What Kind of Fish is That?
What do you think of when you hear the word “species”? For thousands of years, going back to Linnaeus, the founder of modern taxonomy, and even Aristotle, the definition was uncomplicated. A species was a type of organism with a fixed set of traits. Each species had certain essential characteristics that did not change, much like the properties of chemical elements.
Since Darwin, the picture has become more complicated. The fossil record illustrates how new species gradually arise from older ones over long periods of time. More recently, scientists such as Peter and Rosemary Grant, studying Darwin’s finches in the Galapagos Islands, have shown that certain characteristics of a population can even change measurably from year to year in response to environmental factors. That means that an “average” Cactus Ground Finch in 2006 may be markedly different from an “average” member of that species in 2001.
For many years, the Biological Species Concept has been considered the ideal test of species-hood, and it is still the definition most often taught in biology class. Under this model, species are members of interbreeding or potentially interbreeding populations. They share a common gene pool and are “reproductively isolated” from other such groups. However, even this definition presents complications. Populations can become isolated from one another, develop distinctive identifying features, and later come back together to form a hybrid population. Or, populations may be partially hybrid, as is the case with two cricket species in the Eastern U.S.: parents produce viable offspring when the mother is a member of Gryllus firmus and the father is a Gryllus pennsylvanicus, but not when it’s the other way around. The term “potentially” can be problematic when populations are separated in nature by physical barriers; it’s hard to know how the organisms would react if the barriers were removed. And the BSC doesn’t even address the question of organisms that reproduce asexually, or those that we know only from the fossil record.
Biologists in the field have come to rely more and more on the Phylogenetic Species concept, which defines
Given all these complications, species designations start to look less like hard and fast definitions, and more like our best sketch of a tangled web of life in constant motion. The job of the taxonomist, then, is not to mechanically categorize life on earth, but to present hypotheses about the past and present relationships between different organisms.
John Friel, curator of CUMV’s Fish and Herps collections, knows a lot about the difficulties of describing species. In the last seven years, John has traveled to Africa six times to study freshwater fishes, and in the process has identified more than a dozen new species. Friel is a principal investigator with the All Catfish Species Inventory (ACSI), a project funded by the NSF with the goal of discovering and describing nearly all catfish species by 2009, as well as completing the phylogeny of the order Siluriformes. This will be a significant step towards understanding the evolutionary relationships between an important group of freshwater fishes.
Catfish are a particularly diverse clade and an economic resource in many parts of the world. They are found on every continent except Antarctica (where catfish fossils have been found) and display an impressive array of adaptations: there are catfish as small as your finger and catfish bigger than a full-grown man (link to cool photo), catfish that feed on wood and catfish that feed on blood, catfish that live only in waterfalls and rapids and catfish that live only in mud. There are catfish that carry their eggs on their bodies, and catfish that put their eggs in the mouths of other fishes, where they eat the host’s own clutch after hatching. Some catfish are noisy, producing sounds by rubbing their spines against their pectoral girdle, while others produce powerful electric fields. Documenting the diversity of these fishes in a standard format will open the doors for more detailed research on the life histories and evolutionary stories of these fishes. The naming of species is also a necessary first step towards conservation.
The International Commission on Zoological Nomenclature sets the standards for naming and describing new species. Before a new species goes on the record, the description must typically be accepted into a peer-reviewed journal. The actual decision that a given individual is or is not a member of an already described species is often a judgment call - one that may be vetoed when the description is submitted for publication. Taxonomists classify themselves as “lumpers” or “splitters” based on whether they have a high or low threshold for considering variation as substantial enough to warrant a new species title. Mistakes have been made on both ends: scientists have occasionally described what’s now considered the same species multiple times, while others have not been specific enough and encompassed two or more species within a single definition.
To deal with these problems, a specimen must be deposited in a collection as a “name-holder” whenever a new species is described. These specimens are called type specimens, and they serve as a critical reference for scientists working on a particular group of organisms.
In some ways, the type specimen seems like a throwback to a time when it was thought that the ultimate essence of a species could be embodied in one “ideal” individual. Yet type specimens are even more important today than they once were. We now have names for about 1.5 million species, and a better understanding of many organisms that allows us to recognize more subtle differences between populations. Reserving a type specimen means that a standard reference point exists. This resource is especially important because the written description is necessarily incomplete, since it’s impossible to describe everything about an organism.
Type specimens are often used when someone thinks they may have found an undescribed species. Because of the vast number of species already described, new species tend to be closely related to ones that are already on the record. So the key to identifying a new species may be verifying that it is not a member of a known sister species, and the first step in doing so might be to borrow a type specimen for comparison.
In theory, one could look at any specimen of that species as a comparison, but type specimens are ideal because regular specimens may be misidentified. Friel thinks of the label on a specimen as a hypothesis and not a definitive ruling. Sometimes museum specimens may be tagged only to family or genus and not species. This was the case with a catfish from the genus Chiloglanis, which Friel found in Harvard’s Museum of Comparative Zoology. No one had been able to identify it to species before because it was unlike any other species that had been described. Friel kept the specimen in mind when he visited the area in Africa where it had been collected, and on his last day before breaking camp and leaving the area, he found a single individual of the same kind [one or two distinguishing marks of the species?]. Friels descriptions of Chiloglanis xxx will be published [details].
A major difficulty in describing new species is that there may be significant variation between individuals of the same species. Just look at the diversity within Canis lupus familiaris, the dog, which is not even a species by itself, but rather a subspecies of Canis lupus, along with the gray wolf. To demonstrate how much variation is normal within a species, additional specimens may be deposited along with the type specimen. In this case, the specimen that acts as the name-holder for the species is called the holotype, while other individuals are called paratypes.
Generally, it’s best to have as many specimens as possible for reference when attempting to determine if a given organism is part of that species or not. For example, many traits exist on a continuum, so if the only two specimens happen to be from opposite ends of the spectrum, they may be mistaken for more discrete differences representing two separate kinds. In catfish, significant differences may be seen in the skeletons, dentition, and pigmentation.
Changes in sexual dimorphism – when males and females appear considerably different from each other – from one population to another can be a strong indicator that the two populations are different species. This is because sexual dimorphism can play a strong role in mating: it’s necessary to be able to identify a member of the opposite sex to reproduce! Accordingly, the highly developed spines on the cheeks of males but not females, unique among catfish of the Mochokidae family, were a tip-off that several catfish brought back to the CUMV by Cornell researchers working in Gabon were members of a previously undescribed species. Friel and Thomas Vigliotta, a grad student, named the fish Synodontis acanthopera after these “thorny” opercular spines (acanthi means thorn in Greek). This newly recognized species can also be identified by its unique color pattern. (Link to article, photos)
The Museum of Vertebrates contains type specimens for about 50 - 60 different species of fish. The preservation of these individuals greatly reduces the ambiguity in species identification and is vital to the description of new species. Like so much else in science, it’s still an imperfect system for representing this complicated tangle of life on our planet – but the cases that resist easy classification are usually the most fascinating to study anyway.
Last modified 2008-11-11 13:07