A detailed observation of the ejection and retraction of defense tissue acontia in sea anemone (Exaiptasia pallida)

Acontia, located in the gastrovascular cavity of anemone, are thread-like tissue containing numerous stinging cells which serve as a unique defense tissue against predators of the immobile acontiarian sea anemone. Although its morphology and biological functions, such as defense and digestion, have been studied, the defense behavior and the specific events of acontia ejection and retraction are unclear. The aim of this study is to observe and record the detailed process of acontia control in anemones. Observations reveal that the anemone,Exaiptasia pallida, possibly controls a network of body muscles and manipulates water pressure in the gastrovascular cavity to eject and retract acontia. Instead of resynthesizing acontia after each ejection, the retraction and reuse of acontia enables the anemone to respond quickly at any given time, thus increasing its overall survivability. Since theExaiptasiaanemone is an emerging model for coral biology, this study provides a foundation to further investigate the biophysics, neuroscience, and defense biology of this marine model organism.

On the food of the Antarctic sea anemone Urticinopsis antarctica Carlgren, 1927 (Actiniidae, Actiniaria, Anthozoa)

The results of an investigation into coelenteron content of the Antarctic sea anemone Urticinopsis antarctica Carlgren, 1927 are presented. Remains of invertebrate animals and fishes were found in the gastrovascular cavity of anemones. Some of them were damaged by digestion and were considered as food items of U. antarctica. These items were molluscs Addamussium colbecki (Smith, 1902), Laevilacunaria pumilia Smith, 1879, Eatoniella caliginosa Smith, 1875 and one not strictly identified gastropod species from the family Rissoidae; a crinoid from the family Comatulida; sea-urchin Sterechinus neumayeri Meissner, 1900; ophiuroid Ophiurolepis brevirima Mortensen, 1936 and a fish Trematomus sp. In contrast to the prey mentioned above, three specimens of amphipods Conicostoma sp. were not destroyed by digestion. They may represent commensals, which live in the gastrovascular cavity of the anemone.

Life cycle of Craspedacusta sowerbyi xinyangensis

To explore the life cycle of Craspedacusta, the authors collected male and female specimens of the Craspedacusta sowerbyi xinyangensis in a small fire-fighting pond in Ningbo, Zhejiang Province in July, 2005 and 2006. The development of C. sowerbyi xinyangensis was studied from zygote to medusa by means of light microscopy and digital camera. The zygotes of C. sowerbyi xinyangensis are globular and smooth (90 - 105 μm diameter) and have an equal, total cleavage to the two-cell stage 15 min after fertilization. The embryos enter the four-cell stage after another 15 min and become multicellular embryos after 3h 15 min. At this stage the embryos have a diameter similar to fertilized eggs but have uneven surfaces that are distinct from the smooth surfaces of the uncleaved zygotes. Solid gastrulae are formed 7 h after fertilization. These are spherical planulae with short surface cilia that begin to swim in slow clockwise circles. After 12 h, they lose their cilia, cease swimming and become elongated planulae with one end larger than the other. Rod-like planulae, similar in thickness at both ends, are formed after an additional 7 h. After 4 days, the planulae develop into tiny polyps having two germ layers and a gastrovascular cavity. The polyp mouth is 50 - 62 µm in diameter, lacking tentacles but having nematocysts around the mouth. Planulae become mature polyps after 10 days (15 days after fertilization). Medusa buds (45 - 88 μm diameter) are formed by polyp budding, which soon become free-living medusae with 8 tentacles (380 - 620 μm diameters). Sometimes, the movement of frustules, which are formed by the polyps and similar to planulae in morphology can also be observed.

Polymorphism in hydroids: the extensible polyp of Halecium halecinum (Cnidaria: Hydrozoa: Haleciidae)

The study of living Halecium halecinum colonies revealed a new case of zooid polymorphism. Besides the ordinary hydranth, the polyp devoted to feeding, this species is provided with a second kind of polyp, differing only slightly in structure and morphology but most conspicuously in its behaviour. It is named ‘extensible polyp’ in reference to its great extensibility and the resulting filiform shape. There are slight differences in the tentacles: lower number, shorter length, thicker diameter, and the tip slightly swollen and rounded instead of tapering. Their large microbasic mastigophores are abundant and evenly distributed, while the hydranth has a few large ones only on the oral side but has otherwise numerous small ones. When extended and at rest, the tubular column is much longer than that of the hydranth and not delimited from the head of the polyp by a bulge followed by a constriction. Behavioural differences are its capacity to coil and bend during extension and thus being able to move in all directions and exploring a large volume of seawater, and also its ability to produce regional swellings (peristalsis) and to contract by folding and bulging though still extended. Besides a probable role in defence, the extensible polyp exhibits an excretory function and it could also have sensory functions. The extensible polyp type is not classified as a nematophore because it has a functional gastrovascular cavity and a mouth. Polyp dimorphism (hydranth/extensible polyp) is reported in one more halecid and two sertularids.

Cnidaria

The phylum Cnidaria or Coelenterates includes sea anemones, jellyfish, hydras, sea fans, and, of course, the corals. With few exceptions they are all marine organisms and most are inhabitants of shallow water. In spite of the great variation in shape, size, and mode of life, they all possess the same basic metazoan structural features: an internal space for digestion (gastrovascular cavity or coelenteran), a mouth, and a circle of tentacles, which are really just an extension of the body wall. The body wall in turn is composed of three layers: an outer layer of epidermis, an inner layer of cells lining the gastrovascular cavity, and, sandwiched between them, a so-called mesoglea (Barnes 1980). All these features are present in both of the basic structural types: the sessile polyp and the free-swiming medusa. During their life cycle, some cnidarians exhibit one or the other structural type whereas others pass through both. Most Cnidaria have no mineralized deposits. The ones that, to date, are known to have mineralized deposits are listed in Table 5.1. They are found in both the free-swimming medusae and the sessile polyps. Not surprisingly, these have very different types of mineralized deposits. In the medusae they are located exclusively within the statocyst where they constitute an important part of the organism’s gravity perception apparatus. Interestingly the statoconia of the Hydrozoa, examined to date for their major elemental compositions only, are all composed of amorphous Mg-Ca-phosphate, whereas those of the Scyphozoa and Cubozoa are composed of calcium sulfate. Calcium sulfate minerals (presumably gypsum) are not commonly formed by organisms and the only other known occurrence is in the Gamophyta among the Protoctista. Spangenberg (1976) and her colleagues have expertly documented this phenomenon in the Cnidaria. (For a more detailed discussion of mineralization and gravity perception see Chapter 11.) The predominant mineralized hard part associated with the sessile polyps is skeletal. These can take the form of skeletons composed of individual spicules, spicule aggregates, or massive skeletons. They are composed of aragonite, calcite, or both.