Marine Invertebrates

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Animals that lack backbones are known as invertebrates. Over 98% of species on earth are invertebrates that rely on other strategies for support such as hydrostatic pressure, exoskeletons, and in some, even glass spicules. Some invertebrate phylums have only one species, while others like Arthropoda include more than 800,000 species. The most common marine invertebrates are sponges, cnidarians, worms, lophophorates, molluscs, arthropods, echinoderms and the hemichordates.

Sponges

There are approximately 5,000 species of sponges classified under the Phylum Porifera. Sponges are relatively simple animals that originated during the Precambrian times with fossils records found in the Cenozoic, Mesozoic, and Paleozoic eras. The anatomy of a sponge is organized so that flagella inside the sponge pull water into small holes (ostia) in the body and expel waste through larger holes (oscula). Sponge species have a variety of body plans that provide structure, including support by organic fibers, calcareous spicules, siliceous spicules or any combination of these. Some combine organic fibers with siliceous spicules in a body made mostly out of calcite or aragonite. The body plan of a sponge has adapted to consume small food particles allowing them to reside in habitats, such as polar shelves and submarine caverns that may lack nutrients. Sponges are often studied by scientists to find clues about the first life forms on Earth with more than one cell.

Sponges that are composed of organic fibers are often used by humans for cleaning and other purposes. Sponge diving remains a popular business off the west coast of Florida, in the Mediterranean Sea, off the coast of the Florida Keys, and near the West Indies.

The phylum Porifera includes four classes of sponges that are categorized by their support structure. These classes include Hexactinellida, Calcarea, Demospongiae, and Scelerospongiae. In each class of sponges, there are several taxonomic orders with a multitude of species.

Sponges are able to reproduce both sexually and asexually. During sexual reproduction, sperm cells are transferred to the egg in a carrier cell. Asexual reproduction occurs in sponges when favorable environmental conditions, conducive to sexual reproduction, deteriorate. Many species of sponges will fall apart in this event, leaving behind a concentrated mass of amebocyte cells. In some species, the sponge will produce a gemmule, or mass of amebocyte cells surrounded by nutrients. Sponges can regenerate from this mass of cells when conditions improve. In the Tethya species, buds develop on the outside of the body which later fall off and develop a whole new sponge.

Sponges are eaten by chitons, snails, nudibranchs, turtles, fish, and insects. They provide a home to sea anemones, polychaetes, octopuses, copepods, zoanthids, shrimps, brittle stars, amphipods, barnacles, and fish. There are numerous symbiotic relationships between animals and sponges.

Cnidarians

The phylum cnidaria consists of about 10,000 species of simple animals found only in marine habitats. Species in cnidaria have special stinging cells called cnidocytes. Familiar animals in the phylum cnidaria are corals, sea anemones, jellyfish, sea wasps, sea pens, and sea pansies. Cnidarians evolved during the Precambrian era.

Most cnidarians have a very basic body plan which includes a digestive cavity with one opening. This opening functions as the anus and the mouth for the organism. The only true organs in cnidarians are the gonads. Most cnidarians are symmetrical, an observation referred to as “radial symmetry.” Cnidarians also have an ectoderm and an endoderm. The ectoderm is connected to the endoderm by a gell-like substance known as the mesoglea. Cnidarians use a nerve net and very basic receptors for impulses to move. Oxygen is taken in through the tissues.

Organisms in cnidaria capture and kill their prey using stinging cells around their mouth. The cnidocysts, or stinging cells, send out a stinging barb, which immobilize their prey and help protect against cnidarian predators. When prey is stunned by the poison, the tentacles around the mouth bring the animal into the cnidarian’s gastrovascular cavity where it can be digested and pushed back out the mouth.

The four classes of cnidarians are the Anthozoa, the Hydrozoa, the Scyphozoa, and the Cubozoa. Anemones, corals, and sea fans are in class Anthozoa, which was the first to diverge throughout evolution. Portuguese man-o-wars and obelia are examples of animals in Hydrozoa while jellyfish are included in Scyphozoa. Box jellies are in class Cubozoa.

Cnidarian species have a variety of life-cycles. Some alternate between being free-swimming medusae and asexual polyps depending on their environment. In some groups like Anthozoa, organisms never make it to the free-swimming medusae stage, but instead live their whole lives as a non-moving polyp. Organisms in the groups Scyphozoa and Cubozoa spend most of their lives in the medusal stage. Medusae can measure anywhere from a few millimeters to 30 meters long including the tentacles. Some, like the Siphonophores, are individuals but can live in colonies and appear as one organism.

Worms

Marine worms can be placed into more than ten different phyla and come in a variety of colors, shapes, and sizes. Marine worms are often confused with other animals with thin and long bodies. Most marine worms are grouped into the Annelids, a group that includes the Polychaetes, Oligochaetes, Hirudinae, and the Eunice aphroditois. Polychaetes are most often found near the shoreline and swim or crawl using a pair of legs found on each segment of their body. The Oligochaetes, which include earthworms, are found on land and the Hirudinae include leeches that live in freshwater environments. Some marine worm species, such as the Bristle worm, can deliver a sharp sting to humans when handled.

The body structure of an annelid consists of a front end with a prostomium, also referred to as a significantly defined head. Most annelids have two pairs of eyes, three antennae, a pharynx or proboscis used to eat food and tentacle-like cirri for probing the surrounding area. An example of the biodiversity of worm species is the Sipunculid also known as Peanut Worm. This worm digs itself into a hole underneath rocks, eats organic material, has no segments and looks like a peanut when it pulls its proboscis into itself.

In general, marine worms live underneath rocks near the edge of the ocean, in algae, or anywhere there is mud or sand. Species of marine worms can be ringed, segmented, or flat. Species include tube-digging worms, burrow-dwelling worms, ribbon worms, and peanut worms.

Some common annelids include the tube-making Galeoloaria, the stinging Fireworm, the short Scale worm, and the huge Eunice aphroditois. Tube worms actually make a tube with a hard shell and retreat into the shell when threatened. The Christmas tree worm has many brightly colored feather-like tentacles shaped somewhat like a Christmas tree that is used to filter food from the water.

Lophophorates

Lophophorates are characterized by a special feeding organ called a lophophore that appears like a ring of tentacles around the mouth of the organism. Some lophophorates have a U-shaped or coiled ring and others keep the ring on the inside of the mouth. The lophophore is used to grab floating food particles. 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, Ectoprocta, and Brachiopoda and are related to the Mollusca and Annelida phyla. Many lophophorates have tubes, shells, or exoskeletons for protection. They are usually non-moving, benthic (sea floor), small animals most of which live in salt water, although there are a few freshwater lophophorates in the Phylum Ectoproda.

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 under Gastropoda 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 9000 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 huge brains and can move themselves through jet with structures resulting in 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 legs or appendages on every segment with a rigid exoskeleton made out of chitin. Exoskeletons provide protection and maintain moisture.

Although spiders are possibly the most familiar arthropod, lobsters, crabs, barnacles, and shrimp in the class Crustacea are also in this phylum. Crustacea have body plans segmented into a head, an abdomen, and a thorax. Arthropods use their appendages to feed, as sensory mechansims, and for locomotion. Aquatic arthropods use gills for respiration.

Arthopods 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.

Echinoderms

The Echinoderms lack a head and have five-point radial symmetry. These fascinating animals live only in marine environments. They have an endoskeleton made out of calcareous plates, which is often protected by spines. The plates that make up the endoskeleton often support the spines and enclose the coelom, an anatomical feature used for movement, respiration, collecting food, and as a sensory mechanism. The coelom also houses the reproductive organs and alimentary canal.

Echinoderms can be found in all oceans in all zones. There are approximately 6000 species in this phylum. The three main subphyla in phylum Echinodermata are: Asterozoa, Crinozoa, and Echinozoa. Sea lilies and feather stars are in class Crinoidea under subphylum Crinozoa. Starfishes are found in class Asteroidea under subphylum Asterozoa. Brittle stars are also under subphylum Asterozoa, but they are in class Ophiuroidea. Class Echinoidea under the Echinozoa subphylum includes sea urchins, sand dollars and other similar animals.

Mature echinoderms have five points that face outward from the center of the body with a mouth underneath and the anus on top. There are exceptions to this plan however; some echinoderms lack an anus and others, like the crinoids, have both the mouth and the anus on the same side of the body. Scientists refer to the side of the body with the mouth as the oral side and the side with the anus as the aboral side. Crinoids, ophiuroids, and holothuroids have tube feet to help collect food particles floating towards their body. Other types of echinoderms like asteroids are carnivorous and will surround or throw their stomach over their prey. Some echinoids even have teeth used to chew and dismantle plants and small animals found on underwater substrate.

Most echinoderms reproduce sexually producing larvae that feed on phytoplankton until they reach maturity. Some species of echinoderms develop their offspring in embryonic sacs located on the outside of their bodies.

Echinoderms have fascinating water-vascular systems that likely originated from some sort of respiratory system that evolved to include food gathering and movement. They accomplish these tasks through the use of their numerous hollow tube feet that resemble tentacles. There are two rows of tube feet on the outside of the body that fill with seawater so that when the animal expands or contracts, water is drawn into the feet. Once filled, the feet extend outward allowing the animal to walk. Suckers located at the tips of the tube feet are often used to grab prey or to hold onto solid objects when the echinoderm wants remain stationary.

The most familiar echinoderm known to humans is probably the sea star, categorized into the subphylum Stelloridea. There are two classes of sea stars in Stelloridea which include Asteroidea and Ophiuroidea. True sea stars and sun stars in are in class Asteroidea while brittle stars and basket stars are in Ophiuroidea. Echinoderms in the class Asteroidea have arms that are smoothly connected to the body; echinoderms in Ophiuroidea have arms that shoot out from a disk-like center. They are able to regenerate their limbs when one is broken off. In some cases, the limb itself will generaet a whole new sea star, but more commonly the new limb simply replaces the broken one. The small bumps on top of the sea star are referred to as Dermal Branchiae and are used to absorb oxygen from the water for respiration. Pedicellaria are small appendanges used to keep foreign bodies off of the sea star. The madreporite is a hard opening on the aboral side of the sea star used to regulate and filter sea water.

Sea stars also have an eye-like structure at the end of each arm, called the eyespot, used to detect light.

Hemichordates

Hemichordates are a relatively small phylum. These creatures are extremely important to the study of the evolution of vertebrates. They are characterized by a body divided into three main areas: the preoral lobe, the collar, and the trunk. Hemichordates are partial chordates and are closely related to the first chordates. According to DNA analysis, hemichordates are closely related to echinoderms, which is also apparent during observations of hemichordate and echinoderm larval stages. Hemichordates have gill slits, a structure that resembles a notochord but is called the stomochord, a dorsal nerve cord, and a reduced ventral nerve cord.

There are three classes of hemichordates which include Enteropneusta, Pterobranchia, and Graptolithina. The most well-known class is the Enteropneusta or “acorn worms”. Acorn worms have gill slits, burrow into the sediment, and likely feed on dirt and detritus. They can reach up to 2.5 m or 8 ft in length but most are actually quite small. In the Pterobranchia class, there are only a few species notably different from the acorn worms. Pterobranchs live in colonies connected by stem-like stolons. Each tiny individual is referred to as a zooid and has one gill slit. The Graptolithina are most well-known in the fossil record showing up in the Ordovician and Silurian times.

Sources:
Wikipedia: Cnidarians
Introduction to the Hemichordata - Museum of Paleontology, University of California, Berkeley
Wikipedia: Lophophorate
Wikipedia: Arthropod
Sea and Sky: Sea Worms
Worms - Life on Australian Seashores by Keith Davey
Echinoderms-The Spiny Animals! Oceanic Research Group
Karlene V. Shwartz, “Animal Kingdom”, in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.035700, Last modified: November 22, 2004
W. D. Russell-Hunter, "Mollusca", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.431300, Last modified: May 17, 2001.
Andrew C. Campbell, Raymond C. Moore, J. John Sepkoski, Jr., "Echinodermata", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.210700, last modified: January 11, 2002.

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