Larger Organisms-Lecture Notes

This week we will look at a sampling of the animals that live in the ocean. There is a plethora of shapes, sizes, and lifestyles among the many inhabitants of the ocean. As we march our way through the various groups, pay attention to 1) how the critter feeds, 2) where the critter lives, 3) how the critter is shaped, and 4) any other unique features. We can divide the multitude of marine animals into groups in many different ways, but for this lecture, we will split the groups based on their evolutionary groups. The lecture will be divided between one-celled "animals" (the Protozoa), invertebrates, and vertebrates. After Spring Break, we will relook at these groups with respect to where they live. Note that these lecture notes are very brief; more information about each group can be found in the book and many other sources.

Protozoa

Unicellular heterotrophs, includes the foraminifera and radiolarians. Both groups are skeletonized; radiolarians have skeltons of SiO2 and forams can be made of SiO2, CaCO3, or agglutinated sand grains. Some foraminifera harbor photosynthetic algae that they use for nutrient supply. Much of the rock called "chert" common on our beaches is made up of skeletons of radiolarians.

Invertebrates
"Parazoa" or the Porifera

Many biologists differentiate the simplest multicellular animals from all other animals (Metazoa) into the group "Parazoa." Predominantly, this group is made up of the phylum Porifera, or the sponges. Modern sponges are common in reefs, on rocks, on the sandy bottom, and attached to other organisms. All sponges are sessile, meaning they live attached to the sea floor. Unlike metazoans that have cells differentiated into various tissues, the cells of sponges work independently. The collar cells whip water (and the tasty things in the water) into the sponge, digesting the particles as they stick to the surface. The body of the sponge is either a simple or complex bag with lots of holes. Water is drawn into the many small holes, and blown out through the large hole. This bag is held rigid by an interlocking network of skeletal fragments, called "spicules." Various classes of sponges are differentiated by the material of the spicules, which can be simple and SiO2, complex and SiO2, made of CaCO3, or even made of a hard, gelatinous substance called "spongin."

Cnidaria (formerally called the "Ceolentrata")

This groups is united by a very simple body plan that looks like a bag rimmed by little arms. Food is brought into the bag and digested, and the left over residue is spit back out of the same hole. The name reflects the many stinging cells (cnidoblasts) that protect the animal and stun prey. Cnidarians live as either free-swimming upside-down bags (medusae), such as jellyfish or juvenile corals, or live on the seafloor as right-side-up bags (polyp), such as corals and sea anemones. Although jellyfish and sea anemones do not have skeletons, coral polyps can secrete very rigid skeletons that form the framework of most modern reefs. Unlike most animals that have bilateral symmetry (mirror-image), cnidarians have radial symmetry, like a bicycle wheel.

Nematodes and Annelids

Common marine roundworms, nematodes are slightly more complex than cnidarians because they do contain sensory organs. Most nematodes (and other worms) are free-living, but some nasty ones live as parasites within other organisms (can you say, "sushi"?). Another common phylum of worms is the Annelida, or segmented worms. The annelid body is made up of repeating segments that each possess important life systems, such as circulatory, nervous, and reproductive systems. This repetition of body segments is called "metamerism." Within the phylum Annelida is the class Polychaeta which is the most diverse group of marine worms. Polychaetes live as free-swimming worms and are also found living within tubes. Often, you can find jumbles of these tubes on our beaches.

Mollusca

The phylum Mollusca includes many different-but common-marine critters that share a similar body plan. In this body plan, a simple u-shaped digestive system with a mouth and an anus is found within a mass of flesh that is usually encapsulated by one or more shells. The various classes of molluscs are separated by either the form of the shell (s) or the shape of the body. The most common marine classes are (1) the Gastropoda (snails), (2) the Bivalvia (clams), (3) Cephalopoda (octopuses and squids), and (4) the Polyplacophora (chitons). One key feature of molluscs is that they are a very mobile group and have developed many means of swimming or gliding about.

Arthropoda

Most of the modern arthropods belong to the primarily terrestrial class Insecta, but many others live in the ocean. All arthropods share the common segmented body and segmented legs, all closed in by an exoskeleton. Because the animal cannot get bigger inside of an immobile exoskeleton, the exoskeleton is often shed, or molted, and a new exoskeleton is grown. Common marine arthropods include the copepods (already discussed), krill, lobsters, shrimp, crabs, and barnacles. One of the tastier phyla! Note that except for the attached barnacles, all of the other common arthropods are noted for being agile walkers with those segmented legs.

Echinodermata

The last major invertebrate phylum we will discuss is the Echinodermata, or "spiny skin." This phylum includes the common sea stars ("starfish" is no longer PC), brittle stars, sand dollars, sea urchins, and sea cucumbers. The fundamental body plan of the echinoderms is that they posses a radial, five-sided symmetry (although a secondary bilateral symmetry may be superimposed), a water-vascular system that allows movement and feeding by changes in water pressure throughout the body, and a skeleton (except the sea cuccumbers) that is made of many plates made of individual crystals of calcite. The most common classes of echinderms are the Echinoidea (sand dollars and sea urchins), Asteroidea (sea stars), Ophiuroidea (brittle stars), and Holothuroidea (sea cucumbers).

Marine Vertebrates (and near vertebrates)

All vertebrates belong to the phylum Chordata, but not all chordates are vertebrates. Huh? Being a chordate means that you have a long tubular nervous system running up your back called a "notochord." About 5% of the chordates lose the notochord, and hence, become "invertebrates." The remainder retain the notochord and skeletonize it, forming a vertebral column. The tunicates are a group of invertebrate chordates that look like the human heart. Other "near-vertebrates" look like the general fish-shape, but no vertebral column. A classic example is Amphioxus.

A Comment about "Fish"

"Fish" is a misleading term because many organisms that we call fish are unrelated to one another, and many things that are closely related do not look like "fish." A similar problem arises when we think what is a "reptile." Therefore, for this discussion, we will divide the remaining vertebrates into evolutionarily-similar groups.

Jawless Verterbrates (Class Agnatha)

Early vertebrates did not develop a hinged jaw and lived by filtering or acting as parasites. Modern examples include lampreys and hagfish. For many reasons, it is more successful to have a hinged jaw, so nearly all other vertebrates have jaws.

Class Chondrichthyes

Now that I have told you not to use "fish", lets look at "fish." Most things that would sell at the Co-Op as fish fall into one of two groups, the "Cartilagenous Fishes" or class Chondrichthyes and the "Bony Fishes" or class Osteichthyes. Chondrichthyes all have a skeleton made of a hard substance called "cartilage," the same material you have in your ears and nose. This diverse group includes the sharks, rays, and skates. Although there are more modern species of bony fish, the Chondrichthyes are a very dynamic and successful group, living in all realms of the ocean and feeding on everything from plankton to surfers. In all seriousness, humans about one million sharks for every human eaten by a shark…

Class Osteichthyes

The "Bony Fish" have, by definition, a bony skeleton. Most of the modern Osteichthyes belong the very, very successful order Teleosti, or teleosts. Most of the common "fish" belong to the teleost, as well as some eels, and sea-dragons. There have been many adaptations within this group to be able to handle living in the ocean (and moving into fresh water), including dealing with the viscosity of water, buoyancy, and salinity. Osteichtyes (and Chondrichthyes) use elaborate gill systems to efficiently pull oxygen directly from sea water. This elaborate gill system allows some members to swim at astonishing fast rates.

Class Reptilia

Although not as prolific as they were in ancient times, many reptiles still call the ocean their home. This group includes sea turtles, crocodiles, sea snakes, and some BIRDS!! Why birds? Because birds are a specialized group of dinosaurs which are, in turn, a specialized group of reptiles. Reptiles differ from the other vertebrates mentioned so far because they use lungs to gather oxygen and have scaly (or feathery) skin-coverings. Marine reptiles also have fancy salt glands to deal with concentrating and expelling excess salt.

Class Mammalia

Our brothers and sisters! Boy, after 200 million years or so of walking around on land, some of our cousins have gone back to the ocean. Included in this group are the familiar whales, dolphins, and porpoises (Order Cetacea) (note diversity below), seals, sea lions, and walruses (Suborder Pinnipedia), and manatees (Order Sirenia). The Cetaceans are further divided between the "toothed whales," such as dolphins, sperm whales and killer whales (Suborder Odontoceti) and the "baleen whales," such as humpback, gray, and blue whales (Suborder Mysteceti). Being mammals, they have to deal with air-breathing lungs and live-birth. However, marine mammals have had some novel adaptations by developing a skin impervious to fluid loss and by creating internal heat to be able to survive in a multitude of water temperatures (all other vertebrates are ectothermic).