Pogonophora
Pogonophora (Beard-Worms)
A computer model of a pogonophoran or beard-worm, so-called because of the mass of
tentacles (numbering from one to 300 or more, depending on species) - numbering 40 in this
model. The tentacles of almost all species each bear two rows of very fine hairs (pinnules)
which may be no longer than the width of the tentacles or may be elongated to give the
tentacles a feathery appearance. The pinnules may bear microvilli, suggesting an absorptive
function - some pogonophorans can absorb amino acids from surrounding sea water.
Pogonophorans have no mouth and no gut. At the base of each pinnule is a group of cilia, so
that there are two rows of cilia along each tentacle which drive currents of water
back-and-forth over the tentacle, irrigating it for exchange purposes. Illustrations usually omit
the pinnules for clarity.
Pogonophora (Beard Worms)

External characteristics

Very long, slender worms that inhabit a close-fitting tube of their own secretion. Length varies from under 10 cm
to the 2 to 3 m long
Riftia pachyptila. Most are very thin, ranging in diameter from <0.5 mm to 2.5 mm, but Riftia
pachyptila
is 4 cm in diameter.

The tube has a smooth contour, or is made-up of rings or funnel-like pieces, and may show alternating dark and
light bands. The oral end of the tube is thinner, less opaque and may bear a funnel expansion.
Zenkevitchiana
longissima
is 35 cm long, including tentacles, and lives in a 1.5 m tube that sits erect in the bottom ooze.

The body is divided into a
protosome, mesosome and a metasome (trunk). The protosme – mesosome
division is often not externally visible, giving rise to a
protomesosome. A visible constriction separates the
mesosome from the metasome, which marks the position of the muscular mesosome-metasome septum.

The protosome is made-up of the
anterior cephalic lobe, which contains the central nervous ganglion, and 1
to 200 or more tentacles. The tentacles bear a fringe of lateral pinnules on one or both sides and spring from the
base of the cephalic lobe in a single spiral. There is no mouth, anus or digestive tract.

On the mesosome is a region called the girdle or
bridle, which possess a pair of ridges of thickened cuticle,
which may fuse ventrally. These are thought to hold the worm in its tube when it protrudes. Belts are formed from
2 adjacent girdles, and may bear hardened
platelets bearing denticles. This belt divides the worm into
preannular and postannular regions.

A
midventral groove begins behind the mesosome-metasome septum partway along the preannular region.
The edges of this groove bear conspicuous glandular papillae, which may be covered in hardened platelets. It is
thought that these glands may produce the tube secretion or an adhesive secretion. There is also a preannular
dorsal longitudinal ciliated strip. Postannular papillae may be present, grouped either irregularly or in ventral
rows, giving the appearance of segmentation (pseudosegmentation).

Body Wall

The glandular epidermis is more or less columnar and covered with a cuticle. Beneath this there is a thin layer of
circular muscle, followed by a thicker layer of longitudinal muscle, which is especially thick in the mesosome.
There may be 2 longitudinal bundles in the anterior protosome that contribute to the muscles of the tentacles.

The hollow tentacles contain an inner peritoneum lining, enclosing the coelom that houses (usually two) blood
sinuses. A tentacular nerve occurs outside the coelom, and there are 2 muscle bands in each tentacle. The
pinnules are long, slender extensions of epidermal cells. The two adjacent epidermal cells are ciliated, giving rise
to two longitudinal ciliated tracts.

Nervous System

There is an intraepidermal nervous system. The brain (ring-shaped?) in the cephalic lobe thought to project a
posterior middorsal nerve underneath the middorsal ciliated band. The brain also gives off tentacular nerves.
There is a diffuse nerve ring in the mesosome-metasome septum.

Coelom

The eneterocoelous coelom lacks a definite peritoneal lining. The protosome contains a single protocoel, which
is connected to the exterior by one pair of coelomoducts. These coelomoducts may represent nephridia,
although they lack a nephrostome. The protocoel is divided by dorsolateral muscle bands and gives rise to the
tentacle coeloms. The paired mesosomal coeloms lack coelomoducts. The paired metasome coeloms connect to
the exterior via gonoducts.

Circulatory System

The circulatory system is closed and well-developed. A middorsal blood vessel and a midventral blood vessel run
the length of the mesosome and trunk. There are also 2 pairs of lateral vessels in the trunk. The ventral vessel is
enlarged into a heart in the protosome. The heart gives off anterior branches into the tentacles. An afferent and
an efferent vessel supply each tentacle. Both of these vessels give rise to a loop into each pinnule. The efferent
tentacular vessels connect to the dorsal vessel in which the blood runs backward. Blood runs forward in the
ventral vessel. No blood cells have been observed.

Nutrition

The nutrition of pogonophora is still being illucidated, and is extremely unconventional for an animal! The worms
are able to absorb glucose, amino acids and fatty acids direct from their environment, though this alone appears
insufficient. The following focuses primarily on
Riftia pachyptila.

  • No alimentary canal is present at any stage, except for a rudimentary gut in the larva, although the
    trophosome in species like Riftia pachyptila appears to be derived from the midgut, after the rest of the gut
    degenerates
  • No known active feeding processes
  • No extracorporeal enzymes known

  • Assumed to feed by:
            (i) absorbing dissolved organic compounds
            (ii) pinocytosis?
            (iii) symbiotic bacteria

  • Amino acids, glucose, fatty acids - taken up from water and concentrated in blood.
  • Well-vascularised tentacles may enhance absorption
  • Protein and ferritin also absorbed, pinocytosis? - body cuticle?
  • Tube walls contain chitin (= diatom beta-chitin) and are known to be permeable at least to water, NaCl,
    sucrose and phenylalanine.
  • Trophosome tissue is situated in the trunk and may fill most of it. This tissue has a good blood supply
    and consists largely of bacteriocytes, cell that contain symbiotic chemoautotrophic Gram-negative bacteria
    – these oxidise hydrogen sulphide (with oxygen or nitrate) and use the energy to produce organic carbon
    compounds. The plume of tentacles absorb oxygen, carbon dioxide and hydrogen sulphide from the
    surrounding sea water, which bind to carrier molecules in the blood which transports them to the
    trophosome where biochemical reaction fix the carbon - converting it from carbon dioxide into organic
    molecules that serve as fuels and building blocks..
Pogonophorans secrete close-fitting
tubes in which they dwell and into which
they can retract for protection. They have
few enemies, but will rapidly retract their
tentacles into their tube if a crab pinches
a bit off to eat! The tube may be smooth,
composed of rings or of funnel-like
segments. The tube may be 3 or 4 times
as long as the worm's body.

Pogonophorans are mysterious worms,
living sometimes perhaps for centuries in
the dark deeps, with no mouth and no gut
their mode of life is still only partially
understood and new species are still
being discovered.

These worms are of special interest to
astrobiologists, because of their
existence in ecosystems that tap energy
from the deep-sea hydrothermal vents
and which are not directly dependent on
the energy of the Sun.
  • Hydrothermal vents are sulphide rich. Sulphide and oxygen bind to haemoglobin in the tentacle plumes
    and are carried to the trophosome.
  • The bacteria are acquired through the mouth and gut of the young, which disappear in the adult.

Riftia pachyptila is very fast growing, reaching 1.5 m in just two years. Lamellibrachia luymesi, which inhabits
cold seeps, is very slow growing, reaching 2 m in about 200 years, and even longer worms are known.
Reproduction

Pogonophores are dioecious (they have separate sexes). The position of the gonopores is the only sexually
distinguishing character. The gonads are a pair of elongated bodies in the metacoel (one in each half of the
metacoel). The cellular walls of the gonads are all that separates them from the coelomic fluid. The testes open
via a pair of ventral gonopores behind the meso-metasome septum. The sperm ducts are filled with
spermatophores. The ovaries open via a pair of posterior oviducts in the middle of the trunk. The eggs are
large and yolky, and many species brood their eggs in the tube. Each batch consists of 10-30 eggs.

Development

The young are found in the parental tube and resemble young stages of Cephalodiscus buds. The pole of the
egg facing the tube outlet becomes the anterior of the new worm. Cleavage is holoblastic and unequal, forming
larger vegetal blastomeres. No blastopore is formed, gastrulation occurring by delamination. The embryos
show bilateral symmetry and contain a spongy digestive tract rudiment, which is yolky and consumed for
nutrition. Adult worms have no trace at all of a digestive tract.

Ecology

Riftia pachyptila was recently discovered around deep-sea hydrothermal vents, over 1 mile down on the floor of
the Pacific Ocean. These belong to the order vestimentifera, which is sometimes given its own phylum status.
Here they belong to ecosystems dependent on the activity of chemoautotrophic bacteria, and independent of a
direct solar energy input. However, they do require oxygen (and so are indirectly dependent on photosynthesis
far above). Most species of pogonophoran are found in the dark depths with only a few
Classification

The classification of pogonophorans has been problematic. When first discovered, they were classified as
hemichordates due to the regionation of the body and the ventral groove which are similar structures to those
found in
acorn worms. Later they were placed in their own phylum. Currently they are considered by most
zoologists to be members of the polychaete annelids, to which they are apparently closely related (as are the
echiurans).


Class I Frenulata

  • Frenulum or bridle - a ridge of tissue running obliquely round the forepart
  • Usually setae on trunk
  • One to 200+ tentacles
  • Tubes found in soft sediments

O. Thecanephria

  • Anterior 'coelom' of cephalic lobe horseshoe - or corkscrew-shaped, opening to exterior by 2 median
    coelomoducts
  • Flat spermatophores
  • 1-200+ tentacles
  • e.g. Polybrachia, Lamellisabella, Spirobrachia.

O. Athecanephria

  • Body cavity of cephalic lobe sac-shaped, communicating with exterior by 2 lateral coelomoducts
  • Spermatophores cylindrical
  • 1-20 tentacles
  • e.g. Oligobrachia, Siboglinum.


Class II Afrenulata

  • No frenulum, but a pair of lateral tissue folds meeting in dorsal mid-line (overlapping), extend anteriorly
  • No setae on trunk
  • Obturaculum - plug of hardened tissue among tentacles, used to plug tube
  • 1000+ tentacles
  • Tubes attached to hard substrates

O. Vestimentifera

  • e.g. Lamellibrachia, Riftia.

Other classification schemes have 5 orders:

  1. Order Athecanephria
  2. Order Lamellibrachiida
  3. Order Riftiida
  4. Order Tevniida
  5. Order Thecanephria
Summary of Pogonophora

  • About 100 known species
  • Most up to several cm in length
  • Riftia pachyptila, living around hydrothermal vents in the eastern Pacific Ocean, may reach 3m.
  • Marine, free-living, benthic
  • Sedentary, living in secreted tubes
  • No pelagic larva
  • Development: total, unequal and bilateral
  • Adult polymeric (with a number of segments): divided into 4 regions:
           1. tentacular region
           2. vestimentum (forepart)
           3. trunk
           4. opisthosoma
  • Tentaculate
  • No digestive system or alimentary canal
  • Simple nervous system (subepithelial nerve plexus) with a median (ventral) cord
  • Closed blood-vascular system, each tentacle has two vessels
  • Coelomate - a coelomic compartment in each body division and the coelom extends into the tentacles
  • Dioecious
  • Two cylindrical gonads - one on each side in the trunk coelom
  • Two male gonopores at anterior part of trunk
  • Two female gonopores farther back on the trunk
  • Many species brood the eggs within the tube
  • 1-1000+ tentacles, may be arranged in complex patterns
  • Lamellibrachia barhami has 2000 tentacles arranged in 25 concentric lamellae around and attached to a
    double lophophore.
  • Haemoglobin extracellular - in both vascular blood and fluid of the body cavities in Riftia pachyptila -
    similar to annelid haemoglobin.