Distinguishing the Major Gelatinous Groups
At first glance, there appears to be a bewildering array of gelatinous creatures, and it may be difficult to figure out what you are looking at. If, however, you pay attention to certain key features of a gelatinous animal, you should be able to determine its group affinities. The following are some of the diagnostic characteristics you can use to narrow down your determination.
Hydromedusae: Single radially symmetrical swimming bell with marginal tentacles and flap-like velum around margin. Tentacles with nematocysts. Usually small and relatively transparent with little pigmentation and mesoglea that lacks cells. Gastric cavity a simple sac without separate pouches. Predators on various types of zooplankton.
Siphonophores: Colony of polymorphic individuals in the form of a chain, some species exceeding 100 feet in length, but usually only a few inches long. Consists of one or more swimming bells (nectophores), feeding individuals (gastrozooids), protective individuals (dactylozooids), reproductive individuals (gonozooids) and leaf-like, usually transparent bracts. Some with an apical gas-filled float. Long tentacles, which may be difficult to see, that often have a potent sting. Predators on zooplankton and fish larvae.
Scyphomedusae: Single radial symmetrical swimming bell with marginal tentacles and no velum, but usually with scallops at margin. Often large, conspicuous, and with pigmentation. Thick mesoglea that may contain cells. May have long, frilly oral arms that lead to mouth. Gastric cavity with nematocyst-bearing filaments and partitioned into pouches. Tentacles with nematocysts, some species have an uncomfortable sting. Predators on various types of zooplankton, often including other medusae.
Ctenophores: Body with 8 rows of combs (ciliary plates), often seen as shimmering waves of color. May have tentacles, but these are not arranged around a bell margin. If present, tentacles nearly always with sticky colloblasts rather than nematocysts. Usually transparent with little, if any, pigmentation (except for some of the deep-water forms). Predators on various types of zooplankton.
Heteropods: Elongate body with single ventrally placed swimming fin, which is held upward. Sculling motion of the fin moves the animal forward. Well developed pair of eyes on a snout-like head. May have a coiled shell, remnant of a shell, or lack a shell completely. Active predators on salps, doliolids, chaetognaths and other gelatinous zooplankton.
Thecosome pteropods: Body with large pair of lateral plate-like extensions of the foot. Flapping action of these plates as wings propels the animal. Uses mucous web to passively gather planktonic food particles while drifting motionless. Some with a calcareous shell, others with a soft pseudoconch.
Gymnosome pteropods: Body with a relatively small pair of lateral muscular wings. Swims relatively rapidly using a quick beating motion of the wings. Body lacks any kind of shell. Distinct head with two pairs of antennae that may be retracted. Head terminates with a buccal (oral) apparatus housing a radula, specialized hook sacs and a jaw. Active predators on thecosome pteropods.
Salps: Body with incomplete circular bands of muscles (various numbers depending on the species), and an anterior and a posterior opening. Muscular pulsing of body wall pumps water through an internal mucous net that gathers tiny planktonic food. Locomotion by jet propulsion. May occur as single individuals or chains of asexually produced individuals (alternate sexual and asexual generations).
Doliolids: Relatively small, transparent body with complete bands of circumferential muscles (8 or 9), and anterior and posterior openings. Feeds on planktonic particles using currents created by cilia rather than pulsing of body. Hangs motionless until disturbed, and then exhibits a characteristic jumpy motion. Complex alternation of asexual and sexual generations.
Appendicularians: Body in the form of a “tadpole” with tail containing a notochord. Forms mucous house which usually surrounds the animal and collects microscopic planktonic particles for consumption. The inconspicuous animal can be seen inside or beside the much larger mucous house as it beats its tail to create feeding currents.