The Hydromedusae, a subclass within the Class Hydrozoa (link from UC Berkeley Museum of Paleontology), Phylum Cnidaria, include a bewildering variety of mostly small, often inconspicuous medusae. Life histories of hydrozoan species can vary from those that are dominated by the polyp phase (lacking a medusa) to those that are exclusively medusoid. Compared to the usually larger and flashier scyphomedusae, medusa within this subclass typically possess transparent tissue that renders them difficult to see in the water. The smallest species require careful microscopic examination of the contents of a plankton tow in order to see them.
The typical hydromedusa has a globular umbrella, or bell, that like scyphozoan jellies is used for a type of jet propulsion. The convex outer surface is known as the exumbrella while the surface underneath is the subumbrella. A muscular flap of tissue surrounding the margin of the bell, known as the velum, is one feature that distinguishes hydromedusae from scyphomedusae. Other distinguishing characteristics include a noncellular mesoglea (the gelatinous material forming most of the bulk of the bell) and a simple stomach that lacks divisions intoseparate sections. Hydromedusae possess a simple nervous system known as a nerve net in which nerve cells are spread out in the ectoderm of the subumbrella, manubrium and tentacles. With a nerve net, an impulse at any point will propagate in all directions throughout the system. Around the bell margin, between the bases of the tentacles, lie specialized sensory structures. In many species this consists of a number of statocysts, structures with a calcareous ball (the statolith) that determine the orientation of the bell. The rate of pulsing of the bell, and hence the swimming speed and direction of the jellyfish, can be modified by sensory information from the statoliths as they move around within the statocyst sacs. Additional sensory cells are held within swellings at the base of each tentacles, the tentacular bulbs.
Most hydromedusae possess long hollow tentacles for capturing prey such as other jellies, crustaceans, larval fish, eggs and other types of plankton. Tentacles harbor numerous stinging nematocysts for immobilizing prey. Although endowed with various types of toxins, the nematocysts of the vast majority of hydromedusae (and all those inhabiting the West Coast) cannot seriously harm people. From the center of the subumbrella hangs the manubrium, which terminates with a mouth opening. The manubrium leads into the gastric cavity (you can call it a stomach if you like), which then leads to 4 radial canals. Products of digestion are passed by the radial canals through the mesoglea to a ring canal that lines the bell margin. It doesn’t end here yet. The ring canal is in turn connected to the tentacular canals in each of the hollow tentacles. Digestion can take place throughout this system of canals, hence the term gastrovascular cavity for the complete structure. Partially digested food is taken up throughout the gastrovascular cavity by endodermal cells for complete intracellular digestion.
Hydromedusae are typically classified into 5 orders: Anthomedusae, Leptomedusae, Limnomedusae, Narcomedusae and Trachymedusae. Anthomedusae and Leptomedusae are characterized by a sessile (attached) phase of the life cycle known as a polyp, or hydroid. The hydroid phase produces the medusae, although many species in these two orders do not produce free-swimming medusae. Anthomedusae tend to have a bell that is taller thanwide, with gonads located on the stomach. All lack statocysts, which are sensory structures used for balance and orientation. Polyp feeding ends (where the tentacles are located) are not protected by a chitinized cup (the theca), hence the alternative name Athecata for this order. Leptomedusae typically have a more flattened bell, with gonads attached to the radial canals. Most possess statocysts in the bell margin, and some have light sensitive pigment spots known as ocelli. Feeding ends of polyps are enclosed in a theca, so you may encounter the name Thecata for the group. The medusae of a majority of species within these groups inhabit nearshore waters near the surface.
The Limnomedusae is a relatively small group of generally inconspicuous jellies, some of which live in freshwater habitats. Gonads may be attached to the stomach or on the radial canals. Polyps, which may be solitary rather than colonial, are tiny and can produce additional polyps by asexual budding. Medusae are produced by a budding process, with the polyps remaining to create more medusae.
Narcomedusae and Trachymedusae lack an attached polyp phase, and are thus considered to have a holoplanktonic life cycle. Fertilized eggs typically form planktonic larvae that develop directly into the medusa. Among the Trachymedusae this larvae is known as an actinula, which is free-swimming and resembles a polyp without a stalk. Trachymedusae typically possess a bell taller than wide and have gonads attached to the radial canals. Statocysts are typically located on the bell margin. Tentacles tend to be long but very fragile, and are frequently broken off following collection. Narcomedusae can be distinguished by their relatively thick solid tentacles that originate well above the bell margin and may be held above the bell. The bell generally is formed by a stiff central mesoglea with scalloped edges. They also produce an actinula larva, but in most species each actinula buds a number of others before transforming into the adult body form. Most members of these two orders are found in oceanic or deep-water habitats, although some may venture into nearshore surface waters. The number of species is relatively few compared to the number of Anthomedusae and Leptomedusae.
The species listed in the right side-bar menu are among the more common hydromedusae of Central California waters and elsewhere along the West Coast. They by no means include all the species you may encounter.
More about Hydromedusae from Claudia Mills