Arai (1997), figure 1.5 (p. 7); (b) Redrawn from Mayer (1910), Vol. III, figure 352.
Semaeostomae
The semaeostomes include the medusae most typical of the class and most familiar to beachgoers. Medusae are large, typically 5–40 cm, with a bell shape ranging from saucer‐like (Aurelia) to bowl‐like (Chrysaora) and lack a coronal groove. Tentacles are found along or below the margin of the umbrella, which may be divided into eight or more lappets. Most typical of the semaeostomes are the long frilly oral arms that originate at and form the corners of the mouth (Figure 3.10).
Life histories for many of the semaeostomes are known in detail, owing mainly to their abundance near shore. Two life histories have been noted. The sequence of the first (Figure 3.12a), typified by the moon jelly Aurelia, has a planula larva that develops from the fertilized egg and settles to the bottom to form a scyphistoma. The scyphistoma strobilates to form ephyrae that develop into either a mature male or female medusa. In the second life history strategy, seen in the genus Pelagia, the planula develops directly into an ephyra and then into a mature medusa, bypassing the benthic polyp, or scyphistoma, stage (Figure 3.12b).
Figure 3.12 Life cycles of scyphozoans. (a) Aurelia sp. (moon jellyfish); (b) Pelagia sp. (purple‐striped jellyfish). (c) Stomolophus meleagris (cannonball jellyfish).
Sources: (a) Bayer and Owre (1968), figure 161 (p. 105); (b) Bayer and Owre (1968), figure 162 (p. 106); (c) Adapted from Calder (1982), figure 4 (p. 156).
Genera include Aurelia, Chrysaora, Cyanea, Stygiomedusa, and Pelagia. Pelagia, as the name implies, is common in offshore waters
Rhizostomae
Also known as the “cannonball jellies” because the rounded bells of some species (e.g. Stomolophus meleagris) resemble an old‐fashioned cannonball (dangling an old‐fashioned petticoat: Figure 3.12c), the rhizostome medusae lack marginal tentacles and exhibit a proliferation of the oral arms (Figure 3.13). During early development, the lobes of the original four‐lobed mouth in the very young medusa grow and bifurcate (Figure 3.13a–c) to form the (usually eight) “mouth arms” typical of the group. As the oral arms develop, the fringed edges close over to form an inner tube or brachial canal that effectively eliminates the central mouth. Once the canal is closed over, the fringed edge forms an outer groove that communicates at intervals with the inner brachial canal through smaller canals that terminate in “suctorial mouths.” Most often the oral arms are invested with filamentous appendages that bear the suctorial mouths along with nematocyst batteries and mucous cells that aid in prey capture (Hyman 1940) (Figure 3.13d–f). The brachial canals lead to a central stomach that conveys nutrition to the bell via radial canals.
Figure 3.13 Rhizostomeae. Development of the mouth arms of Mastigias: (a) early stage, (b) lobe development, and (c) later stage. (d) Mouth arm of Cassiopeia showing the gastrovascular canals; (e) adult Cassiopeia. (f) Adult Rhizostoma pulmo.
Sources: (a) Adapted from Uchida (1926); (b and c) Kaestner (1967), figure 5‐17 (p. 105); (d and e) Hyman (1940), figure 172 (p. 525); (f) Redrawn from Mayer (1910), plate 73.
The life histories of rhizostome medusae are typical of the Scyphozoa (Figure 3.12c) in having a planula larva that settles to the bottom and forms a polypoid scyphistoma. Scyphistomae may strobilate to form ephyrae or may produce other scyphistomae by budding.
The rhizostomes are chiefly a tropical–subtropical group inhabiting shallow waters, though two genera, Rhizostoma and Stomolophus, are found in temperate climes and may even form blooms. Stomolophus nomurai forms huge blooms in the Sea of Japan at intervals (Mills 2001). Rhizostomes are generally quite large, with sizes ranging from 4 to 200 cm across the bell.
Genera include: Rhizostoma, Mastigias, Cassiopeia, Stomolophus, Cephea.
The Cubomedusae
The Cubomedusae, variously known as the box jellies, sea wasps, or fire medusae, comprise the Cnidarian class Cubozoa. The Cubomedusae were formerly considered to be an order in the Scyphozoa. Now they are considered their own class and comprise two orders, the Carybdeida and the Chirodropida. The group includes about 48 species found in tropical and subtropical latitudes.
Bells of the Cubomedusae are indeed cuboidal, with four flattened sides that are square when viewed from the exumbrellar side or in transverse section (Figure 3.14a). The bell has a simple margin that bends inward to form a velarium (Figure 3.14b). Tentacles arise from the four corners of the bell, either singly or in groups. The base of the tentacle(s), the pedalium, is a blade‐like structure that gives rise to a single tentacle in the family Carybdeidae and a palmate structure that gives rise to several tentacles in the family Chirodropidae (Figure 3.14c and d). Below the pedalium, the tentacle is hollow and armed with rings of nematocysts.
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.
Foraging Strategies
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
