What is a Gelatinous Animal?
Many people know jellyfish and other gelatinous animals only as unidentifiable jelly-like blobs on the beach. Jellyfish are also often treated as a scourge, deadly creatures that should be avoided due to their venomous sting. In reality, jellyfish and other gelatinous animals are among the most beautiful and stunning animals of the sea. Although a few types may present some danger to people, most are harmless. And you won't need to worry about dying after a jellyfish encounter on the Pacific coast - we don't have anything like the deadly sea wasp (box jelly) of Australia.
Take a jellyfish out of water and it collapses into a quivering blob. The disparate creatures comprising the gelatinous animals are united by having bodies lacking hard skeletal components. With tissues composed of 95% or more water, they are usually exceedingly delicate and easily damaged, with a gelatin-like consistency. Unlike a fish, which retains its form when out of water, a jelly requires the support provided by the aqueous environment. Gelatinous animals are remarkable in their abilities to swim and capture food without the aid of hard skeletal parts (with the exception of some pelagic molluscs that have jaws and a radula).
True jellyfish, or medusae, are members of the Phylum Cnidaria. This diverse and widespread phylum also includes corals and anemones. Cnidarian jellies have a simple tissue organization, with a thin outer ectoderm and an inner endoderm layer. The ectoderm serves a protective function, while the endoderm is involved with digestion of food. Between the two layers, forming the bulk of the volume of the animal, is the gelatinous mesoglea. This flimsy material is typically about 96% water, with the remainder about 3% various types of salts and only 1% organic material. The organic content includes a mucopolysaccharide matrix with fibers of collagenous material (sounds nasty!). Although it doesn't appear like much, the mesoglea manages to serve as a scaffolding for the tissue of the jellyfish, including a platform for bell muscles to pull from. Jellyfish lack true organ systems but nevertheless get along just fine without the aid of a heart, brain, liver or kidneys. Unlike the bilateral symmetry that humans and other vertebrates (and most invertebrates) are accustomed to, the cnidarian body plan is based on a radial symmetry. A slice from any angle through the anterior-posterior (oral - aboral) axis of a jellyfish or anemone yields two comparable halves.
Within the Cnidaria are those in the class Scyphozoa, the most familiar of the gelatinous animals, with representatives such as sea nettles and moon jellies. Many are large and conspicuous and typify what comes to mind when people think of jellyfish. All are equipped with the unique cnidarian stinging structures known as nematocysts, which enable them to be effective predators despite their lack of jaws and teeth. Nematocysts contain a variety of toxins and are typically concentrated in the sinuous tentacles and mouth region. Other types of nematocysts are sticky and function to ensnare and hold onto prey. Jellies vary in the number and length of tentacles but generally use them to incapacitate zooplankton prey such as copepods, krill, larval fish, invertebrate eggs and larvae, and other gelatinous animals. Jellies in the orders Coronatae and Semaeostomeae typically use nematocyst-laden tentacles for prey capture. The Rhizostomeae, a tropical order of scyphozoans whose members lack tentacles, utilize fused oral arms with multitudes of mouth openings to form a kind of zooplankton filter.
Less familiar to most people are jellies in the Class Hydrozoa, which includes a bewildering array of smaller and generally inconspicuous species. Many are transparent and easily overlooked. Our most potent stingers, the siphonophores, are within this group, as is the tropical terror, the Portuguese man-of-war (Physalia). One species within the cnidarian class Cubozoa (the box jellies) occurs along the coast of southern California and Baja California, but it does not have a lethal sting like some of its Indo-Pacific relatives.
Cnidarian jellies typically use their bell for a sort of jet propulsion. By contracting the thin layer of muscle tissue in the bell, the margin is moved inward and water is expelled, propelling the animal forward. Acting as a hydrostatic skeleton, the gelatinous bell springs back to its resting state when the muscle contraction ceases. With hydrozoan jellies, the frequency of bell pulsing is influenced by sensory information from the light-sensing ocelli positioned around the bell margin. The signal is transferred by motor neurons to the bell musculature.
A group unrelated to cnidarians, the phylum Ctenophora (ctenophores), is known as comb jellies. Like the true jellyfish, ctenophores are predators on a wide range of zooplankton. Ctenophores typically have a solid globe-like body, rather than the pulsating bell of cnidarians, and utilize unique rows of cilia organized into a series of plates that beat in an oar-like fashion with highly synchronized waves. They also lack the stinging nematocysts of jellyfish and use other means for collecting food, including sticky tentacles, mucous-covered oral lobes and large mouths.
Some groups less familiar to most people have also taken up a gelatinous existence. Among the more abundant gelatinous zooplankton are the various types of pelagic gastropods within the phylum Mollusca. Lacking a hard, dense shell and using modifications of the molluscan foot for locomotion, many of these swimming snails barely resemble their benthic relatives. Few people are aware that some of the most common planktonic animals are gelatinous snails. Predatory heteropods possess a single swimming fin to scull through the water while seeking prey. Pteropods use a pair of swimming wings, and some are known as sea butterflies due to the manner in which they flap them. One group of pteropods uses various styles of mucous nets to graze on phytoplankton and other small particles. Another group is predatory, using complex mouth structures to capture gelatinous prey.
The tunicates are best known by the sessile benthic forms, the sea squirts. Tunicates are a subphylum within the phylum Chordata, to which humans and all other vertebrates also belong. Two groups of less familiar tunicates have taken to the open sea for their entire life cycle. The thaliaceans include salps, doliolids and pyrosomes. Salps are remarkable grazers that are among the fastest growing organisms and are capable of forming incredibly dense populations by a combination of rapid sexual and asexual reproduction. Salps and doliolids employ sets of ring-like muscle bands in their cylindrical bodies. By contracting these muscles while regulating the opening and closing of a pair of body openings, the pelagic sea squirts swim quite efficiently by jet propulsion. Another group of tunicates, the tadpole-like appendicularians (or larvaceans), are generally inconspicuous but ecologically important grazers. Like some of the pteropods, they use a mucous net to collect microscopic planktonic organisms. Swimming and water filtration are accomplished by rhythmic beating of the tail.
Properties of Gelatinous Animals
At first glance it seems to be a rather foolish strategy for a group of animals to have a gelatinous lifestyle. Being somewhat slow swimmers with easily damaged tissue and no hard skeletal parts might appear to be a clear path to extinction. The abundance and wide diversity of gelatinous animals is testimony, however, to the numerous advantages that this body plan presents. When thinking about jellies we need to suspend our bias toward hard skeletons with thick muscles and other relatively dense tissues.
With bodies that are 95% or more water, and lacking bone and thick muscles, gelatinous animals have come up with a solution to the problem of maintaining buoyancy in the water column. Rather than constructing air bladders, harboring lipid-dense bodies or using energetically expensive swimming, jellies in essence become one with their environment since they are mostly water. While some gelatinous animals, such as certain pteropods that retain a remnant of the shell, must swim to compensate for the extra weight, this requirement is far less than would be required for a comparably-sized denser animal. Buoyancy can also be enhanced by the exclusion of certain ions (primarily sulfate) or accumulation of others (ammonium) in body tissues. Lipids, which are less dense than water and stored by zooplankton such as copepods for buoyancy, are not typically used by gelatinous animals.
Gelatinous zooplankton have a far higher water content on a percentage of mass basis than other marine creatures such as fish or crustaceans. With their high water content and relatively low carbon density, there is a greater potential during growth for rapid increase in size on a volume basis. This helps render them a poor source of food for predators lurking about. The low carbon density enables a low metabolic rate to be maintained, which can be an advantage in that food requirements are lower. As a consequence, many gelatinous animals can withstand long periods without food, sometimes shrinking in size during these periods. When presented with an abundance of food, however, many jellies are opportunistic and can respond with rapid increases in size or population density. Certain species can also regenerate parts of their bodies that have been damaged or bitten off by a predator.
A large surface area relative to carbon content also results in an increased area for capture of prey and gas exchange. Certain medusae and siphonophores act like living drift nets, exposing a large tentacle network to passing zooplankton prey. Larvaceans and thecosome pteropods utilize large, delicate mucous webs to gather tiny food particles. With their thin tissues and high surface area exposed to the environment, cnidarian jellies and ctenophores have no need for specialized gas exchange structures. Pelagic gastropods and tunicates may have reduced gills but can also exchange gases over much of the body surface. Associated with the gelatinous body plan are severe osmotic problems in dilute ionic environments. Fortunately for them, with the exception of a few species adapted to fresh and brackish water, most will never encounter such situations.
Another property of gelatinous tissue is that it is often transparent to light. Transparency is useful for avoidance of visual predators in sunlit open water habitats that lack shelter. This is particularly important for delicate gelatinous creatures since the absence of hard body plates and the relatively poor swimming ability renders most quite susceptible to predation. While medusae, comb jellies and pelagic molluscs may have limited escape responses, they are still slow compared to fishes and turtles that are seeking a meal. Only the cnidarian jellies with their stinging nematocysts can mount any serious defense against predators. With their capable defenses, many scyphomedusae such as purple-stripe jellies and sea nettles have abandoned the transparency strategy. Many gelatinous animals are also bioluminescent. Production of intense flashes of light are thought to provide some defense against visual predators, who can be confused or startled by the display.
Although most gelatinous animals are capable of some type of locomotion, their long-distance travels are generally under the control of currents and other patterns of water movement. For this reason they are regarded as having a drifting lifestyle. The disadvantage is that they may be carried into areas that present multiple dangers, such as near shore. The upside is that with little expenditure of energy they can be dispersed over wide areas. A number of fish, crabs, amphipods and other hitchhikers have taken advantage of this by developing associations with wandering gelatinous hosts.
Gelatinous cnidarians, ctenophores and thaliaceans have relatively low carbon to dry weight ratios (less than 15%). Some of the pelagic molluscs are more semi-gelatinous, with values ranging between 20 to 30%. For the purposes of the JelliesZone, we will consider all of these species to be gelatinous.
Perhaps our gelatinous friends are on to something. Simple can be better.