Physiological Processes (Continued)

Nervous System

Arthropods evolved from a common lineage with annelids and mollusks and therefore share some fundamental organizational features of their nervous systems. The most primitive examples of this lineage consist of a bilaterally symmetrical nervous system of one ganglia per body segment, each of which is composed of paired hemi-ganglia, one on each side of the body. The ganglia of adjacent segments are connected by intersegmental connectives, while the contralateral hemiganglia are connected by commissures. This neuronal architecture comprises a ladder-like chain consisting of a brain, two circumesophageal connectives, and a ventral nerve cord.

The supraesophageal ganglion forms the brain, which typically contains integrative centers for the major senses and pre-motor systems controlling many segmental ganglia. This ganglion represents the fusion of several paired ganglia. Circumesophageal connectives run ventrally (towards the tail) around the esophagus and are reconnected with each other by the tritocerebral commissure. In other words, the lobster's gut penetrates its brain. A pair of connective ganglia, known as the stomatogastric ganglia, lie on the circumesophageal connectives. The ventral nerve cord then continues on towards the telson and is made up of the subesophageal ganglion (which is due to the fusion of three cephalic ganglia and the first two thoracic ganglia), five additional thoracic ganglia, and six abdominal ganglia. The subesophageal ganglia controls the mouthparts. Much seemingly normal behavior can occur when the circumesophageal connectives are severed, pointing to many higher level functions of the subesophageal ganglia. The remaining segmental ganglia control postural musculature, appendages on the segments, and some organs, such as the intestine.Lobsters have compound eyes, as do most arthropods, but these are stalked to provide a broader field of view and increased binocular spread. In clawed lobsters, each eye has 13,500 ommatidia which are light capturing, image forming organs. Lobster eyes are adapted for use in low-light environments, but appear to be only monochromatic (no true color discrimination ability). How good is their eyesight? How much do they rely on it, relative to other sensory systems like smell, taste, or touch? Lobsters, with their eyes perched on top of their heads, certainly detect shadows of potential predators looming above them - - whether that detection is perceived simply as quick changes in light intensity or whether an image is formed is not known. Lobsters also use some obvious visual displays in agonistic encounters. Still, chemical and tactile senses appear to be the keys to successful social encounters and prey detection. However, vision in lobsters has not been well studied.

Even though lobsters live in a watery world, they have highly developed systems of both smell and taste. The first antennae, properly known as antennules (little antennae), act as the "nose" of the lobster. Hundreds of fine hairs cover the antennules and are the actual organs of smell. These hairs are incredibly sensitive to amino acids, the building blocks of all proteins, of which animal tissue is made. However, the hairs are densely packed on the antennules and this proves to be a problem in a watery environment. Water is much more viscous (sticker) than air, as oils are more viscous than water. When fine structures are densely packed together and placed in a water environment, the water between these structures is not easily moved - - in other words, a boundary of nonmoving water is formed around the structure. In order for a lobster to be able to smell something, or to be able to walk towards a smell, it has to constantly sample the chemicals in the water to determine their changing concentration. Lobsters do this in the same way that humans do - - they sniff. Sniffing is accomplished by flicking the antennule downward quickly - - this removes the old water and replaces it with new water and a new odor sample. Flicking can be easily observed by watching a lobster in a tank (at an aquarium,restaurant, supermarket, or lobster pound) for just a few moments. Because lobsters have two antennules, they can determine the direction of the smell by comparing the difference in concentrations between the two antennules. Humans use a similar mechanism for distinguishing between different concentrations (noise levels) of the same signal to determine the direction of sounds.

The legs and mouthparts possess the taste organs, which are also hairs, but of different shapes from those found on the antennules. Legs probe the sediment for food items and pass these items to the mouthparts which provide the final determination of whether something should be swallowed or not. Clawed lobsters generally use a combination of claws, legs, and mouthparts while consuming food. The claws are used for crushing mollusks. Spiny lobsters lack claws and use their extremely strong jaws (mandibles) to break open mollusks. Slipper lobsters lack both claws and strong jaws and instead use the incredibly sharp points of their legs to slice open mollusks. If the food is fleshy, the lobster will grip one end in its jaws (mandibles, the final pair of mouthparts before the esophagus) and use the first pair of mouthparts and/or the pincers of the legs to pull the flesh downward. This stretches and tears the muscle fibers of the flesh and the lobster then consumes "strings" of flesh. Lobsters have been misclassified as scavengers - - eaters of rotting flesh and dead animals. Lobsters generally reject rotting material and greatly prefer live or freshly killed food over dead decaying tissue.

Mechanoreception (touch, water movements, equilibrium, motion control, and hearing) is also a very important sense for lobsters. The large antennae house the major touch receptors. Lobster antennae can detect uni- and bidirectional water movements, which would help in orientation to currents or swells. Their carapace has numerous hairs which probably serve to detect water movements similar to the lateral line system of fishes - - this is the case in crayfish, but has yet to be proven in lobsters. This sense would be particularly useful in small lobsters who are prey to many moving predators and to lobsters attempting to capture food when vision may not be possible, as it would provide much information on moving objects. The entire exoskeleton has cuticular receptors that monitor stress (pressure) on the cuticle (shell) itself.

Statocysts (an organ for orientation and equilibrium) lie in the base of the antennules. They are composed of 400-500 hairs distributed in 4 rows that project into the statolith (sand grains cemented together). When the animal shifts its orientation, fluid in the statocyst moves the hairs in a particular direction, relative to the statolith, which moves at a different rate because of its inertia. This differential movement stimulates some hairs more than others and gives the animal a sense of the direction of the movement. Similarly, the organ is used to detect the overall position of the animal - - whether it is right side up or upside down. The purpose of the statocyst is to give the animal a sense of equilibrium and it functions much like our inner ear system.

Lobsters also possess proprioreceptors - - sensory hairs that are internal and provide information about limb movement, posture, and equilibrium. These are generally located at joints and within muscles and are stimulated when the joint is bent or straightened and when the muscles are stretched. Proprioreception is critical to maintaining proper posture and coordination during movements.

Lobsters are also capable of producing and detecting low-frequency sounds. Clawed lobsters produce a growl or rasplike sound by contracting a small sonic muscle in the base of the large antennae. These sounds are not made during social interactions, but have been recorded for lobsters resting in their shelters and can be felt (not heard) when a human pulls a lobster from its tank or natural habitat. Their purpose is completely unknown, but may have something to do with defense. In contrast, spiny lobsters possess a stridulatory organ at the base of the second antennae that makes a variety of sounds: rasps (during aggressive encounters or when predators are nearby), slow rattles/flutters (when secluded), pops (when the lobster is out of its shelter), and mating calls. They do not, however, possess the functional equivalent of ears and thus do not hear these sounds in the same fashion as we do. But sounds are simply vibrations that travel through the medium they are produced in (air or water) and thus, they can be detected by mechanoreceptors that are sensitive to water vibrations.

Do lobsters feel pain? This question has been asked by many a person who tosses a live lobster into a boiling pot or slices the live lobster down the middle to bake stuff. The answer is not at all clear. The lobster's nervous system has been extremely well-studied because it serves as a "simple" model of neural circuitry in something less complicated than the highly cephalized vertebrates. Lobsters do not possess any kind of receptor akin to our pain receptors. However, they do possess stress receptors and certainly perceive the slice of a knife. It is not known whether they possess any kind of temperature sensitivity, although each species is adapted to live in a certain range of temperatures and will eventually die if forced to live beyond its normal temperature range. Scientists have not discovered how a lobster's "brain" processes sensory information from cuticular stress or temperature, so we cannot say for sure if they feel pain or not. Many people have debated how to "humanely" kill a lobster with ideas ranging from electrocution to freezing before boiling (though that also exposes them to an unpleasant temperature and it takes some time before they die). The general consensus is that death is most rapid - - and, if they do indeed "feel" something, it is only momentary - - if they are placed into a boiling pot of water.

The Muscular System and the Lobster's Tail

The muscular system of the lobster brings about movements and constitutes the motor (movement) component of all behavior. It is also the culinary part of the lobster - - particularly as the muscle, or meat, is relatively fat-free. However, it is important for another reason and that is that the lobster, over its lifespan of 50+ years, shows few conventional signs of aging or of muscular disease. They have two basic muscle fiber types which are grouped in bundles: fast and slow and the muscles are composed of single-type bundles (fast or slow) or mixed bundles. In clawed lobsters, the muscles of the prominent claws change from mixed bundles to primarily slow bundles in the crusher (largest) claw and fast bundles in the cutter or seizer claw. Neuron specialized for muscle (called motorneurons) penetrate into the muscles and are either excitatory (elicit contractions) or inhibitory (prevent contraction). This entire neuro-musculatory system undergoes remodeling over the entire lifespan of the lobster and particularly at periods of growth, where the animal sheds its old shell and puts on a new, larger shell.

The abdomen consists of little other than muscle, intestine, ventral nerve cord, and an artery. The two masses of muscles present are made up of small, dorsal (top surface) abdominal muscles which lie above the intestine and artery and function as extensors, and very large, ventral (on the underside) abdominal muscles, which function as flexors. Within the ventral nerve cord that innervates the ventral muscles are two kinds of giant axons: the medial giants and lateral giants, as well as groups of non-giant axons.

The giant axons mediate two types of escape reflexes or rapid tail flips. The first type is caused by the medial giant axons, which project forward to the brain and throughout all of the abdominal segments, and propels the lobster directly backward. This kind of response occurs when the lobster is startled by abrupt rostral visual or tactile stimuli (such as a rap on the head). The second type is caused by the lateral giant axons, which are found in the first three abdominal segments, but not in the last three. This kind of a response occurs when the lobster received abrupt tactile stimulation along its abdomen. Because the last three segments are not innervated, they remain straight, like paddles, and redirect the thrust generated by the flexion of the first three abdominal segments downward. Thus, the lobster flips upward and somersaults. After this somersault flip, there are one or two pitching flips which cause the lobster to land on its back, followed by two to three twisting flips which turn the lobster upright. Slower, swimming tail flips are produced by the non-giant axons, whose processes originate in the subesophageal ganglia, and generally follow the escape tail flips mediated by the medial or lateral giants. (Figure redrawn by Sapir Ad.)

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