Marine Invertebrates
Lophophorates
Lophophorates are characterized by a special feeding organ called a lophophore which is an extension of the body wall into a tentacled structure that surrounds the mouth and is either U-shaped or circular. The lophophore is used to trap floating food particles in passing currents (called suspension feeding). Tentacles surrounding the mouth are usually hollow and the mouth is usually located inside the lophophore. The anus is on the same side of the body but on the outside of the lophophore. Lophophorates include the phyla Phoronida, Bryozoa (Ectoprocta), and Brachiopoda and are related to the Mollusca and Annelida phyla. Many lophophorates have tubes, shells, or exoskeletons for protection. They are usually sessile (non-moving), benthic (sea floor dwellers), and live in salt water, although there are a few freshwater lophophorates in the Phylum Bryozoa.
Phylum Bryozoa contains Class Gymnolaemata (marine bryozoans) and Class Phylactolaemata (freshwater bryozoans ~50 species) and are tiny though visible colonial animals that look a bit like tiny coral colonies that also build skeletons of calcium carbonate (although some species lack calcification and instead are mucilaginous (made of slime)). Members of the Phylum Bryozoa are known as "moss animals" or "sea mats" and they generally prefer warm, tropical waters, but are known to occur worldwide. There are about 8,000 living species, with many times that known from the fossil record. Fossil bryozoans are common throughout the world in sedimentary rocks representing shallow marine habitats, especially in rocks of Paleozoic age.
Bryozoans are usually found on hard substrates such as rocks, shells, wood, blades of kelp, and ships which can become heavily encrusted with bryozoans. Some bryozoan colonies also form colonies directly on marine sediments. Bryozoans have been found at depths of 8,200 m (27,000 ft) though most inhabit shallower warmer waters. Most bryozoans are sessile though a few are able to creep about and some non-colonial bryozoans live and move about in the spaces between sand grains. One species appears to make its living while floating in the Southern Ocean.
Almost all bryozoans are colony-forming animals often with millions of individuals in each colony. The colonies range from millimeters to meters in size, but the individuals that make up the colonies (called zooids) are tiny, usually less than a millimeter long. In each colony, different individuals or zooids assume different functions. Some gather food for the colony (autozooids) while others have specialized for different functions (heterozooids) such as kenozooids which provide structural support and vibracula which have long whiplike structures that they use to clear debris away from the surface of the colony. There is only a single known solitary species, Monobryozoon ambulans, which does not form colonies.
Bryozoan skeletons grow in a variety of shapes and patterns: mound-shaped, lacy fans, branching twigs, and even corkscrew-shaped. Their skeletons have numerous tiny openings, each of which is the home of a zooid. They also have a coelomate body with a looped alimentary canal or gut, opening at the mouth and terminating at the anus. They feed with a specialized, ciliated structure called a lophophore, which is a crown of tentacles surrounding the mouth. Their diet consists of small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish. Bryozoans do not have any defined respiratory, or circulatory systems due to their small size. However, they do have a simple nervous system and a hydrostatic skeletal system. Several studies have been undertaken on the crystallography of bryozoan skeletons, revealing a complex fabric suite of oriented calcite or aragonite crystallites within an organic matrix — see for example Hall et al. (2002).
The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water which drives water together with entrained food particles (mainly phytoplankton) towards the mouth.
Because of their small size, bryozoans have no need of a blood system. Gaseous exchange occurs across the entire surface of the body, but particularly through the tentacles of the lophophore.
Bryozoans can reproduce both sexually and asexually. All bryozoans, as far as is known, are hermaphroditic (meaning they are both male and female). Asexual reproduction occurs by budding off new zooids as the colony grows, and is the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. A colony formed this way is composed entirely of clones genetically identical individuals) of the first animal, which is called the ancestrula.
One species of bryozoan, Bugula neritina, is of current interest as a source of cytotoxic chemicals, bryostatins, under clinical investigation as anti-cancer agents.
The closest relations of the Bryozoa appear to be the brachiopods.
Mollusks
Animals classified under the phylum Mollusca are extremely diverse in form, but all have a fairly simple body plan. Familiar mollusks include oysters, chitons, clams, snails, slugs, octopus, and squid. Most mollusks have a soft body and a hard or “calcareous” shell. Many mollusca use mucous and cilia to eat, move, and reproduce. There are more than 110,000 species in phylum Mollusca, more than every other phylum except Arthropoda. With a few exceptions, all living species of mollusks are categorized underGastropoda or Bivalvia. Another important class is Cephalopoda. Some scientists have determined that there is more biomass from marine mollusks than any other animal on earth.
Mollusks reproduce through external fertilization where the eggs and sperm are released into the water. In some more complex mollusks, fertilization can take place internally after long courtship rituals and mollusk dances. Many of the more sophisticated snails are hermaphroditic. Some go through phases where they alternate gender, others are both female and male at the same time.
Almost all mollusks living in freshwater are gastropods although a few bivalves can be found in brackish water. Some species of mollusks have adapted to living on land but can only live in humid environments. Terrestrial mollusks must have the ability to regulate their temperature, breathe air, make larger eggs, and maintain moisture levels by conserving water. Snails that live in the littoral zone of the ocean often show similar adaptations as terrestrial or land living snails. Snails in the class Pulmonata have adapted to living on land so well that they can be found at high altitudes. Other snails in Pulmonata that once could breathe air have gone back to living in the water.
Mollusks can be found in all habitats of the ocean. Some bivalves like the Protobranchiates, are even found in waters 9,000 m or 29,500 ft deep. The more advanced cephalopods could be viewed as the most sophisticated invertebrates. Animals like squid, cuttlefish, and octopuses have relatively "huge" brains and move about using their arms, fins, and siphons (in a manner similar to jet propulsion).
Arthropods
Arthropoda is the largest phylum in the taxonomic system and is composed of insects, crustaceans, and arachnids. Nearly 4/5 of all living animals are arthropods.
This ancient phylum dates back to the earliest days of the Cambrian period.
Arthropods are characterized by a segmented body plan with a head, abdomen, and thorax and legs or appendages on every segment with a rigid exoskeleton made out of chitin. Arthropods use their appendages to feed, as sensory mechanisms, and for locomotion. Aquatic arthropods use gills for respiration.
Although spiders are possibly the most familiar arthropod, lobsters, crabs, barnacles, and shrimp in the class Crustacea are also in this phylum.
Arthropods are most closely related to the Annelida, or segmented worms. The five main subgroups of the phylum are the Trilobita, Myriapoda, Chelicerata, Crustacea, and the Hexapoda.
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