Invertebrate Differences

Even though they may be caring for relatively large numbers of invertebrate animals, most aquarists have significant misconceptions about these creatures. Invertebrates are often seen as a single group of animals, similar in many regards to the more familiar vertebrates such as dogs, cats or fishes; such a conception is both wrong and misleading and can result in significant problems in caring for them. Even the basic separation of animals into invertebrates and vertebrates is misleading as it implies some sort of division into equal parts. The total number of animal species is not known, but estimates of the number range from about 2,000,000 species to about 50,000,000 species. The number of vertebrate species is relatively well known and stands at about 45,000, thus that group comprises between 0.09% to about 2.25% of the total number of living animal species. In terms of species numbers, vertebrates are rather insignificant.

The inclusion of humans within vertebrates probably drives the separation of the animal kingdom with an "us" and "them" division, with most people assuming that the invertebrates are just "not as good" as the vertebrates. In truth, the vertebrate body plan IS a rather good one, and probably the best design for making large animals. However, most animals are neither large nor vertebrates, and most have a basic architecture that is fundamentally and distinctly different from that found in the vertebrates. The inclusion of humans within the vertebrates has one other, more important, consequence: most people tend to think that invertebrates are just simpler, smaller versions of vertebrates. In truth, they are fundamentally different types of life. I thought I would use this column to explore a few of these differences.

See The Differences

Humans are possibly the most visually oriented of animals. We have a visual system that is second to none; our eyes have exceptional powers of resolution and we can see colors. Outside of the other large primates, no other animals possess eyes that are as capable. Some of the raptors can resolve finer objects, but they live in a monochrome world, and lose all the visual information that is encoded in color. Because of our reliance on vision, we tend to think that all other animals, even those with good eyes, are equally dependant upon vision. Such just isn't the case; for most animals, vision, as we perceive it, is simply irrelevant. Most animals sense their worlds very differently than we do ours.

Figure 1. All of the bristles on this female caprellid amphipod's body are sensory. They respond to
water moving over them, and they are hollow and contain nerves that chemically sense the water
for critical dissolved materials.

We often discuss water flow and how it influences the animals in our systems. Well, folks, as hobbyists we really don't have a clue how it influences them. These animals are living in a system of moving fluids that interact with sensory receptors all over their body surfaces. Most of the animals in aquaria come from environments where the water moves in non-turbulent bulk, a more-or-less laminar flow. Such animals have been shaped by millions of years of natural selection to be able to exist in such an environment. These environments have specific properties of water movement and, for optimal health, the organisms require water movements to conform to the conditions in their natural habitat. Unfortunately, we cannot even approximate such bulk flow in any aquarium. The results of this failure on our part are the abnormal growth forms and behaviors seen in many sessile marine invertebrates in our systems. Most sponges in our systems grow abnormally - if they grow at all. Corals and soft corals are also typically misshapen. Such animals often cannot feed or eliminate wastes normally, and they may be subject to abnormal microhabitats with all of their associated problems, such as unusual chemical distributions, lighting patterns, and feeding restrictions, simply because the water flows in a turbulent rather than a laminar manner.

And then there are eyes…. When most aquarists look at an invertebrate with what appears to be eyes, they see something that is not there. They generally see an animal that they perceive can see them as well as they can see it. With very few exceptions, such a perception is totally false. There are some invertebrates that have camera eyes that have the ability to resolve small difference as well as a vertebrate eye of the same size. Such eyes are found in the cephalopods, such as squids and octopuses, a few snails, and a few polychaete worms. No other eye in the marine invertebrates is capable of forming what we would call a recognizable image.

Probably the most common type of "eye" found in the marine invertebrates is a simple ocellus or "eyespot." This photoreceptor consists of a group of pigment cells and group of sensory neurons. The pigment cells are often found in the shape of a cup or bowl, and the sensory receptors are often located within the bowl. Such a photoreceptor acts to shade the sensory cells from light coming from most or some directions. Because of this, these receptors are often considered to be directional receptors or a sensory receptor that allows the animal to move toward or away from light.

Figure 2. Left is a Polyclad flatworm. The brain is visible as a pair of large white patches to the left
side of the animal. Small eyespots are visible upon them. Right is a close up of the eyespots. These
are simple ocelli and probably allow the animal to sense light and its direction.

Interestingly enough, however, varying the light on many of these photoreceptors seems to have no discernable effect whatsoever. This probably means we don't know what to look for with a specific response, but it could also mean that the photoreceptor acts as a light "accumulator." Such an organ might, for example, produce a small amount of a hormone while it is illuminated. If such a hormone was destroyed at a constant rate by the body's metabolism, the hormone would gradually accumulate with increasing day length in the spring and gradually diminish during the autumn, and hormone levels could be used to synchronize spawning or other behaviors.

The most successful and widespread photoreceptor found in the invertebrates is the compound eye. These are eyes made of several to several thousand discrete functional units called ommatidia. Each of these ommatidia, in turn, is made of several discrete cells and function as independent photoreceptor units. They are not focusable, and do not form images on a retina. Instead, the image is focused on a central sensory structure running the length of the ommatidium. Consequently, any item impinging on the photoreceptor's field of view, regardless of its distance from the lens, triggers a response. Detection of distances by such an eye is impossible, but such an eye is extremely sensitive to objects moving across the field of view. The visual field of each unit of a compound eye is typically small; the whole eye's aggregated image would be pixilated and analogous to a small cathode ray monitor with very large pixels. While incapable of forming a fine image, such a photoreceptor would be very well adapted to detecting movement by the continual flickering of ommatidia on and off as objects passed through their small discrete units.

Figure 3. A marine water flea, Evadne. The units of the compound eye are visible to
the right. The black area is the region of the photoreceptor chemicals, but the tubular
nature of the compound eye units or ommatidia extends upward from this region.

Many of these animals with compound eyes can see colors. In some cases they can even see the same ones we do. On the other hand, some animals have a decidedly different way of looking at the world. Color vision is the result of the brain integrating the responses of visual pigments, called rhodopsins, each responding to distinctly different wavelengths of light. Most animals with color vision have three visual pigments generally responding to colors near magenta, yellow and cyan. The processing of the responses from these photoreceptors produces what we (and most other animals with color vision) perceive of as our colorful world. Well, not in the much maligned mantis shrimps. Some of these superb predators have been reported to have eyes containing as many as 16 different types of photoreceptors, color filters and polarization receptors. They possess 12 narrowly tuned spectral sensitivities that cover the color spectrum. The human eye can respond easily to around 30,000 different colors and hues. Mantis shrimps could conceivably respond to millions of discrete colors, most of which would be impossible for us to even distinguish.

For some more information and illustrations of a mantis shrimp eye click here.

Even the animals with good camera eyes, such as the octopuses and squids, don't see things the way we do. Vertebrate eyes are indirect eyes, the photoreceptor cells, the rods and the cones, face away from the incoming light and in effect respond to the light bouncing off a reflective layer at the back of the retina called the tapetum. This reflective layer is differently colored in various vertebrates and is what is responsible for the various colors that vertebrate eyes reflect in the dark. Be that as it may, the camera eye of cephalopods and vertebrates produce images in much the same manner. However, the two groups of animals do not perceive the same image in the same manner. Much of the processing of an image is done by neural units, sort of like subprocessors, that integrate the responses from the photoreceptor cells, prior to the responses being sent to the brain.

The neural subprocessors, or retinal ganglion cells, of the cephalopod and vertebrate eyes are connected to their receptor cells in decidedly different manners. This results in an image of the same object being perceived of differently. What a vertebrate might perceive as a solid black circle, for example, might be seen by an octopus as a series of checkerboard patterns of alternating light and dark oblique lines. So, even these two eyes might seem similar, they are sending fundamentally different signals to the respective brains.

"Prick us, do we not bleed….blue?"

Everyone is familiar with the circulatory system of vertebrates consisting of a heart which pumps nice red blood through arteries to the tissues and thence through capillaries which carry blood through the tissues and finally into the veins to bring blood back to the heart. This circulatory system is basic to the vertebrates and to the polychaete or bristle worms which have essentially the same pattern. The worm system is an independent variation on the theme, however, as they use a different type of hemoglobin and it is not in corpuscles but free in the blood.

The circulatory system in most invertebrates is wholly unlike that seen in vertebrates. Circulatory systems exist to distribute dissolved materials throughout an animal's body. Specifically, they distribute dissolved foodstuffs such as sugars and amino acids from the guts to the tissues where they are used, and they move dissolved gases to and from points of utilization in the tissues to gas exchange surfaces such as gills, and they move nitrogenous waste products to the excretory organs. That one system can perform all of these tasks simultaneously is marvelous, but equally marvelous is the diversity of different designs that do it successfully.

If the circulatory system is looked at conceptually, there are really two extremes. One extreme is the pattern seen in the vertebrates and polychaete worms. Here, the blood is contained totally with in vessels. The other extreme is seen in animals such as peanut worms or sipunculans. These animals really lack a dedicated circulatory system, but the entire body is a blood filled bag, and the contents just "slosh" around as the animal moves.

Somewhat in between these extremes is the pattern seen in the crustaceans. These animals have a heart that generally is found in the upper central portion of the back. This heart pumps blood forward to the animal's brain through thin walled vessels. From then on the blood flows through channels or spaces in the tissues. Although the blood is not constrained by vessels, the blood flow is not sloppy or haphazard, but is rapid and precise. Crustaceans typically have more blood per mass of the animal than do vertebrates, but for animals of a comparable size the velocity of blood within the tissues is about the same in both groups.

Crustacean blood differs from vertebrate blood in one other characteristic. It is not red as it lacks hemoglobin. Hemoglobin in vertebrate blood carries dissolved oxygen to the tissues and carbon dioxide to the respiratory surfaces. In most small crustaceans, these gasses are simply dissolved in the blood and the relatively larger volume of blood ensures that sufficient gas exchange occurs. In larger crustaceans, a chemical called hemocyanin is found dissolved in the blood. Hemocyanin is a material made of protein subunits complexed with copper ions. It will bind to oxygen in areas of high oxygen concentration and release it in areas of lower oxygen concentration, so it is a respiratory pigment. It is not a particularly good respiratory pigment when compared to hemoglobin, but it does appear to assist in oxygen transport. Like hemoglobin, its color changes depending upon whether or not it is carrying oxygen. Lacking oxygen it is colorless, but while carrying oxygen, it is a beautiful pale blue or cyan; which, of course, gives the pigment its name.

Figure 4. The gills of a Northeastern Pacific shore crab, Hemigrapsus nudus, exposed in
a dissection. The blue color in the gills is due to the hemocyanin in the crab's blood.

The blue blood of the horseshoe crab Limulus polyphemus is extracted and used in biomedical research. For a picture of such blood, follow this link.

Hemocyanin is also found as a respiratory pigment in mollusks, and in cephalopods there is a blending of the closed circulatory pattern such as is found in vertebrates with the blood pigment of the crustaceans and other mollusks. The design of the circulatory and respiratory systems of the cephalopod mollusks such as squids and octopuses is the most efficient design found in any water-breathing animal. Fishes have a closed circulatory system, but it is a low pressure, relatively slowly flowing system. The fishes pump their blood through the gills immediately after leaving the heart. This blood flows through a capillary bed in the gills and then has to flow through the body, but blood pressure is lost in the capillary bed of the gills and from there back to the heart the flow is slow. It wasn't until the double pump heart of the advanced reptiles, birds, and mammals evolved that a high pressure rapid circulatory system evolved in the vertebrates. Such a system isolates the respiratory capillary bed, in the lungs in this case, from the rest of the general body circulation. The general body, or systemic, circulation is high speed and high pressure allowing for the development of high-pressure filtration kidneys and rapid transport of nutrients and gases, which in turn allowed the development of the high metabolic rate characteristic of mammals and birds.

In the seas, a similar circulation pattern is seen in the cephalopods, only it involves three hearts. Each of the two gills has a heart at its base pumping blood through the capillary bed in each gill. From the gill the oxygenated blood flows to a third, systemic, heart which pumps blood to the brain and body. As with the mammals and birds on land, this high pressure system has allowed the development of a higher metabolic rate and efficient high-pressure kidneys. Probably as a result of this efficiency, the evolution of a large nervous system with obvious intelligence has been favored. The intelligence of the cephalopods is well known; but it is from a different blue-blooded base than our own. Interestingly as we learn more about it some functional convergent similarities have been found. For example, using criteria developed for primates, octopuses have recently been shown to play with toys (Mather & Anderson, 1999). Perhaps at the basic level of information processing, it doesn't matter what color your blood is or the source of your neuronal information, once a basic threshold is crossed some properties of intelligent animals may have to share certain functional attributes such as the need, occasionally, to play.

If you have any questions about this article, please visit my author forum on Reef Central.

References Cited:

Mather, J. A. and R. C. Anderson. 1999. Exploration, Play, and Habituation in Octopuses (Octopus dolfleini). Journal of Comparative Psychology. 113:333-338.

Other References of Interest:

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Prosser, C. L. ed. 1991. Environmental and Metabolic Animal Physiology. Comparative Animal Physiology, 4th ed. Wiley-Liss, Inc. New York, NY. 578 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

ReefKeeping Magazine™ Reef Central, LLC. Copyright © 2002

Invertebrate Differences -