July 27, 2008
Cephalopods are the mollusk class cephalopoda (“head-foot”) and are characterized by bilateral body symmetry and usually a large head (Campbell et al. 2004). Cephalopods are found in all the oceans of the earth at nearly every depth. None can tolerate freshwater, but a few of them can withstand living in brackish (where salt and fresh water meet) water. There are two main subclasses of cepahalopds, the coleoidea and the nautiloidea, and are classified by whether or not the mollusk has a shell. Cephalopods are often regarded as the most intelligent of invertebrates as they posses large brains complemented by well-developed senses. The class developed during the late Cambrian, and early cephalopods were likely predators near the top of the food chain. Their lengthy evolutionary history spans an impressive 500 million years and has left a vast record of fossils, making them an excellent speciment to study and track their evolutionary history (Vendetti et al. 2006). Today cephalopods are eaten worldwide and are vital to those who consume them. Playing a key roll in their ecosystem as both predator and prey, cephalopods have shown to have an important role on earth (Piatkowski et al. 2001). Characteristics:
There are approximately 17,000 named species of fossil cephalopods with only 800 species still living. With such a diverse number of species it has been identified that 3 clades have become entirely extinct: Endoceratoidea, Actinoceratoidea, and Bactritoidea. The focus will be drawn upon the two living clades, the coleoidea and the nautiloidea. The coleoidea is characterized by having an internalized shell or no shell at all, and include the octopus, squid, and cuttlefish. The nautiloideas still have a shell and this clade includes the nautilus (Vendetti et al. 2006). The greatest number of known cephalopods are members of the coleoids. Of the coleoids the squids are the most common and are characterized by a flexible internal shell called a pen, a torpedo shaped body, and quick movement. The cuttlefish looks similar to a squid but has a more stout body, a larger internal shell called a cuttlebone, and fins for propulsion. Octopuses are the most evolved of cephalopods and through evolutionary adaptation have lost their shell, indicating successful adaptations to their environment. Nautiloids are characterized by having an external shell, and are among the earliest cephalopods discovered. They have a planispiral shell shape with interconnected internal chambers, allowing the animal to withstand great pressure but resulting in the loss of buoyancy control (Vendetti et al 2006). One prominent feature first noticed by many about the cephalopods is the size of their head compared to the rest of their body. Surprisingly, these heads do in fact hold large brains, and cephalopods are widely regarded as the most intelligent invertebrates. With highly developed eyes and other sensory organs, cephalopods are able to retain information and learn by example or through trial and error. Possessing the most complex nervous system of any invertebrate, giant nerve fibers found in the mantle have been a favorite experimental specimen of neurophysiologists for many years (Wood 2006). Cephalopods have good vision, and all but one species see in black and white. This is strange because cephalopods have the ability to change color in the presence of a predator or when they feel threatened. When changing color they use their chromatophores to change brightness and pattern in accordance with their surroundings. Their ability to blend with their surroundings comes from cells such as iridophores and leucophores that reflect light from the environment (Vendetti et al. 2006). In addition to the highly developed nervous system, cephalopods are the only molluscs with a closed circulatory system. Their vascular system that has two gill hearts that move...