species from polar deep seas that belong to a new genus and family. Antipodactidae fam. nov. is characterized by acontia with macrobasic p-amastigophores; this type of nematocyst has never been reported from acontia. Antipodactis gen. nov. is characterized by a column with a distinct scapus and scapulus, cuticle-bearing tenaculi on the scapus, more mesenteries proximally than distally, mesenteries regularly arranged, restricted and reniform retractor musculature, and macrobasic p-amastigophores in the acontia. Antipodactis scotiae sp. nov. and A. awii sp. nov. differ in number of mesenteries, retractor and parietobasilar muscles, cnidae, and geographic distribution. We discuss the familial and generic characters of Antipodactis gen. nov. and its relationship to other families of acontiarian sea anemones: it most closely resembles members of Kadosactidae in terms of anatomy and some aspects of cnidom, and has a cnidom identical to that of Diadumenidae in terms of the types of nematocysts. Because the morphology of nematocysts is critical to the diagnosis of this family, we review and comment on the nomenclature of mastigophores. The macrobasic p-amastigophores of Antipodactidae fam. nov. conform to England’s (Hydrobiologia 216/217:691–697, 1991) definition rather than that of Mariscal (Coelenterate Biology. Academic Press, New York, pp 129–178, 1974).     

Comparative ultrastructure of catch tentacles and feeding tentacles in the sea anemone Haliplanella

TEM observations of catch tentacles revealed that the tentacle tip epidermis is filled with two size classes of mature holotrich nematocysts and a gland cell filled with electron-dense vesicles. Vesicle production is restricted to upper-middle and tentacle tip regions, whereas holotrich development occurs in the lower-middle and tentacle base regions. Thus, catch tentacles have a maturity gradient along their length, with mature tissues concentrated at the tentacle tip. Occasional feeding tentacle cnidae (microbasic p-mastigophores and basitrichs) and mucus gland cells occur in proximal portions of catch tentacles, but are phagocytized by amoeboid granulocytes and transported to the gastrodermis for further degradation. No feeding tentacle cnidae or mucus cells occur distally in catch tentacles. Unlike catch tentacles, feeding tentacles are homogeneous in structure along their length with enidocytes containing mature spirocysts, microbasic p-mastigophore or basitrich nematocysts distributed along the epithelial surface. Cnidoblasts are recessed beneath cnidocytes, occurring along the nerve plexus. Mucus gland cells and gland cells filled with electron-dense vesicles are present in feeding tentacles, distributed at the epithelial surface. Granular phagocytes are rare in the feeding tentacle tip, but common in the tentacle base.     

The influence of feeding on behavior and nematocyst discharge of the sea anemone Calliactis tricolor

A decrease in basitrichous isorhiza and spirocyst nematocysts is observed in a fully fed sea anemone, Calliactis tricolor, as compared to unfed animals. Discharge of tentacle nematocysts of an intact living animal is inhibited by anesthetics and a decrease in temperature. The chemical (food origin) threshold for discharge of basitrichs is higher than for spirocysts and the implications of these results are discussed. The most probable cause for the observed decrease in nematocyst discharge is a combination of food present in the gastrovascular cavity and a physical stretching of the cavity.     

Morphology,distribution, and evolution of apical structure of nematocysts in hexacorallia

Cnidae are complex intracellular capsules made by all cnidarians. The most diverse of these capsules are nematocysts, which are made by all members of the phylum; spirocysts and ptychocysts are made only by members of some lineages, and they show less functional and structural diversity. In nematocysts, the apex has been shown to be either a hinged cap (operculum) or three flaps that flex outward during discharge. The operculum is known only from medusozoan nematocysts; flaps are known only from nematocysts of members of the hexacorallian order Actiniaria, although they have been inferred to be characteristic of Anthozoa, the group to which Actiniaria belongs. Using scanning and transmission electron microscopy, we discover a third apical morphology in nematocysts, an apical cap, which we find in all nonactiniarian anthozoans examined. This apical cap is identical structurally to the apical cap of spirocysts, and it resembles the apical structure of ptychocysts, whose apex is documented here for the first time. Additionally, a full survey of nematocysts from all body structures of two actiniarians demonstrates that a particular type of nematocyst, the microbasic p‐mastigophore of the mesenterial filaments, does not have apical flaps. The observed variation does not correspond to conventional categorization of capsule morphology and raises questions about the function and structure of capsules across Cnidaria. Despite some ambiguity in optimization of ancestral states across cnidae, we determine that the apical cap is the plesiomorphic structure for anthozoan cnidae and that apical flaps are a synapomorphy of Actiniaria. At present, the operculum is interpreted as a synapomorphy for Medusozoa, but either it or an apical cap is the ancestral state for nematocysts. J. Morphol. 273:121–136, 2011. © 2011 Wiley Periodicals, Inc.     

Epidermal structure of the scleractinian coral Mycetophyllia ferox: light-induced vesicles,copious mucocytes,and sporadic tentacles

Three colony fragments of the scleractinian coral Mycetophyllia ferox Wells from Florida were observed in flow-through seawater aquaria under light and dark conditions. The colonies were then anesthetized and fixed for microscopic examination. Small vesicles formed across the epidermis in response to light as gastrodermis containing approximately 1.9 × 10⁶ zooxanthellae cm⁻² migrated into them. The vesicles flattened in the dark and the gastrodermis retreated to a clumped position. The epidermis is dominated by mucus cells with more than 6300 per mm². In contrast, there are very few epidermal cnidae. The polyps lack tentacles entirely, though small tentacles do occur, albeit sporadically, along the colline walls. Colline tentacles are expanded both day and night, and there is considerable intracolonial variability in the number of cnidae within them, ranging from as few as 316 to more than 3200 per mm² tentacle. There may be several small cnidocyst batteries containing both spirocysts and nematocysts (all microbasic p-mastigophores), but the principal battery is at the tentacle tip where cnidae are much more densely packed. There is considerable variation in the ratio of the two cnidae among tentacles in the same colony. Since the tentacles occur inconsistently and do not appear to expand, their functional role is unclear. Comparisons of epidermal characters are made with other members of the genus Mycetophyllia.     

High level of genetic divergence between sympatric color morphs of the littoral sea anemone Anthopleura orientalis (Anthozoa: Actiniaria)

Using enzyme electrophoresis and nematocyst analysis, the sympatrically occurring "light" and "dark" color morphs of the sea anemone Anthopleura orientalis from the Sea of Japan were shown to be two valid species. The "light" morph was identified as A. orientalis (Averintsev, 1967 Issledovaniya fauny morei: Vyp. 5 (13). Nanka, Leningrad, pp. 62-77), while the "dark" morph was designated as Anthopleura sp. The analysis of 21 isozyme loci revealed high value of Nei's genetic distance (D=1.284) between the two species, which are indistinguishable in their external morphology. The mean values of observed and expected heterozygosities for A. orientalis and Anthopleura sp. are high (H(o)=0.252+/-0.061, H(e)=0.250+/-0.061 and H(o)=0.327+/-0.052, H(e)=0.351+/-0.054, respectively). The species differ significantly in the size of spirocysts and nematocysts, among which the atrichs from acrorhagi and the basitrichs from actinopharynx contribute most to the observed difference. Strong qualitative difference is revealed between distributions of nematocysts in mesenteric filaments of the two sea anemone species studied. The possible conspecificity of Anthopleura sp. with Anthopleura artemisia (Dana, 1848) is discussed and the conclusion made that these are two separate species.     

Microscaling: why larger anemones have longer cnidae

Scaling analysis provides a quantitative method for describing and comparing how qualities of organisms vary as a function of body size. However, cell level phenomena have been notoriously hard to analyze because animal cells and organelles have such irregular shapes. The intracellular cnidae make good models of scaling at the cell level because they are durable and easy to image and measure. The mean length of unfired tentacle cnidae (spirocysts) varies continuously, and reversibly, with body size for three macrophagous anemone species. Significant differences in spirocyst shape and size relative to body mass are related to differences in tissue functions and species ecologies, strongly suggesting that cnida size, shape, and scaling patterns respond to natural selection. Cnida scaling patterns can be treated as features of cnidarian life histories. Spirocyst scaling exponents (slopes of log cnida dimension vs. log body weight) are similar to each other (0.05-0.09) and to reported values for animal somatic cells (0.017-0.17), but are much smaller than reported values for anemone basal diameters (0.30-0.38). I propose, here, a general, mechanical explanation for microscaling of structural secretory cells and their secretions, including the cnidae. Larger bodies require thicker, pliant sheets of sluggishly respiring extracellular support materials such as mesoglea and basement membrane. Thicker mesoglea can support larger, taller epithelial cells, which in turn provide additional maintenance services for these progressively thicker acellular layers. Ultimately, larger, taller cells can secrete and support larger, longer cnidae.     

Nematocysts of the Mediterranean hydroid Halocordyle disticha

The morphology, distribution and function of nematocysts in the hydroid Halocordyle disticha were studied by light and scanning electron microscopy. Two types of stenoteles, two types of microbasic b-mastigophores (one with a capsular inclusion) and one type of desmoneme were identified. Prey-capturing experiments were performed with Artemia sp. nauplii. Stenoteles and microbasic mastigophores (those with inclusions) penetrated and paralyzed Artemia larvae, whereas desmonemes kept them in a firm grip by winding around setae.     

Neural pathways and innervation of cnidocytes in tentacles of sea anemones

Our previously published studies are here reviewed detailing neuro-cnidocyte synapses, demonstrating putative neurotransmitter substances, and identifying complex neural pathways in sea anemones. Synapses were traced to their contacts on nematocytes and spirocytes by transmission electron microscopy of serial thin sections of tentacles. In five animals, cells containing microbasic p-mastigophores had synapses with clear vesicles, whereas cells containing basitrichous isorhizas had synapses with dense-cored vesicles, providing preliminary evidence for a selectivity of neurotransmitter types for different nematocysts. Either clear or dense-cored synaptic vesicles were also present at neuro-spirocyte contacts. Antho-RFamide immunoreactivity occurred in some anthozoan synaptic vesicles and immunogold labeling of serotonin was found at a neuro-spirocyte synapse. Neural pathways included direct innervation of spirocytes by sensory cells, sequential neuro-neuro-spirocyte and neuro-neuro-nematocyte synapses and reciprocal synapses involving axons of both sensory cells and ganglion cells. Such synaptic patterns resemble neuro-effector pathways found in higher animals and lay to rest the independent effector hypothesis for cnidocyte discharge in tentacles of sea anemones.     

Dynamic tuning of hair bundle mechanoreceptors in a sea anemone during predation

The sea anemone Haliplanella luciae (Cnidaria, Anthozoa) detects chemical and mechanical stimuli from prey. Hair bundle mechanoreceptors on the tentacles participate in regulating discharge of microbasic p-mastigophore nematocysts. Properly stimulated hair bundles sensitize the anemone to discharge nematocysts into objects that contact the tentacles. The hair bundle mechanoreceptors are composed of stereocilia derived from a multicellular complex. This complex consists of a single sensory neuron surrounded by two to four supporting cells. The mechanoreceptor is similar in many ways to vertebrate hair cells of the acousticolateralis system. However, anemone hair bundles are adjustable in structure and responsiveness according to the activity of two different chemoreceptors. One chemoreceptor binds N -acetylated sugars and the other binds amino compounds including proline. N -acetylated sugars induce lengthening of the hair bundle and a downward shift in frequencies that elicit maximal discharge of microbasic p-mastigophore nematocysts. Furthermore, N -acetylated sugars shift maximal discharge to smaller amplitude vibrations. Thus, N -acetylated sugars likely tune hair bundles so that small, swimming zooplankton stimulate maximal discharge. Proline leaks into the seawater from the hemolymph of wounded prey. Proline induces shortening of the hair bundle and shifts maximal discharge of nematocysts to higher frequencies and to larger amplitude vibrations. Thus, proline likely tunes hair bundles so that small, wounded, prey stimulate maximal discharge of nematocysts as they struggle to escape. Thus, suitably sized prey stimulate maximal discharge of microbasic p-mastigophore nematocysts upon first contacting the anemone tentacle and again upon attempting to escape.     

The nature of the adhesion to shells of the symbiotic sea anemone calliactis tricolor (Leseur)

The mechanism of tentacular adhesion to gastropod shells has been demonstrated for a symbiotic sea anemone, Calliactis tricolor (Leseur) by means of scanning electron microscopy. Both basitrichous isorhiza nematocysts and spirocysts are involved with the former being much more abundant on the shells. Contrary to its classical characterization, the thread of the basitrichous isorhiza nematocyst possesses, in addition to the large spines at its base, minute spines along its length.     

A study of the spirocytes from the Ceriantharia and Actiniaria (Cnidaria: Anthozoa)

Summary The structure and ultrastructure of ceriantharian and actiniarian spirocysts were compared. In the ceriantharian spirocyst, the apical cap consisted of two layers and the wall consisted of three, whereas in the actiniarian spirocyst, the number of layers was reversed. The inner wall of the ceriantharian spirocyst was smooth in longitudinal section whilst that of the actiniarian was serrated. The apical ends of ceriantharian spirocytes posessed either a set of microvilli or an arrangement that included both microvilli and a flagellum. The actiniarian spirocyte possessed only microvilli. Fine tubules occupying the undischarged tube of the ceriantharian spirocysts were of both moderate and strong electron densities, whilst those of the actiniarian were consistently electrondense. Undischarged ceriantharian spirocytes possessed tubules situated in the pleats and in the centre of the tube, whilst the tubes of undischarged actiniarian spirocytes possessed only centrally placed tubules. Fine electron-dense filaments linking the cell membrane to the spirocyst were present at the apical ends of actiniarian spirocytes. A different electron-dense network of filaments was present in the apical end of ceriantharian spirocytes. All cnidae of ceriantharians lacked tripartite opercula whilst the nematocysts of actiniarians possessed them. Primitive features of ceriantharian cnidae are discussed.     

A new genus and species of isanthid sea anemone (Cnidaria: Anthozoa: Actiniaria) from Chilean Patagonia,Anthoparactis fossii n. gen. et sp

We describe a new genus and species of sea anemone from Chilean Patagonia. Anthoparactis fossii n. gen. et sp. adds another acontiate genus and species to the family Isanthidae Carlgren, 1938. Anthoparactis n. gen. differs from the other isanthid genera in having the same number of mesenteries distally and proximally, acontia with basitrichs only, and a column with verrucae distally. Anthoparactis fossii n. sp. differs from the most similar species, Isoparactis fionae Lauretta et al., 2013, in the number of cycles of mesenteries and tentacles, structures of the column, colour pattern of the oral disc, cnidae, and geographical distribution. Isanthidae now includes seven genera and 11 species.     

The nature of the adhesion of tentacles to shells during shell-climbing in the sea anemone Calliactis parasitica (Couch)

Phase contrast microscopy and scanning electron microscopy show that during the response of the symbiotic sea anemone Calliactis parasitica (Couch) to shells of Buccinum undatum (L.) three times as many spirocysts as nematocysts are discharged. Observations indicate that spirocysts are responsible for the adhesion of tentacles to shells.Discharge levels are not significantly influenced by the nature of the substratum to which the anemones are attached. The reported observation that fewer tentacles adhere to shells when anemones are settled on shells than when they are fixed on a different substratum is re-interpreted in terms of a new model for the control of spirocyst discharge.     

The enidom: an index of aggressive proficiency in scleractinian corals

This study of ten Indo-Pacific and Caribbean scleractinian corals explains their relative aggressive proficiencies in terms of their cnidoms. Species ranging from aggressive to subordinate, on an established hierarchy, were studied. Size, number and distribution of each cnida type were quantified. A marked relationship between number of nematocysts per polyp and aggressive proficiency was demonstrated. The recorded differences in aggressive proficiency between the Indo-Pacific and Caribbean corals are discussed in terms of cnidom differences. In two species a significant trend in linear distribution of nematocysts along mesenterial filaments was recorded, with a distinct zonation of the different nematocyst types along the length of the filament.     

Do the Anemonefish Amphiprion ocellaris (Pisces: Pomacentridae) Imprint Themselves to Their Host Sea Anemone Heteractis magnifica (Anthozoa: Actinidae)?

Juvenile anemonefishes detect their host sea anemone by olfactory stimuli; in order to investigate whether this behaviour is innate or acquired, the anemonefish species Amphiprion ocellaris was bred in two different ways: 1. With no host sea anemone present at all (–A); and 2. With the specific host sea anemone Heteractis magnifica present in the hatching aquarium, so that these eggs were laid and hatched close to the sea anemone, as in nature (+A). The two different types of juvenile A. ocellaris were presented to the odours of the host sea anemone H. magnifica in two sets of short-term experiments with the host (a) visually hidden in a net cage, and (b) visible but physically separated from the anemonefishes. In both cases, a water flow was established between fishes and host. The +A-fishes found their host by olfactory and not by visual stimuli. In both series, the –A-fishes showed a significantly lower affinity behaviour towards the odour compounds from the host sea anemone than the +A-fishes did. A third type of experiment was a direct confrontation between fishes and host; here, the –A-fishes were indifferent towards the host sea anemone for almost 48 h, while the +A-fishes acclimated to the host sea anemone within the first 5 min of the direct confrontation. The results of this study suggest that Amphiprion ocellaris imprints itself olfactorily to its species-specific host sea anemone Heteractis magnifica, and, furthermore, may be genetically disposed towards olfactory recognition of the host sea anemone.     

A new species of Hormathia (Actiniaria, Hormathiidae) from the eastern Weddell Sea, Antarctica

A new species of sea anemone in the genus Hormathia, is described and illustrated based on forty-two specimens collected during the Polarstern cruises ANT XV/3 and ANT XVII/3 in the Weddell Sea. The main features of the new taxon are the crown of flattened and hooked tubercles at the distal end of the scapus, the regular arrangement of pointed tubercles along the column and the cnidom. The new species shares the pointed tubercles, at least in the upper part of the scapus, with two other species of Hormathia in the southern hemisphere: Hormathia spinosa Hertwig 1882 and H. pectinata Hertwig 1882 Received in revised form: 27 December 2000 Electronic Publication