Nematocysts and Taxonomy in Laomedea, Gonothyraea and Obelia (Hydrozoa, Campanulariidae)

The cnidoms of Laomedea flexuosa, Gonothyraea loveni, Obelia geniculata, O. longissima and O. dichotoma were studied by light and scanning electron microscopy to find out whether the nematocysts could be used as taxonomic characters. Three b-rhabdoid heteroneme nematocysts (microbasic b-mastigophore) and three isorhizous haploneme nematocysts (atrichous isorhiza) were distinguished. A small b-rhabdoid nematocyst with spindle-shaped capsule occurred in all the species examined. In the polyp and planula of G. loveni , and the planula of L. flexuosa it was the only nematocyst present. Specific for L. flexuosa was a b-rhabdoid with curved capsule. In the polyp and newly-liberated medusa of O. longissima another b-rhabdoid appeared with bean-shaped capsule and markedly long spines at the distal tube. The polyp and newly-liberated medusa of O. dichotoma were characterized by two different isorhizas. A special type of isorhiza occurred in the polyp of O. geniculata . The curved capsules of the different isorhizas varied somewhat in shape and size. Differences in nematocyst structure and occurrence are presumed to provide characters for taxonomy. Thus, O. dichotoma and O. Iongissima are regarded as separate species due to their distinctly different nematocysts, in conjunction with other morphological and ecological differences.     

Ultrastructure of a novel eurytele nematocyst of Carybdea alata Reynaud (Cubozoa,Cnidaria)

The ultrastructural characteristics of nematocysts from the cubozoan Carybdea alata Reynaud, 1830 (Hawaiian box jellyfish) were examined using light, scanning and transmission electron microscopy. We reclassified the predominant nematocyst in C. alata tentacles as a heterotrichous microbasic eurytele, based on spine, tubule and capsule measurements. These nematocysts exhibited a prominent and singular stylet, herein referred to as the lancet. Discharged nematocysts from fixed tentacle preparations displayed the following structures: a smooth shaft base, lamellae, a hemicircumferential fissure demarking the proximal end of a stratified lancet, and a gradually tapering tubule densely covered with large triangularly shaped spines. The lancet remained partially adjoined to the shaft base in a hinge-like fashion in rapidly fixed, whole-tentacle preparations. In contrast, this structure was not observed in discharged nematocyst preparations which involved multiple transfer steps prior to fixation. Various approaches were designed to detect this structure in the absence of fixative. Detached lancets were located in proximity to discharged tubules in undisturbed coverslip preparations of fresh tentacles. In addition, examination of embedded nematocysts from fresh tentacles laid on polyacrylamide gels revealed still-attached lancets. To examine the function of this structure in prey capture, Artemia sp. laden tentacles were prepared for scanning electron microscopy. While carapace exteriors exhibited structures proximal to the lancet, i.e., the nematocyst capsule and shaft base, neither tubule nor lancet structures were visible. Taken together, the morphological data suggested a series of events involved in the discharge of a novel eurytele from C. alata.     

NEMATOCYSTS OF THE SEA ANEMONE METRIDIUM

Six types of nematocysts and their nematocytes in tentaclesand acontia of the sea anemone Metridium senile fimbriatum werestudied by electron microscopy. Microbasic b-mastigophores, microbasic amastigophores, and basitrichshave one fundamental feature in common: a straight, complexly-foldedshaft with dense spines pointing apically. An additional resemblancebetween a b-mastigophore and a basitrich is the possession ofa long, narrow, coiled thread bearing spines. An amastigophoreis characterized by a short, looped, unspined thread and a cup-shapedgranular matrix. Atrich and holotrich nematocysts have a coiled, spined tubeof uniform diameter which lies in an evenly granular matrixfilling the entire capsule. The above five nematocysts have three flaps at the apex of thecapsule which open upon discharge, and each nematocyte possessesa flagellum with which is associated one or two centrioles anda striated rootlet. The long rootlet of the b-mastigophorebearingnematocyte passes through a circular band of fibrils surroundingthe neck region of the capsule, and the short rootlet of theatrich lies in a dense fibrous sheath surrounding all but theapex of the capsule. The spirocyst differs from the other nematocysts in having athin, ridged, singlewalled capsule; an inverted tube containingbundles of tubules; an apical disk covered only by a thin layerof granular material and the nematocyst membrane; and the absenceof a flagellum in its nematocyte. Theories of excitation and mechanism of discharge of nematocystsand the function of spirocysts are discussed in the light ofthis and other recent studies of the fine structure of nematocysts.Special attention is drawn to the probable role of the foldsin the walls of shaft and thread in increasing the length ofthe tube upon discharge.     

The nematocyst: a molecular map of the cnidarian stinging organelle

Nematocysts or cnidocysts represent the common feature of all cnidarians. They are large organelles produced from the Golgi apparatus as a secretory product within a specialized cell, the nematocyte or cnidocyte. Nematocysts are predominantly used for prey capture and defense, but also for locomotion. In spite of large variations in size and morphology, nematocysts share a common build comprising a cylindrical capsule to which a long hollow thread is attached. The thread is inverted and coiled within the capsule and may be armed with spines in some nematocyst types. During the discharge of nematocysts following a chemical or mechanical stimulus, the thread is expelled from within the capsule matrix in a harpoon-like fashion. This process constitutes one of the fastest in biology and is accompanied by a release of toxins that are potentially harmful also for humans. The long history of research on Hydra as a model organism has been accompanied by the cellular, mechanistic and morphological analysis of its nematocyst repertoire. Although representing one of the most complex organelles of the animal kingdom, the evolutionary origin and molecular map of the nematocyst has remained largely unknown. Recent efforts in unraveling the molecular content of this fascinating organelle have revealed intriguing parallels to the extracellular matrix.     

Ultrastructure of nematocyst discharge in catch tentacles of the sea anemone Haliplanella luciae (Cnidaria: Anthozoa)

The mature nematocyst lies just beneath the cnidodyte plasma membrane. A microtubule array surrounds the nematocyst capsule just beneath the capsule tip. We propose that the array helps to hold the capsule at the cnidocyte cell surface until discharge. The undischarged capsule tip is sealed by three apical flaps, joined together along complex radial seams. The seams are filled with subunits that appear to bind the flaps together. Upon discharge, the flaps separate along the radial seams to permit thread eversion. The everted thread is lined on both sides by subunits that are stained by antimonate, indicating that they bind calcium. We suggest that, together, the subunits hold the uneverted thread in its folded and coiled configuration. Thread eversion would follow subunit uncoupling. The capsule and thread interiors of partially discharged nematocysts are stained by antimonate. In contrast, the capsule and thread interiors of fully discharged nematocysts are not stained by antimonate. Thus, nematocyst calcium might be injected into the target tissue where it is presumed to act in conjunction with nematocyst venom to promote cell death.     

Structural characteristics of nematocyst stinging threads from the parasitic cnidarian Polypodium hydriforme

The structure of discharged nematocyst stinging threads present in free-living individuals of Polypodium hydriforme was studied by scanning and transmission electron microscopy. Not all cnidae of P. hydriforme proved to be atrichous isorhizas (as previously was accepted), but only one of the five nematocyst categories studied. A unique feature of P. hydriforme nematocysts was revealed: their stinging threads possess two strands of spines, rather than 5 or 3, as in Narcomedusae and other Cnidarians, respectively. This fact supplements the evidence in favour of P. hydriforme being a rather isolated branche in the phylogenetic tree of Cnidaria.     

A modified view on octocorals: Heteroxenia fuscescens nematocysts are diverse, featuring both an ancestral and a novel type

Cnidarians are characterized by the presence of stinging cells containing nematocysts, a sophisticated injection system targeted mainly at prey-capture and defense. In the anthozoan subclass Octocorallia nematocytes have been considered to exist only in low numbers, to be small, and all of the ancestral atrichous-isorhiza type. This study, in contrast, revealed numerous nematocytes in the octocoral Heteroxenia fuscescens. The study demonstrates the applicability of cresyl-violet dye for differential staining and stimulating discharge of the nematocysts. In addition to the atrichous isorhiza-type of nematocysts, a novel type of macrobasic-mastigophore nematocysts was found, featuring a shaft, uniquely comprised of three loops and densely packed arrow-like spines. In contrast to the view that octocorals possess a single type of nematocyst, Heteroxenia fuscescens features two distinct types, indicating for the first time the diversification and complexity of nematocysts for Octocorallia.     

Evolution of complex structures: minicollagens shape the cnidarian nematocyst

The generation of biological complexity by the acquisition of novel modular units is an emerging concept in evolutionary dynamics. Here, we review the coordinate evolution of cnidarian nematocysts, secretory organelles used for capture of prey, and of minicollagens, proteins constituting the nematocyst capsule. Within the Cnidaria there is an increase in nematocyst complexity from Anthozoa to Medusozoa and a parallel increase in the number and complexity of minicollagen proteins. This complexity is primarily manifest in a diversification of N- and C-terminal cysteine-rich domains (CRDs) involved in minicollagen polymerization. We hypothesize that novel CRD motifs alter minicollagen networks, leading to novel capsule structures and nematocyst types.     

ELECTRON MICROSCOPE OBSERVATIONS ON THE STRUCTURE AND DISCHARGE OF THE STENOTELE OF HYDRA

Sections of the stenotele type of nematocyst of Chlorohydra hadleyi have revealed that the stenotele, upon firing, completely everts its stylets and spines and the long, thin tubule, much as the eversion of the tubule of the nematocyst of the jewel anemone (Picken, 1953; Robson, 1953). Alternative mechanisms for supplying the energy necessary to forcefully discharge the stenotele contents are discussed as well as the possible significance of several regions containing highly ordered periodic structure. The origin of nematocysts as kinetosomal derivatives is discussed as a possibility suggested by the symmetry of the stenotele contents and the structure, location, and function of the cnidocil.     

The cnidarian nematocyst: a miniature extracellular matrix within a secretory vesicle

Nematocysts are the taxon-defining features of all cnidarians including jellyfish, sea anemones, and corals. They are highly sophisticated organelles used for the capture of prey and defense. The nematocyst capsule is produced within a giant post-Golgi vesicle, which is continuously fed by proteins from the secretory pathway. Mature nematocysts consist of a hollow capsule body in which a long tubule is coiled up that, upon discharge, is expelled in a harpoon-like fashion. This is accompanied by the release of a toxin cocktail stored in the capsule matrix. Nematocyst discharge, which is one of the fastest processes in biology, is driven by an extreme osmotic pressure of about 150 bar. The molecular analysis of the nematocyst has from the beginning indicated a collagenous nature of the capsule structure. In particular, a large family of unusual minicollagens has been demonstrated to form the highly resistant scaffold of the capsule. Recent findings on the molecular composition of Hydra nematocysts have confirmed the notion of a specialized extracellular matrix, which is assembled during an intracellular secretion process to form the most complex predatory apparatus at the cellular level.     

Organelle survival in a foreign organism: Hydra nematocysts in the flatworm Microstomum lineare

Nematocysts are characteristic organelles of the phylum cnidaria. They are designated kleptocnidae when sequestered in animals that feed on cnidaria. Kleptocnidae are known for more than a century. Nevertheless it is still enigmatic how selected nematocyst types survive in the predator and how they reach their final destination in the foreign body. In the free-living Platyhelminth Microstomum lineare the fate of nematocysts of the prey Hydra oligactis was analyzed at the ultrastructural level and by fluorescence microscopy using hydra polyps that had been stained in vivo with the fluorescent dyes TROMI and TRITC. M. lineare digested hydra tissue in its intestine within 30?min and all nematocyst types were phagocytosed without adherent cytoplasm by intestinal cnidophagocytes. Desmoneme and isorhiza nematocysts were digested whereas cnidophagocytes containing the venom-loaded stenotele nematocysts started to migrate out of the intestinal epithelia through the parenchyma to the epidermis thereby traversing the subintestinal and subepidermal muscle layer. Within one to two days, M. lineare began to form a muscle layer basolateral around epidermal cnidophagocytes. Epidermal stenoteles survived in M. lineare for at least four weeks. The ability of epidermal stenotele nematocysts to discharge suggest that this hydra organelle preserved its physiological properties in the new host.     

Die Cnidogenese der Octocorallia (Anthozoa,Cnidaria): I. Sekretionund Differenzierung von Kapsel und Schlauch

The ultrastructural differentiation of capsule and its relation to tube development is described in several Octocorallia species(Alcyonaria: Alcyonium digitatum, Parerythropodium coralloides, Cornularia cornucopiae, Paralcyonium elegans; Pennatularia: Pteroeides spinosum, Veretillum cynomorium; Gorgonaria: Pseudopterogorgia aerosa), all of which have only one type nematocyst. In the Octocorallia, capsule and tube are secreted successively by the Golgi apparatus associated with a primary centriolar complex. During the secretion of the external tube, the outer capsular wall (sclera) is structurally differentiated; inside the capsule the material of the inner capsular wall is separated from the later capsular content (matrix). The primary wall differentiation enables the capsules to grow after capsular secretion has been completed. Following tube secretion, the external tube is completely transferred into the capsule, without the tube wall being transformed into capsular wall, as previously suggested (Westfall, 1966; Ivester, 1977). During early invagination of the tube wall, the coarse, granulated matrix of the external tube is transferred into the internal tube. From this material the spines are developed, which are observed before the tube is completely transferred into the capsule. By a secondary wall differentiation the previously structureless inner capsular wall changes to a complex structure, extending again the capsule, thus mixing the capsular content and enabling the tube to shift to a position, which corresponds with that of mature capsules. These observations demonstrate for the first time the differentiation of the capsule and its close relationship to the differentiation of the tube in nematocysts of Octocorallia.     

Predation resistance and nematocyst scaling for Metridium senile and M. farcimen

Previous studies suggest that large body size reduces the risk of predation for acontiate sea anemones. For two species of Metridium, we found significant increases in the length of the acontial threads and in the mean lengths of the unfired acontial nematocyst capsules, with increasing body size. This supports the hypothesis that more damaging acontial defenses protect larger acontiate anemones from their predators. Metridium is planktivorous, and food size does not increase substantially with body size; so we expected smaller increases in nematocyst size for the feeding tentacles. In fact, scaling exponents were significantly smaller for the tentacle nematocysts than for acontial nematocysts of the same types in 3 out of 4 cases. This suggests that nematocyst scaling responds predictably to selection pressure. When specimens of the same size were compared, the non-clonal, subtidal species, M. farcimen, had significantly larger acontial nematocysts than did its clonal congener, M. senile, which lives at the upper tidal limits for major subtidal predators in the northeastern Pacific. Therefore, larger acontial nematocysts may be particularly advantageous where predation levels are high. These data demonstrate that closely related anemone species can be distinguished on the basis of ecologically and functionally relevant differences in nematocyst scaling.     

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.     

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 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.     

Sequestration of nematocysts by divergent cnidarian predators: mechanism,function, and evolution

Animals have evolved diverse mechanisms to protect themselves from predators. Although such defenses are typically generated endogenously, some species have evolved the ability to acquire defenses by sequestering defensive chemicals or structures from other species. Chemical sequestration is widespread among animals, but the ability to sequester entire structures, such as organelles, appears to be rare. Here, we review information on the sequestration of functional nematocysts, the stinging organelles produced by Cnidaria, by divergent predators. Nematocyst sequestration has evolved multiple times, having been documented in Ctenophora, Acoelomorpha, Platyhelminthes, and Mollusca. For each of these phyla, we review the phylogenetic distribution, mechanisms, and possible functions of nematocyst sequestration. We estimate that nematocyst sequestration has evolved 9–17 times across these four phyla. Although data on the mechanism of sequestration remain limited, similarities across several groups are evident. For example, in multiple groups, nematocysts are transported within cells from the gut to peripheral tissues, and certain types of nematocysts are selectively sequestered over others, suggesting convergent evolution in some aspects of the sequestration process across phyla. Similarly, although the function of nematocyst sequestration has not been well documented, several studies do suggest that the nematocysts sequestered by these groups are effective for defense. We highlight several traits that are common to Ctenophora, Acoelomorpha, Platyhelminthes, and Mollusca and suggest hypotheses for how these traits could have played a role in the evolution of nematocyst sequestration. Finally, we propose a generalized working model for the steps that may lead to the evolution of nematocyst sequestration and discuss important areas for future research.     

Cytological studies of the nematocysts of Hydra. I. Desmonemes,isorhizas, cnidocils,and supporting structures

Entire hydras or tentacles were fixed in OsO(4) or in KMnO(4) and thereafter washed, dehydrated, and embedded in a methacrylate mixture. Ultrathin sections were cut on an experimental model, thermal expansion type ultramicrotome or on a Porter-Blume microtome. The sections were examined in an RCA electron microscope. Type EMU-2 D. "Squash preparations" for light microscopy, were made from the hydra mouth region and the attached tentacles. These were observed with an AO Baker interference microscope. In the mature organism, three of the four types of nematocysts normally found in hydra could be positively identified with the electron microscope. The desmonemes, the smallest type, have a dense matrix and a thin capsule. The two different types of mature isorhizas could not be distinguished with certainty. They are intermediate in size between the desmonemes and stenoteles and have a capsule with a dense matrix. The cnidocil, or triggering hair, which is composed of a dense core and a fibrillar sheath has nine supporting elements arranged in a semi-circle near its base. Twenty "supporting structures" are arranged around the nematocyst capsule and interconnections between the supporting elements and these latter structures have been observed. Development of the nematocysts involves an increase in density of the matrix. Spines can be seen in the interior of tubular structures within the capsules of the holotrichous isorhizas.     

The utilization of cnidarian nematocysts by aeolid nudibranchs: Nematocyst maintenance and release in spurilla

Some nudibranchs that feed on cnidarians are known to store nematocysts within cnidophage cells and use them for their own defense. Most of the nematocysts are in direct contact with the cytoplasm of the cnidophage. Nematocysts are not subjected to lysosomal enzymes because any phagocytic membrane that surrounded the nematocyst after engulfment does not persist. Cnidophage organelles are restricted to regions surrounding the nematocysts and may aid in the maintenance and development of the nematocysts. The release of cnidophages is initiated by a contraction of a dense muscle complex surrounding the cnidosac. Nematocysts do not discharge if the cnidophage membrane does not rupture upon release. A comparison of nematocyst maintenance in Spurilla neapolitana and nematocyst retention in other organisms is presented.     

Cytological Studies of the Nematocysts of Hydra : I. Desmonemes, Isorhizas, Cnidocils, and Supporting Structures

Entire hydras or tentacles were fixed in OsO₄ or in KMnO₄ and thereafter washed, dehydrated, and embedded in a methacrylate mixture. Ultrathin sections were cut on an experimental model, thermal expansion type ultramicrotome or on a Porter-Blume microtome. The sections were examined in an RCA electron microscope. Type EMU-2 D. "Squash preparations" for light microscopy, were made from the hydra mouth region and the attached tentacles. These were observed with an AO Baker interference microscope. In the mature organism, three of the four types of nematocysts normally found in hydra could be positively identified with the electron microscope. The desmonemes, the smallest type, have a dense matrix and a thin capsule. The two different types of mature isorhizas could not be distinguished with certainty. They are intermediate in size between the desmonemes and stenoteles and have a capsule with a dense matrix. The cnidocil, or triggering hair, which is composed of a dense core and a fibrillar sheath has nine supporting elements arranged in a semi-circle near its base. Twenty "supporting structures" are arranged around the nematocyst capsule and interconnections between the supporting elements and these latter structures have been observed. Development of the nematocysts involves an increase in density of the matrix. Spines can be seen in the interior of tubular structures within the capsules of the holotrichous isorhizas.