Exumbrellar Surface of Jellyfish: A Comparative Fine Structure Study with Remarks on Surface Reflectance.

The exumbrellar surfaces of six pelagic cnidarians from three classes were ultra-structurally compared to reveal their structural diversity in relation to their gelatinous, transparent bodies. We examined two hydrozoans (Diphyes chamissonis and Colobonema sericeum), a cubozoan (Chironex yamaguchii), and three scyphozoans (Atolla vanhöffeni, Aurelia coerulea, and Mastigias papua). The exumbrellar surfaces of the mesoglea in D. chamissonis, Ch. yamaguchii, Au. coerulea, and M. papua were covered with a simple epidermis; the shapes of the epidermal cells were remarkably different among the species. The epidermal cells of Ch. yamaguchii and M. papua possessed an array of microvilli on the apical side. The array possibly reduced light reflectance and provided some other surface properties, as seen for the cuticular nipple array in tunicates, considering the length, width, and pitch of the microvilli. The reduction of light reflectance on the array of microvilli was supported by the simulation with rigorous coupled wave analysis (RCWA). Microvilli were sparse and did not form an array in metephyrae of Au. coerulea. The mesoglea matrix beneath the basal side of the epidermis was loose in all of the species. The exumbrellar side of the mesoglea was exposed only in the mesopelagic species, At. vanhöffeni and Co. sericeum, and electron-dense layer(s) covered the surface of the mesoglea. It is uncertain whether the exumbrellar epidermis is absent in these species or the epidermal cells are completely exfoliated during the sampling and handling processes. In the latter case, the electron-dense layer(s) on the mesoglea surface might originally underlie the epidermis.

Mating in the box jellyfish Copula sivickisi--Novel function of cnidocytes.

Within cubozoans, a few species have developed a sexual reproduction system including mating and internal fertilization. One species, Copula sivickisi, is found in a large area of the indo pacific. They have separate sexes and when mature males and females meet they entangle their tentacles and the males transfer a sperm package, a spermatozeugmata, which is ingested by the female fertilizing her eggs internally. After 2-3 days, the females lay an embryo strand that sticks to the substrate and after another 2-3 days, the fully developed larvae leave the strand. We have examined the ultrastructure of the gonads and spermatozeugmata to look for structural adaptations to this specialized way of reproduction and understand how the fertilization takes place. Surprisingly, we discovered that the male gonads were heavily packed with cnidocytes of the isorhiza type and that they are transferred to the spermatozeugmata. The spermatozeugmata does not dissolve in the female gastrovascular cavity but is attached to the female gonad probably using the isorhizas. Here, the sperm cells are partly digested and the nuclei are released. The actual fertilization seems to happen through phagocytosis of the released nuclei by the epithelial cells. The female gonads are likewise packed with cnidocytes but of the eurytele type. They do not mature inside the female and putatively serve to protect the developing larvae once the embryo strand is laid. This specialized way of fertilization is to our knowledge novel and so is this first account of cnidocytes being directly involved in cnidarian reproduction.

Cell proliferation in cubozoan jellyfish Tripedalia cystophora and Alatina moseri.

Cubozoans (box jellyfish) undergo remarkable body reorganization throughout their life cycle when, first, they metamorphose from swimming larvae to sessile polyps, and second, through the metamorphosis from sessile polyps to free swimming medusae. In the latter they develop complex structures like the central nervous system (CNS) and visual organs. In the present study several aspects of cell proliferation at different stages of the life cycle of the box jellyfish Tripedalia cystophora and Alatina moseri have been examined through in vivo labeling of cells in the synthetic phase (S phase) of the cell cycle. Proliferation zones were found in metamorphosing polyps, as well as in juvenile medusae, where both the rhopalia and pedalia have enhanced rates of proliferation. The results also indicate a rather fast cell turnover in the rhopalia including the rhopalial nervous system (RNS). Moreover, T. cystophora showed diurnal pattern of cell proliferation in certain body parts of the medusa, with higher proliferation rates at nighttime. This is true for two areas in close connection with the CNS: the stalk base and the rhopalia.

Ultrastructure of the retinal synapses in cubozoans.

Cubomedusae (box jellyfish) are well known for strong directional swimming, rapid responses to visual stimuli, and complex lensed eyes comparable to those of more advanced multicellular animals. They possess a total of 24 eyes that are of four morphologically different types, yet little is known about the neural organization of their eyes. The eyes are located on ganglion-like structures called rhopalia. Each of the four rhopalia contains an upper and a lower lensed eye (with a cornea, lens, and retina), two pit ocelli, and two slit ocelli. Transmission electron microscopy was used to examine the synaptic morphology of the eyes and pacemaker region of four species of cubozoans (Tamoya haplonema, Carybdea marsupialis, Tripedalia cystophora, and Chiropsalmus quadrumanus). Invaginated synapses were found in all four species, but only in the upper and lower lensed eyes. Density measurements indicated that the invaginated synapses were located close to the basal region of photoreceptor cells, and size differences of invaginated synapses were observed between the upper and lower lensed eyes, as well as between species. Four additional types of chemical synapses-clear unidirectional, dense-core unidirectional, clear bidirectional, and clear and dense-core bidirectional-were also observed in the rhopalia. The invaginated synapses of the lensed eyes may be useful as markers to help sort out the neural circuitry in the retinal region of these complex cubomedusan eyes.

The ring nerve of the box jellyfish Tripedalia cystophora.

Box jellyfish have the most elaborate sensory system and behavioural repertoire of all cnidarians. Sensory input largely comes from 24 eyes situated on four club-shaped sensory structures, the rhopalia, and behaviour includes obstacle avoidance, light shaft attractance and mating. To process the sensory input and convert it into the appropriate behaviour, the box jellyfish have a central nervous system (CNS) but this is still poorly understood. The CNS has two major components: the rhopalial nervous system and the ring nerve. The rhopalial nervous system is situated within the rhopalia in close connection with the eyes, whereas the ring nerve encircles the bell. We describe the morphology of the ring nerve of the box jellyfish Tripedalia cystophora as ascertained by normal histological techniques, immunohistochemistry and transmission electron microscopy. By light microscopy, we have estimated the number of cells in the ring nerve by counting their nuclei. In cross sections at the ultrastructural level, the ring nerve appears to have three types of neurites: (1) small "normal"-looking neurites, (2) medium-sized neurites almost completely filled by electron-lucent vacuoles and (3) giant neurites. In general, only one giant neurite is seen on each section; this type displays the most synapses. Epithelial cells divide the ring nerve into compartments, each having a tendency to contain neurites of similar morphology. The number and arrangement of the compartments vary along the length of the ring nerve.