Cassiopeia xamachana is known as the "upside down jellyfish" because it lies on the bottom with the aboral surface down, against the sand, and the oral surface up, exposed to the overlying water. It inhabits shallow, quiet, sunlit, tropical, marine waters, usually on soft bottoms. It harbors zooxanthellae in its tissues and can exist, and even grow, utilizing photosynthesis as its sole source of nutrition.
The body of a jellyfish is known as the bell (Fig 1) because it is usually shaped more or less like a bell. That of Cassiopeia, however, is flat. The aboral surface (= exumbrella) of the bell is smooth and slightly concave (Fig 1). The oral surface (= subumbrella) bears a complex of frilly arms and appendages and the two surfaces are easily distinguished from each other.
Figure 1. Cassiopeia in side view.
Gently turn the animal over so the frilly oral surface faces up. This is the animal's life position and its orientation when feeding. The oral surface is covered by the feeding apparatus consisting of the manubrium and the oral arms. The manubrium is a short central column emerging from the center of the oral surface of the bell (Fig 1). The short manubrium hangs below it.
Eight frilly oral arms are attached to the free end of the manubrium and radiate outward from it (Fig 1).
Figure 2. Oral surface of Cassiopeia from Big Pine Key, Florida. Only one of the eight oral arms is shown in its entirety. The oral collars have been removed from all but two of the secondary mouths.
Figure 3. Diagrammatic representation of the brachial canals and axes of Cassiopeia.
Cassiopeia rests on its "back" (aboral surface) on the bottom and pulsates the margins of the bell to move fresh water over the surface of the arms. This brings small crustaceans (and oxygen) to the oral (upper) surface of the arms. The food organisms are stung by cnidocytes, entangled in mucus, and moved into the secondary mouths. Food items enter the brachial canals via the ciliated secondary mouths and are moved through the canal by a ciliary current to the stomach. The arrangement of these mouths on the highly branched arms is the basis for the name rhizostome, or "root mouth". If you have a living animal, watch the margins of the bell pulsate. Add some dye/seawater to visualize the currents generated by the pulsations. Do the pulsations move water over the oral arms as stated ?
Look for the small secondary mouths on the oral surfaces of the arms. Each is surrounded by a ruffled lacelike oral collar of tissue. The oral surface of each arm has a frilly appearance due to the presence of these hundreds of collars. The edge of each collar bears tiny papillae. The collar is ciliated and generates the current that moves food into the mouth. The secondary mouths open when food is present.
A brachial canal is visible just under the oral surface of each arm. Find the canal on one of the arms and follow it to the center of the oral surface. The canal is very close to the oral surface. Its position near the surface is consistent with its being formed by closing the brachial groove by an overgrowing tissue layer. Note that its branching pattern reflects that of the arm itself. The secondary mouths open into it the branches. Note that there is no primary mouth at the center of the animal at the point where all the brachial canals converge.
In addition to the mouths, there are numerous flattened, club-shaped vesicular appendages that also open into the brachial canals. The number, size, and color of these appendages varies with sex and age. Their function is not well understood. The vesicular appendages are flat, elongate, and of many sizes. The interior of each large vesicle may be purple, sometimes colorless, bluish, or blue-green. The inner epithelium of the appendages is gastrodermis and bears numerous small, nearly spherical nematocytes and zooxanthellae that are easily seen in a wet mount. The vesicles are used in trapping small crustaceans such as copepods and amphipods which are then stung to death. They also provide surface area for the deployment of zooxanthellae in the sunlight. They may also help camouflage the jelly in grass beds. Small clusters of nematocysts on their outer surface break easily from the vesicle and presumably discourage predators. Disturbed jellies fill the nearby water with these nematocysts which may then sting nearby predators or even swimmers.
The eight arms are joined at their bases to the thick, but short, vertical manubrium that connects with the bell (Fig 1). Remember that the arms are in pairs. The two brachial canals from the two arms of a pair join each other upon reaching the manubrium and form a larger manubrial canal that runs vertically in the manubrium, perpendicular to the brachial canals, to enter the stomach (Fig 2). The stomach is a central space located at the aboral end of the manubrium in the bell. There are four columnar canals, one for each of the original four oral arms. Note that a short vestigial canal runs to the center of the oral surface from the point of confluence of the brachial canals (Fig 2). There are four of these little canals and they join at the point where the central mouth would be if there were one.
The margin of the bell is down-turned (turned toward the oral side) and is slightly scalloped. Evenly spaced around the margin are 16 (usually) sense organs, or rhopalia .
Bigelow, R. P. 1900. The anatomy and development of Cassiopeia xamachana. Boston Soc. Nat. Hist. Mem. 5:191-236, pls 31-38.
Hyman, L. H. 1940. The Invertebrates: Protozoa through Ctenophora. McGraw Hill, New York. 726 p.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.