Describing diversity: too many new species, too few taxonomists.
Since the oceans occupy approximately 71 percent of the earth's surface it is reasonable to suppose that a large proportion of the world's species will be found there, and indeed existing evidence already shows that the oceans harbor a great variety of life. Two-thirds of the world ocean floor is deeper than 2,000 meters (1.2 miles). First evidence for the very great biological diversity of these abyssal depths has come within the past 30 years with the introduction of finer mesh screens (less than 0.5 millimeters, 1/50 inch) for separating organisms from the bottom sediments that most species inhabit (see The Deep Sea: Desert AND Rainforest, Oceanus Fall/Winter 1995). Since most organisms from samples taken before 1965 were lost through the coarse mesh screens previously used, analysis of these samples gave a grossly distorted estimate of diversity in the deep sea. It now has become apparent that the deep sea bottom fauna includes untold numbers of heretofore undescribed species, many never previously encountered. Recent estimates of the numbers of deep-sea species vary a hundredfold, from hundreds of thousands upwards to ten million.
Yet the study of this newly discovered diverse fauna has only just begun. There are on the shelves of laboratories and museums of North America and Europe thousands of unnamed species collected within the past three decades on expeditions of American, French, Russian, and British research ships.
For example, among the superorder pericarida, a major taxon that includes most of the minute bottom-dwelling crustacea of the deep sea, 69 percent among those found in the western Atlantic during the past 30 years are newly discovered species. Though some of these have received attention, most remain to be named and described. Among the 236 species of polychaete worms from the deep western North Atlantic, 64 percent are undescribed, and among bivalve mollusca, one of the better known groups of the deep-sea benthos, 105 new species (about 43 percent of all species collected) have been described over the last 20 years. Only one-third of the collected species of Aplacophoran molluscs have been named and described even though they are an important component of the deep-sea fauna and often appear among the ten most abundant species.
Meanwhile, the number of undescribed new species grows unrelentingly with each additional deep-sea dredge sample - and biologists estimate that a total area of only 500 square meters (635 square feet) of the world ocean floor has been sampled!
Shallower regions off continents are also undercollected in many parts of the world. Recent reports indicate that one-third (124 species) of all polychaete worms collected from Georges Bank are undescribed and unnamed, while a small sample of coral sand from a fringing reef off one of the Hawaiian Island (about as much as fits in six one-quart pickle jars) produced 112 new species (78 percent of all species present in the sample). Ninety-two percent of the flatworms (123 species) collected from two locations on Australia's Great Barrier Reef were new. Shore collections made in tropical lagoons, from coral reefs, and even from rocky shores continue to yield large numbers of new unnamed species.
The question now arises: "Is it really necessary to name and describe every species in order to understand the biological diversity of a particular fauna?" The structures of different bottom communities exhibit striking differences. For example, on the bottom of Buzzards Bay near Woods Hole, Massachusetts, samples from 19 meters depth yielded 79 species of macro-invertebrates exceeding 0.5 millimeter in size. If the species in a sample are ranked in the order of their abundance, two, a bivalve (Nucula proxima) and a polychaete worm (Nephthys incisa), make up three-quarters of the total individuals. If the next five most abundant species are included (namely all species that exceed 1 percent of the total sample) then 91 percent of all individuals will be included. Thus the remaining 72 species make up only 9 percent of the fauna.
Compare these percentages with those from dredge samples taken at abyssal depths averaging 2,000 meters off the east coast of the United States. A total of 798 species were collected, approximately ten times the number found in Buzzards Bay. The most abundant species in the abyssal samples is a polychaete worm, Aurospio dibranchiata, which on average is only 7.1 percent of the total number of individuals. The first two most numerous species make up only 11.7 percent of the fauna, in contrast to Buzzards Bay where they make up three-quarters of the total sample (76 percent). The first seven species in the deep-sea sample still make up only 28 percent of the total, whereas in Buzzards Bay they represent 91 percent of all individuals. Consequently, to understand the biodiversity of the deep-sea fauna, it will be necessary to identify more species than in shallow coastal waters in order to understand how the community functions. Indeed, some species that occur in small numbers may nevertheless have a profound effect on the community.
How many species must we describe? There is, of course, no simple answer. In the deep sea, where biodiversity is unusually great, it is necessary to know as much of the fauna as practically possible since no species there is overwhelmingly dominant. In the deep-sea example above, the difference between the most abundant and rarest species is atmost only 7 or 8 percent. By contrast, in shoal waters this difference may exceed 90 percent. Understanding Marine Biodiversity, A Research Agenda for the Nation, published in 1995 by the National Academy Press (see Oceanus, Fall/Winter 1995), observed, "The level of investigation of biodiversity [i.e., the number of species] should be dictated by the basic scientific question asked, and the urgency of the perceived environmental threats...."
Many species found in marine inshore waters are known to have a planktonic, free-drifting larval stage that differs radically from and is not readily related to its adult parent. The importance of the larval stage is that it may influence the distribution of a species both in space and time, according to success in settlement, metamorphosis, and survival of the larvae. However, the larvae are known for only a small fraction of all species. Among the 380 most common species listed in Key to Marine Invertebrates of the Woods Hole Region (R. I. Smith, ed., Systematics Ecology Program, Marine Biological Laboratory, 1964) the larval forms of only 96 species have been described. Consequently, three-fourths of the most common species can be recognized only by their adult forms. This presents the taxonomist with a perplexing problem - it is rather like knowing the identity of a butterfly but not of its caterpillar!
Taxonomy is a precise descriptive science. Over the past half-century both interest in and opportunities for training and advanced graduate study have declined catastrophically. The increase in gray heads among taxonomists is alarming; for many animal groups there are but few experts actively working with or able to authoritatively describe and identify species. Species identification among many marinebenthic groups is almost impossible because of the lack of competent taxonomists. There are for some groups only two or three living experts who can authoritatively describe, name, and identify species; for others there are none at all! As older taxonomists retire, there are no new young biologists to replace them.
Recognition of this problem by the National Science Foundation has resulted recently in the initiation of a new program, "Partnership for Enhancing Expertise in Taxonomy" (PEET), designed to support research on poorly known groups of organisms and to train a new generation of taxonomists in such taxa. The grants provide for funds to publish monographs and to computerize data bases for the dissemination of information and aids to identification. Though modest, the efforts of PEET are appropriate and important steps toward recognizing the need for trained taxonomists in the "pursuit of biodiversity."
Understanding the sea's biodiversity will require a greater effort in order simply to describe and name species. This information is a prerequisite for understanding biological processes in the ocean, and it is the taxonomist who must provide this knowledge.
RELATED ARTICLE: What is Taxonomy, Anyway?
taxonomy - 1. the science of classification; laws and principles covering the classifying of objects 2. Biol. a system of arranging animals and plants into natural, related groups based on some factor common to each, as structure, embryology, biochemistry, etc.: the basic taxa now in use are, in descending order from the most inclusive, phylum (in botany, division), class, order, family, genus, and species.
- Webster's New World Dictionary
Genus and species are the terms commonly used to designate animals. A complete heirarchical schema for the American oyster distributed along the east coast of the United States is: Phylum: Mollusca Class: Bivalvia Order: Pteriodea Family: Ostreidae Genus: Crassostrea Species: virginica
The American oyster is referred to as Crassostrea virginica: Crassostrea is the genus of a group of oysters; virginica is the specific name that designates a particular kind or species of oyster. Although they may vary significantly in size and shape, all members of the species Crassostrea virginica exhibit certain characteristics. In the case of Crassostrea virginica these include a left or attached valve larger than the right, production of large numbers of eggs (up to 50 million) that are externally fertilized, muscle scar colored deep purple, and a long, strongly curved beak.
The taxonomist who describes a species selects and deposits an individual specimen, called a "type," in a museum for the reference of other taxonomists. There is, however, more to taxonomy than accurately describing and naming species. The taxonomist also makes inferences about speciation and evolutionary relationships.
Rudy Scheltema has been a biologist at WHOI since 1960. His studies on the life histories of invertebrates have led him to consider the importance of passive dispersal to the biogeography and evolution of benthic invertebrate species and taken him to remote parts of the world, most recently tropical South Pacific islands and Antarctica.
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|Title Annotation:||Marine Biodiversity; includes related article on taxonomy|
|Author:||Scheltema, Rudolf S.|
|Article Type:||Cover Story|
|Date:||Mar 22, 1996|
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