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Chapter 13

Radiate Animals

Position in Animal Kingdom n Both phyla Cnidaria and Ctenophora make up the radiate animals.

n Biradial symmetry is radial symmetry limited to two planes that create mirror images.

n Other eumetazoans have bilateral symmetry or their radial symmetry is derived from a bilateral ancestor.

n Neither Cnidaria nor Ctenophora have advanced beyond tissue level of organization although a few organs are seen.

Biological Contributions n Both phyla have two well­defined germ layers: ectoderm and endoderm; mesoderm may be derived from ectoderm.

n There is an internal body cavity: the gastrovascular cavity.

n Extracellular digestion occurs in the gastrovascular cavity; gastrodermal cells accomplish cellular digestion.

Biological Contributions n Most have tentacles, which are extensible projections for food capture.

n Radiates are the simplest animals with nerve cells; there is no central nervous system.

n Radiates are the simplest animals with sense organs: statocysts and ocelli.

n Locomotion is by muscular contraction or ciliary comb plates.

n The polyp and medusa forms allow wider ecological possibilities.

Definition n The Cnidaria are tentacle­bearing Metazoa, of the tissue grade of construction, with primary radial symmetry, composed essentially of two epithelia with a gelatinous matrix (mesoglea) between them, with nematocysts and with only one internal cavity, the digestive cavity, with a mouth, but no anus.

13­6

Phylum Cnidaria n Some ctenophores, molluscs and flatworms eat hydroids and use the stinging nematocysts for their own defense

n Four classes of Cnidaria n Hydrozoa n Scyphozoa n Cubozoa n Anthozoa

n A fifth class, Staurozoa, has been proposed n No medusae in life cycle but polyp topped by medusa­like region

Characteristics

n Entirely aquatic, some in fresh water, but mostly marine.

n Primary radial symmetry around a longitudinal axis with oral and aboral ends; No definite head

Characteristics n Two basic types of individuals: polyps and medusae.

Figure 13­2

Characteristics n Special cells, cnidocytes (from which the phylum gets its name), with stinging organelles, nematocysts, in either epidermis, gastrodermis or both.

n A primitive nervous system consisting of a nerve net at the base of the epidermal and gastrodermal layers, with impulse transmission tending to be radiating and with typically nonpolarized synaptic junctions.

Characteristics n Most feed on zooplankton, although some utilize larger animals and some are suspension feeders on fine particulate matter.

n Incomplete gut. Prey is caught, immobilized and killed with the nematocysts on the tentacles and digestion is initially extracellular, then intracellular.

Characteristics n Sense organs for balance (statocysts) and photosensitivity (ocelli)

n Asexual reproduction by budding (in polyps) or sexual reproduction by gametes (in all medusae and some polyps).

n Sexual forms monoecious or dioecious; Primitive gonads; Ciliated free­swimming planula larva in life cycle of most. (Image is from the Biodidac image data base maintained at the university of Ottawa.)

n No respiratory or excretory system and no coelomic cavity.

Form and Function n Polymorphism (poly = many; Morph = form). Many members of phylum display dimorphism and polymorphism. Two major morphological types are polyp and medusa, but there can be many different types of polyp in a colony creating polymorphic colonies.

Basic Body Forms n Polyp = hydroid form, adapted for sedentary or sessile life. Tubular body w/ mouth at one end surrounded by tentacles. Aboral end usually attached to a substratum by a pedal disc or by skeletal secretion or by stolons (root­like outgrowths).

Figure 13­1 B

Basic Body Forms

n May live singly or in colonies. Colonies of some species w/ more than one kind of individual (zooid or "person"), each specialized for a certain function, e.g. Feeding, reproduction or defense.

Colonial Hydroids (Class Hydrozoa)

n Usually have life cycles featuring both polyp and medusa stages ­ can take advantage of feeding in a bottom environment as a polyp and in the open water as a medusa.

Complex Hydrozoan Colonies

n The Portuguese Man­o­ war is actually a polymorphic colony of individual zooids serving differing functions.

Basic Body Forms n Medusa = jellyfish form, free swimming and sexually mature form, n Bell­shaped or umbrella­ shaped bodies,

n tetramerous symmetry, n Mouth centered on concave side, tentacles and one or more types of sense organs borne on rim of umbrella.

Physalia from Jamaica

Basic Body Forms n Medusa provides dispersal mechanism offspring don’t compete with parents. Mesoglea much thicker than in polyp ­ constitutes bulk of animal ­ makes it buoyant = "jelly" of jellyfish. n Mesoglea

n Gelatinous (at least 95% water) n Continuous in polyps, extending through body and tentacles

n Supports body n Thick in anthozoans, and scyphozoan medusae n Thinner in hydromedusae

Nematocysts

n Functions: n Hold prey. n Provide adhesive which aids in locomotion by attaching tentacles to substrate.

n Penetrate and anchor in prey and poison it.

Figure 13­3

cnidocil

Nematocysts • Chemical and mechanical stimuli

• Discharge 2 m/sec • Penetrates most biological substances

• Tips open, closed, some wrap

• Contents injected, adhesives, toxins

Nematocyst Poison n Most dangerous to man is Cubozoan

jellyfish, the sea wasp (Chironex fleckeri). Caused more human suffering and death off Australian coasts than Physalia (Portuguese man­ of­war) has in any of its home waters.

n Symptoms range from burning pain at site of contact with tentacles through skin lesions and eruptions of various sorts, often severe enough to leave scars, to generate great pain, fever, prostration, and respiratory interference. Most severe rxns due to shock = allergic rxn. "Sting" of most is imperceptible to humans. Problems usually occur only when repeatedly stung.

Nervous System

n First animals to possess one, so of much research interest.

n Diffuse system = nerve net. Conduction radiates. Synaptic vesicles on both sides of many synapses, so bi­ directional.

characteristics

n Cnidarian bodies extend contract, bend, and pulse

n No mesodermally derived muscle cells n Have epitheliomuscular cells that form most of epidermis,c over the organism and function in muscle contraction

Sense Organs / Statocyst

n Balance organ. n Ball of sandy or stony material shifts when animal moves.

n It stimulates surrounding neurons and provides animal with information about its position. caspar.bgsu.edu/.../Neuroet hology/ labs/images/Statocyst.jpg.

Sense Organs /Ocelli and Rhopalium

n Significant that first organs are for perception of external world.

n Ocellus is first primitive light­sensing organ.

n Usually found on bell of scyphozoan and Cubozoan medusae.

n Often combined with statocyst into rhopalium.

Class Hydrozoa

n 700 species, majority marine and colonial, a few fresh water (F.W.) Species (the only F.W. Cnidarians).

n Typical life cycle includes both asexual polyp and sexual medusa (some with no polyp and some with no medusa). Includes hydra which is not typical, but much studied because easy to get and maintain.

Class Hydrozoa n Colonies with polymorphism very common (e.g. Hydractinia) and Physalia. What is adaptive value of colonies? n If can’t specialize body areas into organs (no true mesoderm), specialize individuals in a colony. Get “superorganism”.

Class Hydrozoa – Life Cycles n There is general trend to reduced medusa stage and increased colonialism and polymorphism.

n In the previous scheme, the medusa is the most primitive form because it is sexual and therefore represents an adult form.

n Polyp is larval stage. Budding produced more polyps asexually. The budding is equivalent to formation of medusa buds in present polyps.

Class Hydrozoa – Life Cycles

n With the evolution of polyp, some groups reduced the medusa. First it remained attached, then the polyp epidermis itself produced the gametes.

Scyphozoa, Cubozoa and Anthozoa

n Jellyfishes (classes Scyphozoa and Cubozoa) are predominantly medusae w/ polyp stage very reduced or completely absent.

n Corals and anemones (class Anthozoa) are entirely polyps, no medusa.

Class Scyphozoa (the Common Jellyfishes)

n Pelagic cnidarians in which the medusa is the dominant and conspicuous form.

n Often seen in coastal waters and usually feared by local swimmers.

n Only about 200 species, but very abundant, so play important ecological role.

n Found everywhere in oceans, extending to depths of 3000 m or more.

Class Scyphozoa

n Larger and more brightly colored than hydromedusae. Compensate for increased size by increasing complexity of gastrovascular system (oral arms and radial canals) and by better control of swimming (no velum around bell).

Class Scyphozoa­Life Cycle n In some species the scyphistoma transforms directly into a young medusa, (additional evidence that the polyploid form was derived from a larval stage in the evolution of the cnidarians, or evidence that scyphozoans evolved from anthozoans?).

n In most species of scyphozoans young medusae are budded off transversely from the oral end of the scyphistoma which has become a strobila.

Class Cubozoa.

n Formerly classified as order in Scyphozoa. Cuboidal medusa with tentacles hanging from corners of medusa. Polyps very small and most often unknown. Active swimmers and feeders in warm tropical waters. Many poisonous (see sea wasp above and in text).

Class Cubozoa

n It is interesting to note that the margin of the bell of the cubozoan medusa has a fold of tissue analogous, but not homologous, to the velum of hydrozoans.

Class Anthozoa

n Either solitary or colonial polypoid cnidarians in which the medusa stage is completely absent.

n Include sea anemones, corals, sea fans and sea pansies. Largest Cnidarian class with over 6000 species.

Class Anthozoa­Polyp Structure n More complex than hydrozoan polyps. n Pharynx extends more than half way into the gastrovascular cavity.

n Mesenteries = longitudinal sheets that divide the gastrovascular cavity into radiating compartments. The edges of the mesenteries bear nematocysts. May be for internal gas exchange.

Class Anthozoa­Polyp Structure n Many traits in common with Scyphozoa (cellular mesoglea, gastrovascular cavity divided into compartments, cnidocytes in gastrovascular cavity and gastrodermal gonads) so probably more closely related to them than to hydrozoa.

Anthozoa

n Anemones have no secreted skeleton, are solitary and tend to be much larger than the corals.

n Corals are colonial and are often of extreme ecological importance because of their reef building.

Phylum Ctenophora

n A minor group n Fewer than 100 species

n All are marine n Have 8 rows of comb­like plates for locomotion

n Nearly all free­swimming n Many are bioluminescent

Compare to Cnidaria n Similarities with Cnidaria

n the “coelenterates” n mesoglea n symmetry – biradial and radial n Tissue Grade of Development

n Differences: n gastrulation ­ n ctenes ­ plates of fused cilia n rows of ctenes = costae n Do not have nematocysts (one exception)