How Scientists Classify Marine Life... And Why, And What It All Means to You
Story and photos by Marty Snyderman
Note: This article is the first in an ongoing series about the classification of the marine animals you are likely to see when you dive. The series is designed to help you gain practical knowledge about the natural history and behaviors of the creatures that draw us into diving. We promise not to bore you by being too highbrow, but we do want to help you understand how to find the answers to questions like, "What is that?" and, "Are those animals closely related?"
Suppose you walked into an introductory class called Biology 101, and on day one your teacher presented you with specimens of at least one male, female and juvenile of every living species found in the world's oceans. Now, let's add in all the fossil evidence known to science. Your semester-long assignment: "Start sorting, make sense of it all and create a system of classification that will be accepted worldwide." With just under 2 million living species and far more extinct species, it is pretty easy to believe that a lot of us would be headed for the nearest exit.
As sport divers, it is highly unlikely we will ever face this scenario, but it is easy for anyone to understand that the task would be both complicated and incredibly painstaking. But it is a task that biologists have faced for centuries and that they continue to confront every day. They try to make sense of the natural world, and organize their knowledge in a way that has universal acceptance and meaning.
Now, put yourself in a typical setting on a dive boat. On a trip to explore California waters, you might find yourself on a boat with someone who tells you he just saw "a beautiful giant sea star," only to have his buddy chime in and say, "No, that was a knobby sea star." Someone else might say, "I saw the animal you were looking at, and it is called a giant sea star, I think. No, maybe it is a giant-spined sea star." So you and the rest of the gang go off to look the animal up in a reference book to see who is right. A few minutes later, each diver returns with a book that "proves" he or she is correct.
That's right! One species of sea star can have at least four accepted common names. Now that can get confusing! It's equally confusing when different species share the same common name. That is the case with "the" red crab found in California waters. The term red crab is used to describe two species. One is commonly found in reef communities in central and northern California, and is highly valued as a food source, while the second species is much smaller, of no food value to humans and only rarely encountered near shore. If your dive buddies told you they just saw a red crab, you really wouldn't be able to be sure what animal they encountered without further clarification.
Ready for another scenario? Try this one. You are getting ready to make a dive when another buddy team climbs up the dive ladder and excitedly announces that they just saw a white shark under the boat. Quickly, you ask yourself, do they mean a whitetip reef shark, an oceanic whitetip shark or a great white shark Ñ the legendary star of the Hollywood hit Jaws and the Discovery Channel's Shark Week. Are you going to proceed with your dive plan? My bet is that if your IQ is as high as the water temperature, you will want more than a little clarification and assurance before you let go of the boat.
From these scenarios alone, it is easy to understand the potential for confusion and miscommunication caused by the use of common names, those names used by laymen in everyday conversation. Knowing that members of the scientific community must be able to communicate precisely with one another without any room for misunderstanding and error, it is also easy to understand why the use of common names doesn't work for scientists. They need to be able to clearly distinguish each species from every other.
To date, almost 2 million currently living species of plants and animals have been described and classified by scientists. In order to study these organisms and make sense of it all, specialists group these organisms into various categories based on commonly shared anatomical, or morphological, traits. This organizational system is referred to as the system of taxonomic (taks uh NAH mik) classification. It has been the scientific standard ever since being introduced in 1758 by a Swedish scientist named Carolus Linnaeus.
What might prove surprising is how often you hear terms from the taxonomic system used in discussions between sport divers. While it is possible that in some instances someone is trying to impress, in many situations laymen use familiar taxonomic terms so they can communicate clearly without any one-upmanship intended. The scenarios with the red crab and white shark are good examples of times that some knowledgeable diver might use a taxonomic, or scientific, name to prevent a misunderstanding.
You are probably more familiar with the system and how it works than you suspect. Surely it hasn't been too long since you heard someone refer to our species by our genus and species names, Homo sapiens. Derived from the Latin "same wise," the name Homo sapiens is the name used to describe human beings in the taxonomic system. In correctly written language, both the genus and species are italicized, and the genus name (Homo) is capitalized, while the species name (sapiens) is not.
I am sure you remember at least some of the basic principles learned in your high school biology classes, such as the fact that plants and animals are given scientific names called their genus and species. These names are the all-too-often ridiculously long, impossible-to-pronounce Latin and Greek italicized words that you had to memorize in order to pass a test your teacher told you would likely determine your professional future. This, despite the fact that by the time the weekend rolled around the only person on the planet who seemed to care about any of those words was your biology teacher. Even so, it really shouldn't come as a surprise when you are reminded that scientists worldwide accept the language of taxonomic classification. Its usage allows them to communicate with each other with the exactness required when there is no room for error.
Rest assured, no one is suggesting that you start communicating in Latin and Greek when chatting with your diving pals. I am simply pointing out 1) why scientists need an agreed-upon system and language, and 2) why it will prove helpful for you to have some familiarity with the classification system used by scientists when you use reference sources, even those written in lay language for divers.
Once you refamiliarize yourself with the taxonomic system, you will probably be surprised by how quickly you begin to use and understand certain terms and scenarios. For example, you will soon realize why stinging animals such as jellyfishes, corals and anemones are so closely related. All are members of a group, or phylum (the taxonomic term), of animals called cnidarians. All creatures described in this phylum possess stinging cells, and it is this characteristic that distinguishes them from animals described in other phyla.
If you are like a lot of divers, long ago you happily forgot the bulk of the information you once memorized about phyla, classes, orders and families. If you fit into this category, don't despair. The system isn't difficult to understand, and you will probably surprise yourself by how quickly some dormant brain cells snap back to life. If you would like to remember at least some of the basics, the information in this article will prove helpful. In some cases it will enable you to positively identify the plants and animals you see during your dives, and help make it much easier for you to gain insight into the natural history of many of these organisms.
The first major division in the taxonomic system is that of kingdoms, the kingdom of plants and the kingdom of animals. At least that is how it was 30 some-odd years ago when I was in high school. Today, there are at least three additional kingdoms, but they are not very relevant to our diving interests. Just the same, their existence illustrates the fact that the process of classifying all the species on Earth is the subject of spirited ongoing debates. As the renowned biologist Edward O. Wilson stated in 1992 in his text, The Diversity of Life, "systematics is mostly science, but also a bit of art."
Many species of plants play vital roles in the ecology of the world's oceans, but with the exception of the alga known as kelps, most sport divers don't pay too much attention to smaller marine plants. As a result, this article and this series will focus on the creatures described in the animal kingdom. Within the animal kingdom, scientific specialists have grouped, or classified, various species according to the following hierarchy:
(Of course, to complicate matters, there are also categories known as subphyla, subclasses, suborders, subfamilies, subgenera, and subspecies, but it is unlikely, with only an occasional exception, that you will need to get involved at this level of classification. I am pointing out the existence of subgroups only so encountering these terms won't someday throw you for a loop.)
A specific organism is classified and placed in each of the just-mentioned categories in the taxonomic hierarchy, and in the end is named according to its genus and species. Adult males and females of the same species should be able to reproduce, while in most cases successful reproduction is not possible with members of different species. However, this is not always the case, but let's assume you are taking Taxonomy (taks AW noe mee) 101, not the advanced class. Most of the animals that divers are interested in are described in only nine of the approximately 35 major phyla that comprise the animal kingdom.
Figure 1 provides a detailed example of the taxonomic classification of four animals whose common names are the California spiny lobster, garibaldi, schoolmaster snapper and humpback whale.
Organisms that are different species often share many common features. As a result, these creatures would be described in the same categories, such as their class, order and family. It is even possible for animals to be members of the same genus and to differ only with regard to their species. If you examine the chart, you can see that the fishes named the garibaldi and schoolmaster are described in the same kingdom, phylum, subphylum, class and order, but they are then distinguished by being described in different families. From that point on in the hierarchy, these species will be found in different categories, such as their genera (plural of genus) and species.
Being included in more of the same categories within the hierarchy indicates that animals are more closely related to each other than they are to animals that are described in different groups. So it is easy to see that a garibaldi is more closely related to a schoolmaster snapper, another fish, than it is to a California spiny lobster. While all three species are included in the kingdom of animals, the lobster is described in the phylum Arthropoda, while the fishes are described in a different phylum, Chordata. (See sidebar, "Classifying Marine Life.")
The humpback whale is described in the same phylum as the garibaldi and schoolmaster, but they are members of different classes. As you would suspect, whales and fishes are not all that closely related. It logically follows that other fishes are more closely related to, and share more common traits with, garibaldi and schoolmasters than they do with spiny lobsters or whales. Conversely, other species of whales and dolphins, as well as all other mammals, all of which are described in the class Mammalia, are more closely related to each other than they are to fishes or to lobsters.
The benefit to you in understanding the nature of these relationships and the fundamentals of the taxonomic classification system is that you can safely make some generalizations about a given animal's natural history if you know about the natural history of other species described in the same class, order, family or genus. For instance, all the species in the family Pomacentridae (damselfishes) are relatively small, feisty fishes that occur in grass beds and reef communities in temperate and tropical seas around the world. Most species are shorter than 6 inches (15 cm) in length with bodies that are characterized by their round to oval shape while being flattened from side to side. All damselfishes have a single nostril on each side of their snout, as opposed to the pair of nostrils located on each side, as is the case with most other fishes.
As a rule, the approximately 275 species of damselfishes found worldwide are strongly territorial and readily defend their chosen turf against all intruders, from the tiniest reef fishes to full-grown divers. Their pugnacity is especially evident in nest-guarding males during their mating seasons. In some species, the females also vigorously defend their nests.
In the case of garibaldi (comparatively large members of the damselfish family that occur in southern California), you can safely make a lot of generalizations about their natural history if you know that garibaldi are damselfish and you know something about other damsels, such as sergeant-majors. The same is true for many other animals described in the same class, order, family and genus.
If you encounter another fish that you are able to identify as some kind of damselfish, you can reasonably expect to observe behaviors that are associated with this family as a whole. In many instances you will be satisfied by knowing more or less where an animal fits into nature's overall scheme. This is likely to be especially true in the case of many similar-looking, small invertebrates such as worms, snails, bryozoans, shrimps and crabs.
Just being able to group different species that share common characteristics to the family level can provide you with a lot of information about how and where an animal fits into the kingdom of animals, the varieties of behaviors you might reasonably expect to observe and how to find out more information if you want to use scientific texts as reference sources. In addition, the topics in many marine life guides written specifically for divers are organized by the taxonomic system. In the final analysis, while the taxonomic system was created by and for scientists, sport divers also commonly use it.
Understanding Phylum, Class, Order, Family and Genus
The simplistic definition states that in order to be considered a member of the same species, adult males and females must be able to successfully produce offspring when they mate. But what about all the other categories in the taxonomic hierarchy such as phylum, class, order, family and genus? How do given species get included or excluded? Scientists explain that all the divisions above that of species are artificial. They are created in order to enable scientists to conveniently label groups of similar organisms. Of course, the categories are not randomly made. They are organized to reflect known or assumed evolutionary relationships between various life forms.
Ideally, every genus consists of a group of very similar, closely related, but genetically isolated species. Families contain related genera whose members share many common features. The family of angelfishes provides a familiar example.
Orders include related families based on shared generalized characteristics. Classes and phyla (plural of phylum) are even larger categories based upon more generally described and shared characteristics.
To date, almost 2 million currently living species of plants and animals have been described and classified by scientists. In order to study these organisms and make sense of it all, specialists group these organisms into various categories based on commonly shared anatomical, or morphological, traits. This organizational system is referred to as the system of taxonomic classification.