Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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comprise two orders, the Carybdeida and the Chirodropida. The group includes about 48 species found in tropical and subtropical latitudes.

      A four‐sided manubrium leads to a simple, central stomach that is located at the apex of the sub‐umbrellar surface. The stomach differentiates into four gastric pockets that occupy the flattened sides of the umbrella.

      The life history of the Cubomedusae is much like that of the hydromedusae and scyphomedusae with one important difference: the polypoid stage of the Cubomedusa does not strobilate. Rather, each polyp metamorphoses into an individual medusa. Arneson and Cutress (1976) described the development of Carybdea alata in Puerto Rican waters as proceeding from a released blastula stage to a swimming planula in 1 day, settlement of the planula in an additional 4 days, growth and maturation of the polyp for about 60 days, and the metamorphosis culminating in a liberated medusa taking an additional week, for a total of about 75 days for the entire process. Temperature during development was 26–29 °C.

      Genera include: Carybdea, Tripedalia, Tamoya, Chirodropus, Chiropsalmis, and Chironex.

      The subject of foraging strategies covers a lot of ground, from diets and prey selectivity to models of encounter rates and predatory behavior (e.g. Gerritsen and Strickler 1977). Our chief concern is to describe what is known about feeding in medusae, including both diet (favorite foods) and elements of the feeding behavior itself. What is important to a weakly swimming, tentaculate, predator?

      In most studies of feeding in medusae, hydromedusae are grouped together with scyphomedusae, siphonophores, and sometimes even ctenophores. Clearly, though differing in complexity and size, many elements of hunting will be highly similar between the hydromedusae, scyphomedusae, and cubomedusae because of their similar body shape. The section on foraging strategies will regard the medusae as a whole, though crossing taxonomic boundaries, as will the discussion of locomotion and energetics.

Schematic illustration of cubomedusae.

      Sources: (a–c) Conant (1898); (d) Redrawn from Mayer (1910), plate 47.

      General Considerations

      Perspective is important when evaluating the foraging behavior of medusae. Equipped with rudimentary sensory systems and limited locomotory capabilities, they forage in a profoundly three‐dimensional environment. Prey are captured on tentacles deployed in a stationary ambush or a slowly moving array as the animal swims forward. Stinging cells (nematocysts) on the tentacles paralyze the prey, which are then conveyed to the mouth and digested. Since both locomotion and the sensory field are quite limited, feeding success of a medusa will be determined by the number of its physical encounters with prey and the effectiveness of its tentacles in subduing the prey item.

      The Cnidae

      The stinging organelles, or cnidae, that give the phylum Cnidaria its name are highly complex intracellular structures unique to the phylum. They are formed inside cells called cnidoblasts (Brusca and Brusca 2003), which are formed from interstitial cells in the epidermis and gastrodermis. The mature cnida in its cell is a cnidocyte. The majority of cnidocytes are located on the tentacles in small, blister‐like groups called “batteries” or in the epidermis of the oral region.

      Their older name, nematocysts, is still very much in use, and the term cnidocyte then becomes nematocyte. In the newer terminology, only the stinging cnidae are termed nematocysts, to distinguish them from other types of cnidae that, for example, stick to prey (spirocysts) instead of envenomating them (Brusca and Brusca 2003).

      Nematocysts, or cnidae, are considered to be “independent effectors”: their discharge is not governed by the nervous system of the medusa but will discharge when stimulated directly by prey contact. The nematocyst has a “lid” or operculum (Figure 3.15) that covers the capsule and acts as a trapdoor. When the cnidocyte discharges, the operculum is flung open. The cnidocil, a bristle located next to the operculum, is believed to be the mechanoreceptor or “trigger” responsible for nematocyst discharge. Though the cnidocytes are considered independent effectors, their sensitivity threshold can be modified by the nutritional state of the medusa. A starved medusa will have a lower threshold for discharge than a well‐fed one.

Schematic illustration of nematocyst structure.
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