A comparison of the structure and function of nematocysts in free-living and parasitic cnidarians (Myxozoa)

Myxozoans are obligate parasites that have complex life cycles requiring alternate vertebrate and invertebrate hosts, with transmission via microscopic waterborne spores. Unusually for parasites, they belong to the phylum Cnidaria, alongside thousands of free-living corals, sea anemones, jellyfish and hydrozoans. Their cnidarian affinity is affirmed by genetic relatedness and the presence of nematocysts, historically called "polar capsules" in myxozoan research. Free-living cnidarians utilise this cellular weaponry for defence, predation and adhesion, whereas myxozoans use it to anchor to their hosts as the first step in infection. Despite the ~650 million years of divergence between free-living cnidarians and myxozoans, their nematocysts retain many shared morphological and molecular characters. Both are intra-cellular capsules with a single opening, and contain a coiled, evertable tubule. They are composed of unique nematocyst proteins, nematogalectin and minicollagen, and both likely contain an internal matrix of metal cations covalently bound to the anionic polymer poly-gamma glutamate. The rapid dissociation of this matrix and the resulting increase in internal osmotic potential is the driving force behind tubule elongation during discharge. In this review, we compare the structure and function of nematocysts in Myxozoa and free-living Cnidaria, incorporating recent molecular characterizations. We propose that terminology for homologous myxozoan structures be synonymized with those from other Cnidaria, hence, "polar capsule" as a taxon-specific nematocyst morphotype and "polar filament" as "tubule." Despite taxonomic divergence, genome reduction and an evolution to parasitism, myxozoans maintain nematocysts that are structurally and functionally homologous to those of their free-living cnidarian relatives.

In vitro and in vivo assays reveal that cations affect nematocyst discharge in Myxobolus cerebralis (Cnidaria: Myxozoa)

Myxozoans are parasitic, microscopic cnidarians that have retained the phylum-characteristic stinging capsules called nematocysts. Free-living cnidarians, like jellyfish and corals, utilize nematocysts for feeding and defence, with discharge powered by osmotic energy. Myxozoans use nematocysts to anchor to their fish hosts in the first step of infection, however, the discharge mechanism is poorly understood. We used Myxobolus cerebralis, a pathogenic myxozoan parasite of salmonid fishes, and developed two assays to explore the nature of its nematocyst discharge. Using parasite actinospores, the infectious stage to fish, we stimulated discharge of the nematocysts with rainbow trout mucus in vitro, in solutions enriched with chloride salts of Na+, K+, Ca2+ and Gd3+, and quantified discharge using microscopy. We then used quantitative polymerase chain reaction to evaluate the in vivo effects of these treatments, plus Mg2+ and the common aquaculture disinfectant KMnO₄, on the ability of M. cerebralis actinospores to infect fish. We found that Mg2+ and Gd3+ reduced infection in vivo, whereas Na+ and K+ over-stimulated nematocyst discharge in vitro and reduced infection in vivo. These findings align with nematocyst discharge behaviour in free-living Cnidaria, and suggest phylum-wide commonalties, which could be exploited to develop novel approaches for controlling myxozoan diseases in aquaculture.

Adhesive papillae on the brachiolar arms of brachiolaria larvae in two starfishes, Asterina pectinifera and Asterias amurensis, are sensors for metamorphic inducing factor(s)

It has been hypothesized by Barker that starfish brachiolaria larvae initiate metamorphosis by sensing of metamorphic inducing factor(s) with neural cells within the adhesive papillae on their brachiolar arms. We present evidence supporting Barker's hypothesis using brachiolaria larvae of the two species, Asterina pectinifera and Asterias amurensis. Brachiolaria larvae of these two species underwent metamorphosis in response to pebbles from aquaria in which adults were kept. Time-lapse analysis of A. pectinifera indicated that the pebbles were explored with adhesive papillae prior to establishment of a stable attachment for metamorphosis. Microsurgical dissections, which removed adhesive papillae, resulted in failure of the brachiolaria larvae to respond to the pebbles, but other organs such as the lateral ganglia, the oral ganglion, the adhesive disk or the adult rudiment were not required. Immunohistochemical analysis with a neuron-specific monoclonal antibody and transmission electron microscopy revealed that the adhesive papillae contained neural cells that project their processes towards the external surface of the adhesive papillae and they therefore qualify as sensory neural cells.

Myxozoan transmission via actinospores: new insights into mechanisms and adaptations for host invasion

Various mechanisms that enable and improve transmission success of myxozoan actinospore stages towards fish hosts are described, based upon a combination of experimental data and functional analysis of morphological characters. For this purpose, laboratory-reared actinospores of Myxobolus cerebralis, Myxobolus parviformis, Henneguya nuesslini and Myxobolus pseudodispar were employed to exemplarily investigate aspects of host attachment and invasion. The process of polar filament discharge of M. cerebralis actinospores was analysed, showing that full discharge occurs in less than 10 msec. Additionally, a mechanism that rapidly contracts the discharged filament after discharge is described for the first time. Its purpose is most likely to bring the actinospore apex rapidly into intimate contact with the surface of the host. Unlike M. cerebralis, M. parviformis actinospores did not discharge polar filaments after mechanical and chemical stimulation, suggesting a different mode of triggering. For H. nuesslini actinospores, experimental results indicated that polar filament discharge is independent of external calcium-ion concentration but is influenced by osmolality. After attachment of an actinospore and prior to penetration into the host, an ensheathed unit ('endospore'), containing the sporoplasm, was emitted from the valves in a manner which varied from species to species. Experimentally induced sporoplasm emission was time-dependent and was found to be independent of polar filament discharge in H. nuesslini. Remarkably, it could be concluded that the sporoplasm is able to recognize host-stimuli while still within the intact spore. An updated summary of the sequential course of events during host recognition and invasion by actinospores is given.

Larval behavioral, morphological changes, and nematocyte dynamics during settlement of actinulae of Tubularia mesembryanthemum, Allman 1871 (Hydrozoa: Tubulariidae)

The marine colonial hydroid Tubularia mesembryanthemum produces a morphologically unique dispersive stage, the actinula larva. Detailed observations were made on the behaviors and nematocyte dynamics of actinula larvae during attachment and morphogenesis by employing microscopic and time lapse video techniques. These observations produced four primary results. (1) Actinula larvae demonstrated two forms of attachment: temporary attachment by atrichous isorhiza (AI)-nematocysts discharged from the aboral tentacle (AT) tips-and permanent settlement by cement secretion from the columnar gland cells of the basal protrusion. (2) During larval settlement, numerous AIs were discharged from the AT tips with sinuous movement and rubbing of the tentacles onto the substrata, leading to "nematocyte-printing" around the settlement site. (3) Simultaneous with the discharge of the AIs, migration of stenoteles, desmonemes, and microbasic mastigophores occurred, resulting in a dramatic change of nematocyte composition in the ATs after larval settlement. This was in parallel with changes in larval behavior and the tentacle function. (4) Nematocyte-printing behavior during settlement could be recognized as metamorphic behavior responsible for irreversible changes in AT function, from attachment to feeding and defense.

A sea anemone's environment affects discharge of its isolated nematocysts

Nematocysts were isolated from individuals of Calliactis tricolor maintained under different feeding schedules or in different salinities in an attempt to determine how these culture conditions influence the discharge of isolated nematocysts. In addition, the discharge frequencies of nematocysts isolated from two different populations of sea anemones found in two different environments were also compared. Undischarged acontial nematocysts were isolated by extrusion into 1 M sodium citrate and were then treated with 5 mM EGTA to initiate discharge. Nematocysts isolated from anemones maintained under three different feeding schedules showed significantly different responses to the test solution. Nematocysts isolated from anemones maintained in two different salinities did not differ significantly in discharge frequency. Nematocysts isolated from individuals from two separate populations of C. tricolor responded significantly differently to 5 mM EGTA and to deionized water, and these responses also depended upon the isolation solution used. Environmental conditions are known to have an impact on the physiological state of most organisms, but this is the first study providing evidence that the environment or feeding state of an anemone affects discharge of isolated nematocysts. Inherent differences in ionic and osmotic characteristics among nematocysts could explain some of the ambiguities when comparing past studies of isolated nematocyst discharge.

Occurrence, distribution, and localisation of metals in cnidarians

The Cnidaria are simple organisms that have remarkable physiological features susceptible to microscopic investigation. As a group they produce cnidae, the most complex intracellular organelles known, form symbioses with a range of unicellular algae, contain mucocytes that account for a very substantial fraction of their body mass, and form complex skeletal structures of calcium carbonate. This review summarises contributions dealing with the distribution and localisation of metals of physiological and pathological importance within soft tissues and skeletons. Whilst there have been detailed studies of microscale metal distribution, using X-ray microanalysis, in the stinging organelles or cnidocysts and in mucocytes, other cells such as symbiotic algae and the epithelial cells have received little attention. In the skeleton-producing scleractinian corals X-ray microanalysis has provided tenuous, but persistent, evidence of Ca associated with intracellular vesicles or granules in the skeletogenic epithelium, even though the investigations were technically limited. These observations may be germane to the intriguing and intransigent problem of the mechanism of coral calcification. Metal localisation in coral skeleton at the resolution of annual growth rings has been concerned with the validity of Sr/Ca and Mg/Ca ratios as thermometers for paleoclimatic studies. It is not clear whether these ratios are influenced primarily by environmental or biological parameters. Microscale analyses by X-ray microanalysis and ion microprobe indicate a much greater variability of metal ratios which suggests biological control of metal deposition. New data are provided on the elemental composition, measured by X-ray microanalysis, of cells and cell compartments in the coral Galaxea fascicularis and zooxanthellae in the anemone Aiptasia sp. New information is also presented on changing Ca/Sr ratios at the skeletal interface in Galaxea fascicularis.