Marine Science Chapter 8 Marine Worms

8.1 FLATWORMS AND RIBBON WORMS The vertical zone of water that extends from the top of the ocean to its bottom is called the water column. Flatworms exist in both fresh and saltwater The flat-body form is the distinguishing trait used to classify these worms in the phylum Platyhelminthes (meaning “flatworms”).

8.1 FLATWORMS AND RIBBON WORMS There are 10,000 species of flatworms, ranging in size from microscopic to 20 meters in length. The planarian is a macroscopic flatworm. These worms have a mouth on their ventral surface. They take in food as they move along rocks and other substrates. Unlike other worms, planarians have a two-way digestive tract. The nutrients diffuse throughout the body also, there are no circulatory or respiratory systems. The undigested materials are discharged from the mouth (there is no anus).

8.1 FLATWORMS AND RIBBON WORMS Since the structures on the right side of its body are the same as the structures on the left side of its body defines bilateral symmetry. Bilateral symmetry, which first appears in the flatworm, is a characteristic feature of all other worms and more structurally complex animals.

8.1 FLATWORMS AND RIBBON WORMS Another feature first seen in the flatworm is the possession of three cell layers: ectoderm, endoderm, and mesoderm. This is important for the development of organ systems, also first seen in the flatworms.

8.1 FLATWORMS AND RIBBON WORMS The planarian, like most flatworms, is a hermaphrodite — it contains both male and female reproductive organs. A parasite is an organism that obtains its food by living in or on the body of another organism. The organism that is fed on is called the host. The tapeworm is sometimes found in the intestines of fish and other animals, including humans. The tapeworm attaches itself to the intestinal lining of its host and absorbs nutrients directly through its thin body wall. As a result, it has no need for a digestive system. Some can grow to more than 18 meters in length!

8.1 FLATWORMS AND RIBBON WORMS The trematode, or fluke, lives in the bodies of mollusks, fish, birds, and other animals. The liver fluke, which lives as a parasite in mammals. The blood fluke is a parasite that attaches to a fish’s skin and then forms a cyst in which it lives within the fish’s tissues. Trematode parasites that are accidentally eaten in raw fish (think sushi) may then reproduce in the digestive tracts of people.

8.1 FLATWORMS AND RIBBON WORMS When the trematode larvae are looking for a host, they may attach to skin, causing swimmer’s itch. The largest free-living worm in the sea is the ribbon worm (Cerebratulus lacteus). The nearly 1000 species of ribbon worms are classified in the phylum Nemertea. Ribbon worms have more “highly” developed circulatory and digestive systems.

8.1 FLATWORMS AND RIBBON WORMS The ribbon worm spears prey such as small fish and polychaete worms with a sharp, sticky extension called a proboscis. Ribbon worms can reproduce asexually by breaking into pieces; each piece is capable of regenerating into a whole new worm. They can also reproduce sexually.

8.2 Roundworms & Segmented Worms The most numerous of all worms in the sea are the roundworms. There are more than10,000 species of roundworms. Typical whipping movements of the roundworm’s body propel it through the spaces between sand grains.

8.2 Roundworms & Segmented Worms Roundworms are classified in the phylum Nematoda. They are often referred to as nematodes. Roundworms are characterized by a cylindrical body shape, which is tapered at both ends. Reproduction is sexual. Fertilization is internal; development of eggs is external.

8.2 Roundworms & Segmented Worms Segmented worms, or annelids, earthworms and all other annelids are classified in the phylum Annelida (meaning “little rings”). There are three classes of annelids, comprising more than 10,000 species. Earthworms, and their aquatic relatives that feed on organic matter in sediments, belong to the class Oligochaeta (meaning “few bristles”).

8.2 Roundworms & Segmented Worms A common marine annelid is the clamworm, or sandworm (Nereis). Sandworms belong to the class Polychaeta (meaning “many bristles”). The sandworm’s nervous system coordinates the movements of its body. They wriggle through the wet sand by using their paddle like appendages called parapodia.

8.2 Roundworms & Segmented Worms Another sign of evolutionary advancement in worms is the process for waste elimination. Waste products of metabolism are excreted from a pair of coiled tubes located in each body segment. These organs, which carry out excretion for the annelids as our kidneys do for us, are called the nephridia.

8.2 Roundworms & Segmented Worm Another segmented worm found in marine sediments is the bloodworm (Glycera). The bloodworm is a polychaete. The bloodworm has an open circulatory system, where the blood circulates through the tissue spaces rather than through blood vessels. You can see a bloodworm’s blood through its skin, hence its common name.

8.2 Roundworms & Segmented Worm A segmented worm without bristles is the leech, which is placed in class Hirudinea. The leech attaches to its host by means of two suckers, located at the anterior and posterior ends of its body. Sharp teeth in the sucker at the anterior end pierce the host’s skin. The leech then draws blood from its host. The leech secretes a chemical anticoagulant, called hirudin, into the wound. This prevents the host’s blood from clotting. Leeches can increase their body weight threefold.

8.3 GIANT TUBE WORMS AND ARROW WORMS A species of worm belonging to the phylum Pogonophora was discovered living near hot-water vents on the deep seafloor, thousands of meters below the ocean’s surface. These giant tube worms (Riftia pachyptila) measure up to 1 meter long and live clustered in water that is rich in hydrogen sulfide.

8.3 GIANT TUBE WORMS AND ARROW WORMS The process by which the bacteria produce energy-rich compounds from inorganic chemicals is called chemosynthesis. The giant tube worms live by utilizing carbohydrates made by the bacteria, which get their energy from the oxidation of hydrogen sulfide.

8.3 GIANT TUBE WORMS AND ARROW WORMS In the water column, near the surface of the ocean, lives the tiny, transparent arrow worm (Sagitta). The arrow worm is classified in the phylum Chaetognatha (meaning “bristlejaw”). Arrow worms are just a few centimeters long. They have tiny fins that enable swimming, but they mostly drift as part of the plankton community.

8.3 GIANT TUBE WORMS AND ARROW WORMS An active hunter, the arrow worm uses its mouth bristles, which are modified as hooks, to prey on other animal plankton such as copepods, fish eggs, and fish larvae.