ALVARO E. MIGOTTO/USPThey look like gelatinous mushrooms – red, blue, green, orange, yellow, and even luminescent – that move through the sea as if they were dancing. Appreciate them, but do not get too close: there are poisonous species, and a simple brush may cause fatal poisoning. These are the cnidarians, a group of invertebrate animals formerly call coelenterates, represented by polyps, jellyfish, hydras, and medusas.
“Aren’t they fascinating?”, observes biologist Antonio Carlos Marques, in front of the image of a transparent medusa on the screen of the computer in his room, at the Biosciences Institute of the University of São Paulo (USP). “In a way, it all started with them.” Arising about 600 million years ago, the cnidarians – a group made up today by about 11,000 species – were one of the first multicellular animals on Earth, already with organized tissues and an outline of a digestive apparatus.
Marques identified a new class of cnidarians, baptized as Staurozoa – a class gathers together animals with similar characteristics, even though they may be very different between themselves, since they must have originated from one and the same ancestor (elephants and rats belong to the same class, the mammals, although they are distinct species). There are about 50 species with living representatives, not counting fossils, grouped together into two orders, at one stage below in the classification of living beings.
The medusas from one of these orders shows a basic difference in relation to the dancing species: those from the new class live clinging to rocks and seaweed, by means of a structure similar to a sucker, the peduncle. His work – carried out in collaboration with Marcello Simões, from the São Paulo State University (Unesp) in Botucatu, and with an American, Allen Gilbert Collins, from the Veterinary School (or ITZ, its acronym in German), of Hannover, in Germany, was the principle study published in the issue of the Invertebrate Biology magazine at the beginning of the year.
The identification of the Staurozoa, which includes fossil representatives, is the first step in the direction of Marques’ most audacious plan: to put together the genealogical tree of the group regarded as the basis for the evolution of animals – the metazoans – and to define what one of the first beings to inhabit the Earth was like. “We now believe that the common ancestor of this group must have been a strain that lived fixed, without moving”, says, happily, the 34-year-old biologist, contracted four years ago by USP. “It’s quite a step forward.” His studies are based on the cladistic theory, a current of thought created about 50 years ago by a German, Willi Hennig, which adopts evolution as a point of reference for classifying living beings, based on common external characteristics.
“For cladistics”, Marques comments, “only a group with an exclusive common ancestor is considered valid, not shared with any other group.” Before Henning, the school known as phenetic predominated according to which living beings should be grouped together without necessarily respecting the origin of the groups. This line of thought, looked on today as simplistic, made, for example, all fish to be grouped together in the Pisces class, for having scales and looking like a fish. If, however, other elements for classifying are adopted, there are fish, called osseous, which come closer to all other vertebrates than to the other fish, the cartilaginous ones.
Actually, the concern for finding ways of gathering animals together is far older, and interested philosophers like Aristotle, in ancient Greece. Sweden’s Carolus Linnaeus, in Systema naturae, of 1735, proposed a standardization of the names of living beings that persists even today: the names written in Latin, always using two words, the first just for the genus; the two together, the species. The current scale for classifying living beings, which was born with Linnaeus, works like a funnel with seven stages, which goes from the most general, the kingdoms, which represent the largest sets, until arriving at the particular, the species. Classes are amongst the more wide-ranging compartments, below the first two stages, kingdoms and phyla, and above orders, families, genera, and species. In the phylum of the arthropods, for example, there are the classes of spiders, crustaceans, and insects.
Marques realized that there was something wrong in the classes of the cnidarians during his doctorate, concluded in 1997: very different animals, probably with distinct ancestors, were in one and the same group. It was by pulling the strand from the ball of wool that he managed to identify and characterize the Staurozoa, which won this name because of one of their main characteristics: the peduncle in the shape of a cross (stauros, in Greek), which fixes the animal to the rocks. Halyclistus octoradiatus, the species stamped on the cover of the Invertebrate Biology magazine, displays a bright red peduncle, which recalls a tongue sticking out of a mouth. Its eight yellow tentacles, with their pink tips, give this invertebrate from the seas a flower-like shape.
“The Staurozoa bring basic information about the first steps of the evolution of life on Earth”, comments Allen Collins, a collaborator of Marques in Germany. Until the two began to work together, in 1999, the majority of the studies on the cnidarians emphasized only the description of the species, in a retail approach, without a broad vision. “Even today, works on the largest sets of living beings are rare, and they are fundamental, because the greater the scope, the more significant the impact”, says the researcher from USP.
Rarities in Brazil
The new class is formed by two orders. The first, the Conulatae, covers only fossil representatives, which may have existed from 550 to 200 million years ago, between the Cambrian and Triassic geologic periods. The species of this order had the shape of an ice cream cone and lived stuck to marine surfaces, today fossil-bearing deposits, located mainly in the present-day Amazon and Paraná river basins, occupied by salt water millions of years ago.
The 50 or so living representatives form the other order, the Stauromedusae. In this group are the fixed medusas, with up to 5 centimeters in length, and a body made up of a cylindrical base with a sucker at one of the ends and tentacles at the other. Their colors vary from a bright red or orange to a pallid brown.
Solitary and carnivorous, feeding on the larvae of crustaceans and other animals, they are common in cold waters off the coast of Japan, Canada, and the United States. Brazil has just one species, the Kishinouyea corbini, with 1 centimeter in length at the most. “Up until today, only two specimens of this order have been found, one on the coast of Espírito Santo and the other in Cabo Frio, in Rio de Janeiro”, says Marques. “They are extremely rare over here.”
Until the recognition of the Staurozoa, there were four known classes of cnidarians, so called for having stinging cells known as cnidocytes on their tentacles. The first is the Anthozoa, represented by corals and sea anemones, abundant in shallow waters.
The Scyphozoa are the group of jellyfishes, capable of moving and swimming freely, like Aurelia aurita, one of the best known species, which is some 15 centimeters in diameter, and has a shape that is reminiscent of a bluish, almost transparent, umbrella. The hydras, including the most famous of them, the Portuguese man-of-war (Physalia physalis), are part of the Hydrozoa, the third class. In their larva stage -or as a polyp, before becoming adult or a medusa –, the animals from this class look like a miniature tree with very thin branches, like Corydendrium parasiticum.
If these three classes were proposed back in the 19th century, the fourth one, the Cubozoa, was described only in 1975 by a German, Bernhard Werner. It is made up by about 25 species, known generically as jellyfish – some of them poisonous, like the sea wasp (Chironex fleckeri), which inhabits the seas close to Australia. On the Brazilian coast, it is possible to find Chiropsalmus quadramanus, which comes to as much as 15 centimeters in diameter, and the Tamoya haplonema, both similar to a bell in shape – and poisonous.
Before getting a life of their own, the representatives of the Staurozoa class belonged to the Scyphozoa, but the molecular analyses by Marques and Collins, combined with analyses of morphological evolution, showed that there were more differences than similarities between the two sets of animals. It was as if a bird like a hen were in the same set as a bat, which is a mammal.
The reclassification of the cnidarians resulted from the comparative analysis of 87 characteristics of the four classes then already described, such as the structures of the cells and of the bodies themselves, the types of life cycle (some do not show the polyp stage, others do not become medusas), and the ways of reproduction (fertilization is not always direct). There were almost five years of intensive work until arriving at a demonstration that the four classes ought to be five. Now, with the work ready, the prospects have opened up for understanding a little better how the history of the animals on the planet began. “We are trying to document a small part of a world that is very close to us”, says Collins cheerfully.
Phylogenetic Relations in the Cnidaria, with an Emphasis on the Medusozoa (nº 96/10544-0); Modality Young Researcher; Coordinator Antonio Carlos Marques – IB/USP; Investment R$ 120,799.43