According to tradition, the answer to the old English riddle, "Why is a duck?" is "Because one of its legs is both the same." A similar whiff of good, healthy surrealism is often present in discussions of invertebrate animals in the marine reef aquarium hobby. The majority of invertebrates are decidedly odd and unfamiliar to most people, so perhaps the many examples of such wackiness that occur from time to time may be excused. However, given the familiarity that I thought most aquarists have with fish, I have always expected discussions of fishes to show a sounder grasp of reality. Although many aquarists seem able to relate to and understand the practices of fish husbandry, I have seen numerous instances in the recent past indicating that, while the level of misunderstanding that permeates the discussions of invertebrates may be exceptional, there is plenty of misunderstanding about the fishes as well. As a result, for a number of years now, I have wanted to do a short article, or three, on the attributes of fishes simply to discuss some of their basic features with the hope of piquing, in some small way, further interest in them as marvelous examples of functional morphology.

I suppose some readers may be asking themselves, "Why should an invertebrate zoologist feel qualified to discuss fishes?" After all, being an invertebrate zoologist, it might seem obvious that I don't have a background in ichthyology. Well, just as obviously, seeing much of what is written and discussed in the hobbyist "literature," it is certainly apparent that having expertise in a topic is no prerequisite to writing about the topic. Indeed, it may be a hindrance; if someone actually has the background, his or her ethics may prevent becoming involved in the "speculations under the guise of fact" that seem to permeate the hobby, particularly in some of the online forums, where common sense is uncommon and supposition and superstition take the place of fact and true scholarship. But I digress; I think that all professional zoologists probably got their start studying vertebrates. Most undergraduate university programs are organized to teach vertebrate biology first, and then, finally, ascend to the true and glorious path of righteous enlightenment, the study of invertebrates. So, like most other zoologists, I was originally trained in the ways of vertebrate animals, and I think that training has given me, as an author, sufficient background to cast a few lines into the pond of ichthyological lore in the hope that I might hook a tale or two.

"If one could conclude as to the nature of the Creator from a study of his creation it would appear that God has a special fondness for stars and beetles."
John B. S. Haldane

Haldane, a rather famous atheist and, probably, the pre-eminent physiologist in the middle-third of the twentieth century, responded with the above statement to a question about what he could discern about God from his study of biology. One might reckon, in addition, that He (She? It?) also had a special fondness for fishes. If one were to pick a typical animal from that part of the Earth covered by water, it would likely be a fish. I don't mean to imply that fishes are the most abundant types of animals in the seas, far from it, that distinction probably resides with the copepods; however, fishes as a whole are probably the most diverse group of animals living in the water, as opposed to the benthos, of the oceans.

A Bit of Variety

Coral reef aquarists tend to oversimplify things. We often hear statements such as, "Corals do this," or "Fish do that," as if there were only one species of either corals or fishes. This simplification is, in the best case, a convenient shorthand for dealing with a diverse group of disparate and dissimilar animals. Too often, however, such a simplification seems to be a declaration of ignorance that reflects a profound misunderstanding of the animals involved and how to care for them. Nonetheless, in the aquarium hobby, fishes probably fare better in this regard than do most animals simply because most reef fish species are distinctive and may often be easily distinguished from other species. Often, probably because shallow-water fishes are visually oriented, color patterns have developed that are specific to individual species. After all, many advantages accrue to fish from being able to distinguish animals of one's own kind, not the least of which is the ability to find a mate. We tend to take for granted that fish can see pretty well; that is to say that they can visually assess a distant object with some fine resolution and perhaps color. There is no indication, however, that this was a property of the animals that were early twigs on the bush of chordate evolution. Quite to the contrary, the most basal animals on the chordate lineage, the lancelets (Branchiostoma = Amphioxus), lack eyes. The most primitive living fishes, such as lampreys, have rudimentary eyes, while their distant cousins, the hagfishes, lack these sense organs altogether, although in this latter case the absence of eyes may represent a secondary loss from an ancestor that once possessed them. Nevertheless, good vision is a characteristic of reef fishes, and such vision would have facilitated the development of species-specific color patterns. As humans also have good vision, we can distinguish the same (presumably) color pattern signals that the fish themselves are using to discriminate differences in gender and species. Consequently, in these cases, the aquarist can tell a lot more about them than is possible to easily discern from corals or many other animals.

In fact, the variety of fish types is rather bewildering; many different types of fishes are alive today, including several thousand species found on various coral reefs alone. Such a diversity of form and function could make any article discussing them as a group a futile exercise in triviality. Notwithstanding the diversity apparent in coral reef fishes, in terms of types of body forms, ancient fishes were probably more diverse in general types, even though each of these types was likely less species rich than the types found today. Recognizable protochordates, including fossils called Haikouella (2) and Pikaia (2), were something like "super" lanclets, having been found in the fossil record from rocks formed from sediments deposited as long ago as the geological period referred to as the early Cambrian epoch, roughly half a billion years or so ago. These animals were not fishes, but they gave hints of what was to come. Evidence of early true fishes has been found in rocks of the Ordovician (1, 2) period, around 400,000,000 years ago. Most of this evidence consists of fossilized enameled teeth (only vertebrates can make enamel), and bony scales. Whole skeletons are rare, and many of these early fish were small, a centimeter or less in size.

Although a lot of other different lineages of fishes have evolved, most of these have gone extinct, and living fishes fall into a relatively few major lineages (see, for example, Turner, S. and Miller, R. F. 2005. New Ideas About Old Sharks, American Scientist 93: 244-252). Fortunately, by a process of elimination, an article's focus may be narrowed considerably by eliminating those types of fishes not commonly found in aquaria. The jawless fishes, such as lampreys and hagfishes, certainly are uncommon in even large commercial aquaria. This is, I suppose, a pity; these primitive fishes have some "interesting" and odd attributes. Additionally, watching a hagfish feed "up close and personal" is a profound and emphatic way to impart a lasting visual definition of the word "disgusting." Most fishes with a cartilaginous skeleton, such as sharks, rays, and ratfishes are simply too large to be suitable for most aquaria. Lobe-fin fishes are almost vanishingly rare today, and, being very large (2, 3) as well as from deep water, are essentially impossible to collect or maintain. Almost by default, aquarists are left with only the classical "bony fishes" as the fishes of choice, or phrased another way, as the primary group of fishes that may be easily kept in the home aquarium. Bony fishes are bilaterally symmetrical vertebrates with a few distinctive characteristics. Roughly 30,000 species of such fishes have been scientifically described. These species are taxonomically arranged in about 600 families in approximately 40 orders. Characters used to distinguish the various taxonomic groupings include mouth structure, dentition, position of the pelvic fins, lateral line shape and position, scale characters and the number and type of dorsal fin rays. Vertebrae, the bones surrounding and protecting the spinal cord, define the vertebrates, a group which includes amphibians, reptiles and their avian and mammalian descendents as well as fishes. Given the importance of bones to the scheme of things, I thought it would be worthwhile to briefly discuss some of their attributes.

Bones

Fishes, indeed all vertebrates, are rather peculiar animals biochemically, and this is probably related at least in part to their particular line of descent. Recent genetic studies have shown that the chordates, including the vertebrates, are descendent from some primitive ancestor through a lineage that includes echinoderms. One general property of this lineage, more so than the other major animal groups, is the ability to metabolize and secrete calcium salts as endoskeletal components: calcitic ossicles in echinoderms and calcium phosphate bones in vertebrates.

In many ways, the most obvious characteristic that sets the fishes, indeed all vertebrates, apart from all other animals is the presence of an internal skeleton, or endoskeleton, composed of bone. Many other animals, for example, crabs and shrimps, have complicated exoskeletons; in those cases, however, this skeleton is mostly proteinaceous or chitinous and, additionally, is continuous with the exoskeleton covering their body. Although they are mineralized, bones are unlike the calcite-containing echinoderm ossicles, as bones are not simply crystalline material. They are composed of a calcium-phosphate salt called hydroxyapatite along with a significant amount of proteinaceous material. All aquarists have seen bones and can easily visualize them. However, I think few aquarists think about them being inside a living animal, and particularly, they are not visualized inside our aquarium occupants. One major feature that tends to be overlooked is that unlike a coral's skeleton or a mollusk's shell, vertebrate bone is a living, dynamic, tissue constantly getting deposited, remodeled and altered in response to stress and use. The material comprising a fish's skeleton is rapidly and continuously metabolized and moved around, depending on the state of the organism. From an aquarist's perspective, it is important to consider the metabolism involving bone, as not only does bone contain calcium, but it also contains very large amounts of phosphate.

All animals need a lot of phosphate in their metabolic reactions, but vertebrates need much more of it than just about any other animals. All organisms use phosphate as part of the structural backbone of the DNA and RNA that constitute their genetic material. Additionally, all organisms use large amounts of phosphate as a component of the vast number of adenosine triphosphate, or ATP, molecules, the common "carrier" of energy throughout living organisms. Adenosine triphosphate is a part of just about every type of metabolic function, as it transfers energy either to, or from, the chemical reactions that constitute those functions. In addition to these uses of phosphate common to all animals, vertebrates need what might be termed excessively large amounts of phosphate because their skeleton is largely made of phosphate salts. It often is said that one way to cut down on the phosphate level in an aquarium is to use food low in phosphate. This is a particularly poor argument as it simply has the effect of starving the animals, particularly the fish. If they can't get enough phosphate from their food for basic metabolic processes, they will scavenge it from their own bones, resulting in weak bones. Eventually, if phosphate deprivation persists, the animals will die.

Functions

In addition to being phosphate reservoirs, bones have two additional functions. They serve as protection and they function as levers. The protective function of bones is probably best seen in the bones of the skull. Largely fused and immobile, these bones appear to serve two major purposes in bony fishes: they form a protective vault around the brain, and form points of articulation with the gill arches and the jaw (a modified gill arch). Interestingly enough, it appears that the bony plates that have been adapted to become part of the skull had their evolutionary origin as bony plates in the skin of some of the first small fishes. These fishes, called ostracoderms, lacked jaws and were covered in enough bony armor to make a turtle green with envy. Typically small animals, ostracoderms were found in ancient environments where the dominant predators appeared to be Eurypterids or sea scorpions, relatively large arthropods related to horseshoe crabs. It has been proposed that vertebrate dermal bone developed in response to the predation threat such animals posed.

The axial skeleton of vertebrates probably developed in response to natural selective pressures resulting from inefficient locomotion. Fishes can move in a number of ways, but the one most aquarists are probably familiar with is the use of fins as "oars" or "sculling" devices. This locomotion is particularly evident in fish with more-or-less immobile bodies, such as puffers, boxfishes and seahorses. It is much more common, though, for fish to move by rapidly oscillating their tails. This oscillation is caused by the contraction of muscles on opposite sides of the animal. Such contractions tend to flex the body of a soft-bodied animal into a series of undulations that pass down the body in a wavelike manner. When moving in this way, forward locomotion results from the motion of the wave passing down the body acting against the water surrounding it. As the wave propagates to the posterior end of the animal, the animal moves forward. This type of locomotion is seen in eels, sea snakes and in many types of worms. It is quite inefficient. A lot of energy is lost simply moving the body from side to side. On the other hand, if the body's axis is strengthened and held almost rigid and a terminal tail fin is moved in response to these oscillations, a very large component of the energy is spent moving the fin, which in turn moves the animal. Such locomotion depends on a stiff axial skeleton, which is present as the vertebral column, along with strong lateral muscles and, often, strengthening bony struts, such as ribs. The net result can be very rapid locomotion, indeed; tunas, for example, can sprint to speeds in excess of 75 km/hour (47 mph).

Small reef fish can't swim as fast as tuna, of course, and chief among other reasons is that they simply don't have the muscle mass to generate the forces necessary to propel themselves at that speed. Even in the smallest aquaria, however, small bursts of speed are common when fishes get startled. Among the fastest fishes commonly kept in reef aquaria are the fusiliers, Caesio species. These are planktivorous fishes that are normally found swimming some distance from the reef face nabbing small morsels brought to them on the currents. Fusiliers share many of the structural attributes of their high-speed cousins, and given a large enough tank, fusiliers can be seen to move at impressive velocities.

Speed isn't everything, though. The precision movement caused by fin locomotion alone is often a joy to watch. The delicacy with which a Mandarin dragonet positions itself before it strikes at a minute prey item is a testament to the advantages of binocular vision and good eye-fin coordination. For such locomotion to work at all, the fins must have lightweight, but strong, internal struts. These fine bones are termed fin rays. Bones function as levers, and when pulled by a muscle, they transmit force to some other place; the muscles that move a Mandarin dragonet's fins are located in the base of the fin, yet the stiff bony fin rays transmit their energy throughout the fins. It is well to remember, as well, that bones do function as levers. When pulled by a muscle, they have to transmit that force to some other place, and in doing so they may stretch out an antagonistic muscle. Muscles, anywhere and in anything, can only contract. Once contracted, they will remain contracted unless they are actively re-extended. The many small bones of a reef fish allow the fish to bend, flex, and turn because they transmit the force of muscle contraction through a bony lever. In doing so, they stretch out an opposing and antagonistic muscle.

Some aquarists become intimately familiar with other properties of fins when they contact the stiffened fin rays of a Lionfish (Pterois spp). Fitted with a potent venom apparatus, such fins may be used either in a defensive mode or to herd small prey into position prior to eating them. Lionfish are not the only fish with venomous spines, of course; that property is characteristic of the entire family of scorpion fishes. Many other fishes also have projecting bony spines, often at the leading edges of their fins. While not necessarily venomous, such spines certainly can do a lot to deter predation.

Quite possibly, though, the most important attribute of the existence of fishes, in particular, and vertebrates in general, is so obvious that it often escapes notice: bones continuously grow. Being internal to the tissues they have to grow without damaging the surrounding tissues. Having a skeleton growing inside its tissues may be vitally important to the animal, as the bone may respond to repeated stress by growing stronger. This means as the animal grows larger, the skeleton not only grows larger, but it changes its shape to match the increase in stresses put on it by the organism's increased size. In the final outcome this means that animals with an internal skeleton made of bones may become very much larger than animals with an exoskeleton.

Both endo- and exo-skeletons have to change proportions as they grow; in the final analysis skeletal functional strength is related to the surface area where muscles and viscera are attached as well as to the thickness of the skeletal material that provides structural strength to the skeleton. The surface area available for muscle attachment is measured by some factor of the square of the skeleton's linear measurement. Skeletal mass, however, increases as the animal's mass increases, and this growth is in response to the cube of the linear measurement. Additionally, muscle power is also proportional to muscle mass, and muscle mass grows in proportion to the cube of the linear measurement. Consequently, skeletons need to be proportionally larger and more robust in larger animals than in smaller ones, as the forces that the muscles can exert per unit area of skeletal surface increase with the cube of the length, but the skeletal surface area increases only by the square of the length.

As a result, the exoskeleton of a large crab, for example, is much thicker and more robust than is the exoskeleton of either a smaller crab species or a smaller individual of the same species. It has to be. The forces exerted upon it by the muscles are much greater. However, for an animal with an exoskeleton, increases in size can also spell trouble. As the exoskeleton gets progressively more robust, the amount of internal space for muscles and the viscera gets proportionally smaller. This is probably best visualized by considering the limb of a crab as a cylinder. In cross section, as the animal gets bigger, if the exoskeleton is constructed of the same material, the walls of the cylinder have to get progressively thicker and the free volume of the cylinder gets smaller. However, the muscles that move the limb have to lie within the volume of the cylinder. As growth proceeds, soon there isn't enough room in the appendage's lumen for the muscles to move the appendage.

A crab's exoskeleton weighs about as much as the endoskeleton of a fish of the same mass. This means that a crab the size of a baby elephant would have to have a skeletal weight roughly equivalent to the skeletal weight of the elephant. However, because the elephant's muscles are not confined in all directions by the surrounding exoskeleton, they can develop the more extensive muscle mass necessary to move the heavier bones. In other words, conceptually as well as actually, elephants can "work;" large vertebrates can and do exist. On the other hand, large arthropods on the same scale simply can't exist. Their exoskeleton would soon be too big to move with the muscle mass inside of it. There are a great many ecological and evolutionary reasons that becoming large is a "good" thing. The vertebrate body plan, based on an internal endoskeleton made of bones, makes such growth possible. The converse is also true, however. Such a body plan is not a particularly good way of making a small animal. The smallest vertebrates are reef fishes that reach an adult size less than one centimeter long, roughly the size of a neon tetra. Below that size, the necessary adaptations favor an animal with an exoskeleton over one with an endoskeleton.

Parting Thoughts

For those aquarists who are so inclined, reef aquaria are ideal places to examine the problems of animal form and function. The basic form of the vertebrate body plan and the reasons for the successes inherent in this form as it relates to larger animals are really self-evident. In terms of the number of species, fishes constitute the most successful vertebrates, and a large number of those are found on reefs. Such fishes are generally larger and faster than reef invertebrates in the area. The primary reason for this impressive radiation of species is the presence of an endoskeleton made of bones. Interestingly enough, the one pelagic invertebrate group that has evolved primarily to prey upon fishes, the squids, also contains large animals, and also is immensely successful (the world biomass of squids is increasing due to global warming and overfishing of larger fish that prey on small squids, and now exceeds the biomass of humans) and, more importantly for the subject of this article, squids also have an endoskeleton.

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

Useful References:

Hildebrand, M. 1974. Analysis of Vertebrate Structure. John Wiley and Sons, Inc. New York. 710 pp.

Wardle, C. S. and J. J. Videler. 1980. Fish Swimming. pp. 125-150. In: Elder, H. Y. and E. R. Trueman. eds. Aspects of Animal Movement. Cambridge University Press. Cambridge. ix and 1-250 pp.