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CTENOPHORA. Common Names. Ctenophores are often called “comb jellies,” “sea gooseberries,” “sea walnuts,” or “Venus’s girdles.” . Examples. There are two classes in the phylum Ctenophora : Tentaculata , of which the most characteristic species is Hormimorpha plumosa
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Common Names Ctenophores are often called “comb jellies,” “sea gooseberries,” “sea walnuts,” or “Venus’s girdles.”
Examples There are two classes in the phylum Ctenophora: Tentaculata, of which the most characteristic species is Hormimorphaplumosa Nuda, of which the most characteristic species is Beroepunctatus. The most commonly seen ctenophores are of the genus Pleurobrachia. There are about one hundred species of Ctenophora that have been discovered.
Characteristics All organisms in Ctenophora have comb rows that are actually large cilia used for transportation. These are the largest cilia any organism uses. Ctenophores lack the ability to sting and instead use tentacles covered by colloblasts . All ctenophores are bioluminescent, meaning they are capable of producing light. Some have the appearance of rainbow coloration. This is caused by the beating of their combs, causing the diffraction of light to form a rainbow. Classification is generally done through comparative morphology.
Characteristics (continued) • Ctenophores have two digestive openings, but rarely use the anus for defecation • Biradial symmetry and three germ layers (epidermis, gastrodermis, mesoglea if included) • Mesoglea holds muscle cells and amoebacytes • Digestive system consists of mouth, pharynx, and stomach with a canal system • Have no CNS or brain, but a nerve net • Use an aboral sense organ with a statocyst to maintain balance • Balance is maintained by mood (turn toward prey if in danger) • All Ctenophores have a distinct larval form before adulthood
Morphology Most ctenophores are round or spherical in body shape. They generally have two large tentacles and eight rows of comb plates. Their tentacles have the adhesive colloblasts to capture prey and they have a stomach, mouth, pharynx, and anal pores. They have a nerve network that is rather complex and an apical sense organ. All ctentophores are hermaphrodites and most reproduce sexually. They generally range in size from .6 cm to 30.5 cm.
Colloblasts (Morphology) Colloblasts are microscopic, sticky structures used to adhere to the prey of a ctenophore. Usually the classes Cydippida and Lobata have colloblasts on their tentacles. After a prey is caught by a colloblast, the tentacle is contracted and the prey is brought by the predator’s mouth. There the prey is released and ingested.
Cydippids (Morphology) • Egg-shaped bodies • Order includes pleurobrachia • Tentacles on opposite sides of body • Body can be flattened to increase range of tentacles • Tentacles have tentilla (small tentacles) with colloblasts • Generally coil around prey to kill • Combs are spaced evenly around the body • From each statocyst balancer, two combs are moved at once • Movement is dependent on water disturbances
Lobata (Morphology) • Order also called Lobates • Have pair of muscular lobes extending from mouth • Tentacles in grooves on lobes, have auricles between lobes and mouth • Auricles have cilia to create current and flush prey into the mouth • Two comb rows on each lobe and two on each side between lobes • Comb movement dependent on nerves rather than water movement • Some can clap combs to push water rapidly and move opposite • Generally prey on plankton
Beroids (Morphology) • Order also known as Nuda • No feeding appendages, but pharynxes have macrocilia • Marcocilia are large bundles of cilia that work to bite off pieces of whatever is being consumed • Prey on other ctenophores • When not eating, a ridge shuts the mouth by connecting with the other part of the ridge
Embryology When the egg is fertilized, development of the embryo begins. Cleavage occurs completely but unequally, first producing two cells, then four, then eight, and so on until the embryo is full developed. The embryo forms within the egg cover. It develops double rows of cilia, a pair of lateral tentacles, and a large, apical sense-organ. The ectodermal layer of the gastrovascular system undergoes Epiauxesis. Epiauxesis involves the flattening and extension of the ectoderm germ layer in the gastrovascular system. Soon, the cilia begin to function and the developed larva breaks the egg shell and enters the water. The following pages are detailed steps to the process of initial cleavage and development.
Biochemical Evidence Little biochemical evidence exists regarding the phylogeny of Ctenophora. However, by using 18S ribosomal RNA for comparison, it has been determined that the ctenophores are a distinct monophyletic group that is closest related to the cnidarians. The ancestral ctenophore seems to have been like Cydippida. Thus, it was determined that Cydippida is a polyphyletic group, all other orders having been secondarily derived from the ancestor. The relatively close distance between the 18S ribosomal RNA sequences leads to the conclusion that ctenophores are derived from a recent common ancestor.
Life History Mnemiopsis leidyi is a ctenophore of the order Lobata. These have a distinctive level of bioluminescence that makes them appear to glow even more than others. Locomotive flappers hide the phosphorescent cells within the meridional canal, perpendicular to the plane of the comb plate. It exists in various morphological forms, chiefly where transparence is inversely proportional to body size. They can be up to 5-7 cm long and always live in a marine environment.
Life History (continued) M. leidyi is a hermaphroditic and are capable of self-fertilization. They are considered developed when capable of producing offspring, even though they have not ceased growing yet. When reproduction begins, the gonads in the meridional canals produce one spermatophore and one to four eggs. The auricalular gonads produce only one egg. The egg is covered in a thick membrane one minute after seawater contact.
Life History (continued) When embryogenesis begins, the eggs are spherical and soft. After the larvae is formed, the egg is covered with a thick membranous cuticle. The embryo experiences epiboly, gastrulation, ctenes appear, apical organs grow, tentacles grow, and the embryo enters the tentacular stage. Here, the larval form hatches and continues development. Complete embryonical development takes 20-24 hours.
Life History (continued) After hatching, the M. leidyi enters the lobate stage. Next, the meridional canal meets and the auricles begin to grow out. At this point, the M. leidyi is capable of reproduction and considered an adult.
Life History (continued) M. leidyi are capable of regeneration of damaged body parts. After damage is done, the cells begin dividing to reform the organ by doubling. M. leidyi feeds on fish eggs and larvae, phytoplankton, and holoplankton. It ingests any organism it can grasp with its oral lobes. They are generally considered carnivores but have been known to consume phytoplankton when needed. Their feeding on phytoplankton and zooplankton has reduced their populations as well as those of the kilka in the Caspian ecosystem. Thus, the economy in Caspian countries has suffered due to the loss of kilka.
Life History (Digestion) • Enzymes and contractions of pharynx liquidize prey • Cilia move the resultant mush into the stomach canal system • Nutritive enzymes break down the mush in the stomach • Ciliary rosettes in canal transfer nutrients to muscles in mesoglea • Most waste is ejected through the mouth, rarely through the anal pores. • Exact mode of excretion is unknown
Summary Are ctenophores carnivorous, herbivorous, or omnivorous? How do ctenophores capture prey? How do ctenophores reproduce and what organs do they have for such? To what phylum are ctenophores most closely related? What is the function of a colloblast? When are ctenophores considered fully developed?
