A colony of giant beardworms (phylum
Pogonophora) at great depth near a
hydrothermal vent along
Trench, eastern Pacific Ocean.
Phylum Pogonophora (po´go-nof´e-ra)
, beard, + phora
or beardworms, was entirely unknown
before the twentieth century. The first
specimens to be described were collected
from deep-sea dredgings off the
coast of Indonesia in 1900. They have
since been discovered in several seas,
including the western Atlantic off the
U.S. eastern coast. Some 145 species have been described so far. We recognize
two classes, Perviata and Vestimentifera,
but some authorities consider
Vestimentifera a separate phylum.
The usual length of perviatans is from
5 to 85 cm, with a diameter usually of
less than a millimeter. Vestimentiferans,
however, live around deepwater
hydrothermal vents and grow much
larger: up to 1.5 m long and 5 cm in
diameter (Figure 21-6).
These elongated tube-dwelling
forms have left no known fossil record.
Their closest affinity seems to be to
Most pogonophores live in ooze
on the ocean floor, always below the
intertidal zone and usually at depths
of more than 200 m. This location
accounts for their delayed discovery,
for they are obtained only by dredging.
They are sessile animals that
secrete very long chitinous tubes in
which they live, probably extending the anterior end of the body only for
absorbing nutrients. The tubes are
generally oriented upright in the bottom
ooze. The tube is usually about
the same length as the animal, which
can move up or down inside the tube
but cannot turn around.
Beardworms have a long, cylindrical
body covered with cuticle. The
body is divided into a short anterior forepart
; a long, very slender trunk
and a small, segmented opisthosoma
(Figure 21-7). At its anterior, the cephalic
lobe bears from 1 to 260 long
tentacles (the “beard” that gives the phylum its name), depending on
species. Tentacles are hollow extensions
of the coelom and bear minute
pinnules. For a part or all of their
length, the tentacles lie parallel with
each other, enclosing a cylindrical
intertentacular space into which the
pinnules project (Figure 21-8).
Diagram of a typical pogonophoran. A, External features. In life, the body is much more elongated than shown in this
diagram. B, Position in tube.
Cross section of tentacular crown of pogonophore Lamellisabella. Tentacles arise from ventral side of forepart at
base of cephalic lobe. Tentacles (which
vary in number in different species) enclose a cylindrical space, with the
pinnules forming a kind of food-catching network. Food molecules may be
absorbed into the blood supply of
tentacles and pinnules.
The long trunk bears papillae and,
about midway back, two rings of shorttoothed
setae called girdles
grip the tube wall, allowing the two
halves of the body to contract or
extend independently in the tube. Posterior
to the girdles, the trunk is very thin and easily broken when the animals
are collected. In fact, the segmented
tail end of the animal, or
opisthosoma, was not found and
described until after 1963! It is thicker
than the trunk and has 5 to 23 short
segments that bear setae.
Cuticle, epidermis, and circular
and longitudinal muscles compose the
body wall. The cuticle is similar in
structure to that of annelids and sipunculans.
Pogonophores are remarkable in
having no mouth or digestive tract, making
their mode of nutrition a puzzling matter. They absorb some nutrients dissolved
in seawater, such as glucose,
amino acids, and fatty acids, through
the pinnules and microvilli of their tentacles.
Most of their energy, however,
apparently is derived from a mutualistic
association with chemoautotrophic bacteria.
These bacteria oxidize hydrogen
sulfide to provide energy to produce
organic compounds from carbon dioxide.
Pogonophores bear the bacteria in
an organ called the trophosome
which is derived embryonically from the
midgut (all traces of the foregut and
hindgut are absent in adults).
Sexes are separate, with a pair of
gonads and a pair of gonoducts in the
trunk section. Cleavage is unequal
and atypical. It seems to be closer to
radial than to spiral. Development of
the apparent coelom is schizocoelic,
not enterocoelic as was originally described.
The worm-shaped embryo is
ciliated but a poor swimmer. It is
probably swept along by water currents
until it settles.
Because the first specimens of
Pogonophora that were dredged up
lacked the segmented opisthosoma,
Ivanov and other early workers, who
believed they were working with
whole specimens, described the coelom
as trimeric (composed of three
parts), like that of the hemichordates,
and assumed that the organisms were
deuterostomes. Ivanov also described
the larval coelom as being trimeric.
The later discovery of the segmented
posterior end caused some revision of
the hypothesis. The adult coelom has
proved to be polymeric, not trimeric.
That fact and schizocoelic development
of larvae point toward an affinity
with protostomes rather than deuterostomes.
Pogonophore tubes were originally
thought to resemble those of
hemichordate pterobranchs, but analysis
of their amino acid and chitin
content shows no relationship to pterobranchs.
Pogonophores have photoreceptor
cells very similar to those of
annelids (oligochaetes and leeches),
and structure of the cuticle, makeup of
the setae, and segmentation of the
opisthosoma all strongly suggest a relationship
with the annelids. However, the phylogenetic position of the
Pogonophora must be considered still
unsettled until the embryology of more
species of more than one family is
Adaptive radiation has not been
extensive. The chief areas of diversity
are in the structure of the tentacular
crown and the tube.