been of special experimental interest to students of aging.
The Sixth international Symposium on Tardigrada met in Cambridge, England,
August 22 to 26, 1994 (my heart warms to the thought that even minor phyla
can spawn such multiple and cosmopolitan celebrations by their human devotees).
At this meeting, Walossek, Muller, and R. M. Kristensen, of the Zoological
Museum of the University of Copenhagen, presented a show-stopping paper
entitled "A More Than Half a Billion Years Old Stem Group Tardigrade
from Siberia." They. had found the first indisputable tardigrade
fossil,- and this species dated right back to the verge of the Cambrian
explosion.
These specimens look just like tardigrades and range from 0.25 to 0.35 millimeters
in length (a middling size for modern tardigrades). But the key to identification
lies not in similar sizes or general appearances, for such basic features
can be evolved by convergence in independent lineages, but in a large and
striking set of unique and complex traits found only in these fossils and
in living tardigrades. These marks of genealogical affinity include a distinctive
pitlike mouth, limbs with paired hooks, or claws, that can be withdrawn
at their out edges, and minute plate-shaped knobs between the limbs.
The pentastomes provided a much better potential case--the classic
instance among minor phyla, one might even have said--for continuing
origin of major groups after the Cambrian explosion. All of the 100 to
110 species of this phylum are obligatory parasites of vertebrates (almost
all are terrestrial, although a few species live on fishes). Like many
parasites, pentastomes have a complex life cycle, moving from an intermediate
to a final host. Larvae bore through the gut wall of the first host, where
they mature to their infective stage. When another vertebrate eats this
first host, the mature pentastome moves to the respiratory tract, either
by crawling from the stomach to the esophagus and boring through, or by
tunneling through the intestinal wall and into the blood stream. The parasite
then attaches to the lungs, nasal cavity or oral cavity (some pentastomes
have even been reported from human eyes), by means of hooks at the end
of the two pairs of limbs surrounding the mouth. In this (now) permanently
attached feeding stage, the pentastome uses its mouth to suck the host's
blood. (For most people, nothing in biology sounds more, well, to use
the contemporary vernacular, yucky than the life styles of parasites;
but such creatures do form a major component of life's diversity and ecology,
and we do need to understand them, although I advance no case for loving
them.)
Like many parasites, pentastomes are extremely simplified in anatomy
(for the safe and sheltered environment of a host specifies little advantage
for retaining the complex features needed for life in the tougher external
world). The specific organs of parasitic life--the means of finding,
attaching to, and exploiting hosts--are present and complex (in this
case, the five star arrangement of the stalked mouth and two pairs of
legs at the front end), but the rest of the body is secondarily simplified.
Pentastomes have, for example, no internal organs for respiration, circulation,
or excretion. The gut is a simple straight tube, with a muscular pumping
apparatus at the front end, obviously useful in extracting the host's
blood.
This extreme anatomical simplification of ordinary organs, combined with
elaboration of highly specific devices for exploiting hosts, makes the taxonomy
of parasites, and their genealogical placement into the evolutionary tree
of free-living forms, particularly difficult. Pentastomes have long provided
a classical example of this general nightmare in taxonomy. The range of
available hypotheses spans nearly all conceivable solutions, with links
to annelids (segmented worms), arthropods of one subgroup or another, and
separate status (often joined with onychophores and tardigrades) as the
favored solutions.
In recent years, however, a consensus has arisen for allying pentastomes
with crustaceans of the arthropod phylum. Several authors had presented
evidence of similarity between larvae of pentastomes and a group of crustaceans
known as branchiurans. Fine structure of the external cuticle and morphology
of the sperm cells also seemed to affirm a crustacean link. Then, in 1989,
a sealing argument seemed to emerge from the laboratory of my friend and
colleague Larry Abele, of Florida State University (see L. G. Abele,
W. Kim, and B. E. Felgenhauer, "Molecular Evidence of Inclusion of
the Phylum Pentastomida in the Crustacea," Molecular Biology and
Evolution, vol. 6). Abele and associates used the most powerful and
appropriately fashionable technique of comparing DNA sequences (in the
commonly used and highly informative molecule 18S ribosomal RNA) in pentastomes
and representatives of several candidate phyla for relationships--segmented
worms and all major groups of arthropods, including insects, horseshoe
crabs, millipedes, and crustaceans. The evolutionary tree reconstructed
from molecular distances revealed a closest tie of pentastomes with crustaceans.
These data led Brusca and Brusca to argue in their textbook for "convincing
cases that pentastomids are actually highly modified crustacean parasites."
Moreover, the current life of pentastomes in terrestrial vertebrates
led all
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