Marine Science Chapter 7 Cnidarians

Cnidaria Animals with stinging tentacles are classified in the phylum Cnidaria. There are more than 9000 species in this phylum, divided into three classes—Scyphozoa, Anthozoa, and Hydrozoa.

7.1 Jellyfish Jellyfish are characterized by two cell layers, a saclike digestive tract, and tentacles. Jellyfish tentacles, which are long, flexible appendages, are arranged in a ring around a central mouth, thereby giving these animals radial symmetry. Jellyfish have no brain, so there is no central coordination of movement. A network of nerve cells and receptor cells make up the nerve net, which is a simple nervous system; when one part of the body is stimulated, the whole animal responds.

7.1 Jellyfish A jellyfish is an invertebrate. Jellyfish are members of the class Scyphozoa. All members of this class have an umbrella- shaped structure called the medusa, with tentacles hanging down from it. The medusa is composed of two membranes: an epidermis, and a gastrodermis (“stomach skin”). Lying between the two membranes is a jellylike mass called the mesoglea.

7.1 Jellyfish The low density of the mesoglea enable a jellyfish to float. Jellies are considered part of the plankton population, because of their limited abilities of locomotion. Oxygen diffuses directly from the water into the animal’s cells. Carbon dioxide diffuses from the cells into the water. This gas exchange occurs over the entire surface of the jellyfish.

7.1 Jellyfish Jellyfish use their stinging tentacles for defense and for getting food. The tentacles contain stinging cells, called cnidoblasts. Inside each cnidoblast there is a coiled thread with a barb at the end, called a nematocyst. (See Figure 7-3.) The barb contains a paralyzing toxin. The nematocysts can be discharged in response to either a mechanical or chemical stimulus.

7.1 Jellyfish Once a nematocyst is discharged, it cannot be used again. New nematocysts are constantly replaced. Paralyzed fish are brought up to the mouth by contracting tentacles located in the center of the medusa. Most of the food is digested in the saclike digestive cavity by enzymes secreted by its lining. Digestion in the jellyfish is both extracellular (in the digestive sac) and intracellular (in food vacuoles). Waste products are expelled, through the mouth. The surface of its medusa produces a sticky mucus. Tracts of ciliated cells move the captured plankton from the medusa to the mouth, where it is ingested.

7.1 Jellyfish Know figure 7-4 page 176 Jellyfish have separate sexes. Their life cycle includes both sexual and asexual reproduction phases. In jellyfish, the ovaries and testes are organized into a four-leaf-clover pattern in the medusa. Some jellyfish release eggs and sperm directly into the water, where external fertilization occurs. A swimming jellyfish larva is called a planula.

7.1 Jellyfish – Comb Jellies The comb jellies are classified in their own phylum, Ctenophora. They all have eight rows of long, fused cilia that look like combs. Comb jellies usually float on or near the surface of the water. They are bioluminescent when disturbed. Comb jellies release their gametes into the water for external fertilization.

7-2 Sea Anemones Sea anemones are members of the class Anthozoa (meaning “flower animal”). Sea anemones possess stinging tentacles, radial symmetry, and a nerve net. The adult sea anemone lives as a polyp, attached to a substrate by means of a muscular foot. An animal capable of only limited movement is called sessile.

7-2 Sea Anemones Tiny organisms and small fish that swim into the sea anemone’s tentacles are paralyzed by the discharging nematocysts. Large sea anemones can have little shrimp and clownfish that live unharmed among their tentacles. The clownfish have a protective skin coating, so they are not injured by the stinging tentacles.

7-2 Sea Anemones By living in the tentacles, clownfish gain protection from predatory fish; at the same time, the clownfish attracts fish for the anemone to catch. The shrimp help keep the anemone clean and may also help protect it from predators. This kind of mutually beneficial relationship between different species is known as symbiosis. The participants are referred to as symbionts.

7-2 Sea Anemones Digestion in the sea anemone is mostly extracellular. Digestion in sea anemones and jellyfish occurs in what is called a two-way digestive tract, where both ingestion and elimination occur through the mouth.

7-2 Sea Anemones The dominant structure in the sea anemone is the polyp. Some sea anemones can reproduce asexually by splitting in half. They can regenerate from pieces of the polyp’s base that are broken off. Sea anemones can also reproduce sexually. They produce embryos that develop into planula larvae, which settle and develop into adult polyps.

7-3 Corals The largest coral reef system is the Great Barrier Reef off the coast of Australia, which is about 2000 km long and 80 km wide. Some of the coral polyps that makes up this reef system are so tiny that some can be seen only through a microscope.

7-3 Corals There are two types of corals: stony (hard) corals and soft corals. The stony corals are made up of limestone (CaCO3, calcium carbonate), and form massive structures. Examples of hard corals are the brain coral, staghorn coral, and star coral.

7-3 Corals The soft corals are composed of a fibrous protein, which gives them flexibility. Examples of soft coral is are the sea fan, sea whip, and sea plume. The basic structure of the coral animal is the coral polyp The coral polyp resembles a very small sea anemone. Life activities are similar in both anemones and coral polyps.

7-3 Corals Unlike sea anemones, which live alone or in small groups, coral polyps live in large groups as colonial animals, attached to one another by a thin membrane. The membrane connects the polyps’ digestive systems, so there is nutritional sharing among them.

7-3 Corals In hard corals, each polyp sits in a cup-shaped depression that it forms on the surface of the reef. The stony depressions are layered over year after year by the current polyps that live on the surface. This how coral builds up over time.

7-3 Corals Coral gets its energy from the tiny plankton trapped by its tentacles at night. Also, the coral gets energy from the food that is made during photosynthesis during the day. This photosynthesis is actually carried out by tiny symbiotic algae called zooxanthellae, which live inside the tissue of each polyp. The coral get the excess energy and the zooxanthellae gets a secure home.

7-3 Corals To build the reef, cells within the polyp absorb calcium (Ca) from the seawater. The zooxanthellae take up carbon dioxide (CO2) that is produced by the polyp during respiration. These two substances combine inside the tissue of the polyp, to form the CaCO3 needed to build the limestone reef. Coral reefs are found in tropical areas, where the waters are warm and clear. “Coral bleaching” occurs, when the living polyps die off.