Animals that lack backbones are known as invertebrates. Over 98% of species on Earth are invertebrates that rely on other strategies than a backbone for support such as hydrostatic pressure, exoskeletons, shells, and in some, even glass spicules.
Some invertebrate phyla have only one species, while others like Arthropoda include more than 83% of all described animal species with over a million species. The most common marine invertebrates are sponges, cnidarians, marine worms, lophophorates, mollusks, arthropods, echinoderms and the hemichordates.
There are between 9,000 and 15,000 species of sponges classified under the Phylum Porifera. Sponges are relatively simple animals that originated with the first animal life in the Precambrian times.
The body plan of a sponge has adapted to filter small food particles from the passing water allowing them to reside in most habitats, including polar shelves and submarine caverns that often contain very few nutrients.
Like other animals, sponges were found to also grow extremely slowly in cold waters such as those of the Antarctic. Age estimates based on growth rates of one glass sponge (Scolymastra joubini) in the Ross Sea were between 15,000 and 23,000 years, which means that specimen appears to be the longest-lived animal on earth yet recorded. Sponges are often studied by scientists to find clues about the first life forms on Earth with more than one cell.
Sponges are hermaphroditic and are able to reproduce both sexually and asexually. Most sponges usually reproduce by sexual reproduction, where sperm cells (spermatocytes) develop from choanocytes (collar cells) and eggs develop from oocytes. When environmental conditions are favorable, spermatocytes are ejected in out-going currents and the eggs, once fertilized inside the sponge in some sponges, develop into flagellated larva that swim about as plankton until they find a suitable place to settle and grow into adults. Asexual reproduction occurs when favorable environmental conditions deteriorate and includes both regeneration (regenerating from fragments), budding (groups of cells differentiate into small sponges that are then released externally or expelled through the central canal (oscula)), or the formation of gemmules (“survival pods” of unspecialized cells that remain dormant until conditions improve and then either form completely new sponges or re-colonize the skeletons of their parents).
Sponges that are composed of organic fibers (demosponges) have been used by humans for thousands of years for cleaning and other purposes. Sponge diving has declined significantly due to overfishing and most sponges these days are now synthetic.
Most cnidarians have a very basic body plan which includes a digestive cavity with one opening. This opening functions as both the mouth and anus 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 (tissue that covers the outer body surfaces) and an endoderm (inner layer of cells forming the gastrointestinal and respiratory tracts, and inner organs). The ectoderm is connected to the endoderm by a gel-like substance known as the mesoglea. Cnidarians use a nerve net and very basic receptors for impulses to move. Oxygen is taken in directly from the water through the tissues.
Organisms in cnidaria capture and kill their prey using cnidocysts, or stinging cells, around their mouth which send out stinging barbs which immobilize their prey and help protect against predators. Once prey is captured the tentacles move it into the central gastrovascular cavity where it is digested. Waste is then expelled back through 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, jellyfish are in class Scyphozoa, and 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.
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 (bristle worms), 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 mainly found on land and the subclass Hirudinae include leeches that usually live in freshwater environments. Some marine worm species, such as the bearded fire worm, can deliver a nasty burning 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 the 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 and 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 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.
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 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).
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.
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 with approximately 6,000 described species.
The two main subphylums in phylum Echinodermata are Eleutherozoa and Pelmatozoa.
Subphylum Eleutherozoa conatins the superclasses Asterozoa and Cryptosyringida.
Superclass Asterozoa contains the sea stars/starfishes in Class Asteroidea and the extinct Class Somasteroidea.
Superclass Cryptosyringida contains Class Echinoidea (heart urchins, sand dollars, and sea urchins), Class Holothuroidea (sea cucumbers), and Class Ophiuroidea (basket stars, brittlestars, and snake stars).
Subphylum Pelmatozoa contains the Class Crinoidea (feather stars and sea lillies).
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.
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 to remain attached to something.
The most familiar echinoderm known to humans is probably the sea star, categorized in the superclasses Asterozoa and Cryptosyringida. There are two classes of sea stars which include Asteroidea and Ophiuroidea. True sea stars and sun stars in are in Class Asteroidea while brittle stars and basket stars are in Class 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. Both are able to regenerate their limbs when one is broken off. In some cases, a lost limb can generate a whole new sea star. 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 appendages 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 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.