The family siboglinidae, also known as Pogonophora or merely the tube or the beard worms, is a unique type of marine worms comprised of various kinds of worms. Pogonophora is sedentary, non-colonial, tuberculous marine coelomates with an elongate, bilaterally symmetrical, cylindrical body divided into segments. These types of worms live sedentary lives, spending most of their time on the seafloor. Their name originates from the beard-looking mass of pinnate tentacles (numbering between one and three hundred) at the anterior end of most species. The species in this family are unique and distinctive in various ways. These tentacles of almost all species bear a feathery appearance, with the tentacles bearing two very fine hairlines. These feathery tentacles bear microvilli, which give a suggestion of the absorption of amino acids from the surrounding seawater. However, there is little information that has been documented about this family, which lacks significant data. Research is going on about the mystery that surrounds this family of organisms. These organisms are of great interest to marine biologists and astrobiologists as they do not depend on the energy from sunlight but rather energy derived from the deep-sea hydrothermal vents which house them.
Some of the earliest Pogonophora sightings took place in early 1900 in several specific places, but it took several decades before the organisms could be classified in their own family. This controversy arose from the debate about the newly discovered organisms, with their close relation to other families like the Annelida due to the possession of their ventral nerve cord and the homology of their chaetae. However, siboglinidae could not satisfy either of the two classifications of either protostome (ventral nerve cord) or deuterostomes (dorsal nerve cord). The 1900 strange and unique tube-dwelling worm grudged from the deep waters of Indonesia lacked the obvious segmentation of annelid and an alimentary gut. One of the most significant discoveries of the organisms occurred in 1977. They discovered the giant red tubeworm in the heated, sulfur-rich waters around volcanic vents of the Pacific Ocean. These organisms were placed in their own family of organisms with similar characteristics, known as the Pogonophora or siboglinidae.
The external characteristics of beard worms depict a very distinctive and unique class of organisms, with a varied length of between ten centimeters and three meters. The tube-like organisms are made up of ring-like or funnel-like pieces, with alternating dark and light bands. The tubeworm’s physical characteristics are the most critical part of the worm living in this highly hostile and hot environment. For instance, the giant red tubeworm has a bright red plume that is crucial for the exchange of compounds such as oxygen, carbon dioxide, and nutrients. The red plume is associated with the massive concentration of hemoglobin within that particular organ. The tube’s outer part is made up of chitin, the tough natural substance that makes the exoskeletons of crabs. The hardened material is vital in protecting the tube worms from the harshness of the environment in which they reside. Inside this columnar chitin or cuticle, there is a thin layer of circular muscle, followed by a thicker layer of a longitudinal muscle and an inner peritoneum lining that encloses the coelom. The coelom is essential in the family as it is responsible for housing the two blood sinuses accountable for the organism’s primary functions.
The pogonophoras is divided into four main parts or sections. The anterior region is often called the cephalic lobe and supports the tentacles, which vary between one and 300. The short glandular region is short and contains cells that secrete chitin, the primary secretion that is used to build the tube where this animal lives. These tubes are composed of a high concentration of proteins, and chitin for the structure is firm. The glandular region consists of some ridged thickened cuticle in some species and is known as a bridle. They are all made to ensure the strength of the tube and protection from the hostile and harsh environment. After this bridle, there is the trunk, which makes up most of the body of the organism. The trunk bears two rows of papillae along the full lengths of the tube. In most species, there is toothed girdle or setae somewhere in the midpoint of the trunk. The setae hold the animal in its place within the tube. Lastly, the animal’s body ends with a short holdfast or opisthosoma of about 5 to 30 segments. The opisthosoma is thicker than the trunk section and is buried in the substrate beyond the animal’s tube.
Locomotion of the pogonophorans is limited as these organisms do not move much, but only extend their tentacles from their tubes into the sulfur-rich seawater. In vestimentiferans, a quick retraction of the anterior end is observable as only the most significant form of movement from these organisms. Another type of Pogonophora, the perviates, does not have a lot of locomotion or movement. They have been recorded only moving inside their tube, and only using the opisthosoma to burrow down into the sediment. This tube lengthens as the burrow extends. However, the lack of motion these organisms exhibit is understandable, as the environment they live in covers for the lack of mobility. These deep vents where these organisations live in are constantly served with fresh, nutrient-rich water from the vents. The lack of depth can be attributed to the lack of direct sunlight as a source of energy, thus leading the organism requiring to conserve energy.
Understanding these animals requires researchers first to understand animal functions, appearance, and environment. First off, beard worms have a very complex mode of feeding, known as chemosynthesis. These multicellular animals do not have a mouth or an anus in their adult stage—the question of how these worms’ feeds can be sufficiently by analyzing them carefully. First and foremost, these organisms do not have an alimentary canal in all development stages and do not exhibit any sign of a feeding process. Marine biologists studying these organisms have unearthed a unique and rather interesting mode of how these organisms feed. Chemosynthesis is the answer for these organisms. For instance, the giant tube worm that resides in the boiling volcanic vents of the deep seas. When the water emerges from these vents, the richness in chemicals and minerals create the feeding environment these worms crave. The dosage of these minerals and chemicals can be lethal for most of the other species, not considering the boiling temperatures of the waters.
This stage is where chemosynthesis arises, as these organisms do not have the mechanism to synthesize these minerals on their own. These eight-foot long worms do not have a mouth or a digestive track but rather depend on the millions of bacteria that live inside them for food. These symbiotic bacteria are housed in the specialized organ known as the trophosome, which develops from the embryonic gut of the gutless worms. This creates a type of a mutually symbiotic relationship between the two. The bacteria convert the minerals from the hydrothermal vents into organic molecules that can provide food for the worm. This symbiotic behaviour is hugely responsible for most of the nutrition for these pogonophorans. On the other hand, the symbiotic bacteria oxidize the sulfur-containing components for the pogonophorans’ diet. The bacteria derive energy from the sulphur oxidation, which the bacteria use to fix carbon into larger organic molecules for the pogonophoran nutrition.
The nervous and sensory system of the Pogonophora is as complicated and sophisticated as the organism themselves. They have an intra-epidermal nervous system, with a ring-shaped brain in the cephalic lobe. The ring-shaped brain projects to the posterior mid-dorsal nerve underneath the mid-dorsal ciliated band, and thus making up most of the organism’s nervous system. Form the brain emerges several tentacular nerves which diffuse into the nerve rings. The circulatory system, on the other hand, is closed and well-developed. There are two pairs of the trunk’s lateral vessels, the mid-dorsal efferent and afferent vessel supply lines. The ventral vessel enlarges into a heart in the protostome, which grows into two branches of tentacles. Both of the afferent and efferent vessels give rise to a loop into each pinnule. However, there are no blood cells observed within the blood o these organisms. The blood vessels, like in the Annelida, well defined and surrounded by a basal membrane. Some of the blood vessels contain podocytes and are responsible for the ultrafiltration of the primary urine to the coelom. Hirudeans are highly specialized vascular system, consisting of confluent segmental coelomic cavities that later advance and become modified into some narrow canals.
The excretory organs of the various organisms in this family and the related family or Annelida shows excellent variation. The excretory system also varies between adults and larvae, with protonephridia being visible in several types of both the larvae and the adult stages of the organisms. Larval protonephridia or head kidneys vary from the various organisms of the family, with some having a single, monociliated terminal cell while others have complicated organs. The inner loops of the excretory organs are closely related, and adherent to an enlarged and are thin-walled part of the ventral blood vessel. In some larger perviate species, the connection of the secretory ducts and the anterior coelomic cavity allows the exchange of excretory fluid. The removal of nitrogenous wastes from the blood is principally completed by the trophosome bacteria, aiming to satisfy their nitrogenous requirements. Mineral wastes are excreted more straightforwardly, as insoluble granules in the trophosomal cells, and some are stored in these cells for the whole of the organism’s life.
Another hugely significant form of secretion takes place within the exocrine glands, a particular type of epidermal secretory cells. The most critical cells and the most essential are those cells that are involved in the production of secretions for tube construction. The secretions of the multicellular pyriform glands enable chitin or cuticle and other mucopolysaccharides that are used for the building and developing these tubes. The glands opening open up on the epidermis, and the papillae carry the scretips to the cells. For instance, in perviates, the glands are more pronounced and abundant on the forepart of the organisms. At the same time, they are scattered in the trunk of the opisthosoma segments of the vestimentiferans. These single-celled epidermal glands secrete the proteinaceous component that helps the organisms build their tubes.
Majorly, all the Pogonophora reproduce sexually, although monilifera shows asexual production. Reproduction in beard worms is not as complicated as some of the other functions of the organisms. Pogonophora is dioecious and thus has separate sexes, and this is the only distinguishing feature of these organisms. The sexes are different, with the gonads lying in the trunk segment of most organisms. Perviates, for instance, have paired gonads in the anterior part of the trunk, oocytes at the front end of the ovaries, the diaphragm at the posterior, and oviducts connecting these through the girdle area. Males usually possess a pair of testicles in the post-annular region. The complexity of the reproduction system belies its simplicity in many ways. First is the production of the spermatozoa, and the development of the juveniles that emerge as a product of the production.
The testes open via a pair of ventral gonophores behind the meso-metasome septum while the ovaries open via a couple of posterior oviducts in the middle of the trunk. These gonads of the mesodermal origins are the principal reason for the maturation of the sperms. Once the sperms are released into the seawater, the coiled tail uncoils to form a long sticky filament which assists the sperm to get entangled into the Pogonophora. The large yolky eggs that are laid are released in batches of 10-30 eggs. When the eggs are released into the water, it drifts to the open tubes of the females. Fertilization takes place inside these tubes, and the embryos are held there for long periods before being released into the seawater. The growing embryos are released as juveniles, and quickly settle at the bottom of the ocean and start secreting their own tube. The internal fertilization in siboglinidae was observed by Bakke, where the eggs are removed from the oviducts of the frenute.
Ecology and behaviour
Perviate ponogophora lives in the deep sediments of the seas, while vestimentiferans live in the volcanic vents within this sulphur-rich environments. These organisms depend on chemosynthesis as the primary mode of feeding, carried out by the symbiotic chemo-autotropic bacteria. These more profound layers of the ocean, devoid of the sunlight or sun energy, become the optimum environment for these organisms. The Pogonophora tubes are permeable to the breathable gases and molecules that are essential in the life of the microorganisms. The animal’s length allows it to become a bridge of the redox reaction that is important in transferring oxygen-rich blood to the bacteria for oxidation. Sediment pore water from the sediment particles and the overlying seawater provide the necessary carbon dioxide that is dissolved. The vestimentiferans are unique with their ability to reside on these hydrothermal regions of the ocean, where temperatures exceed 300 degrees Celsius.
The controversy and affiliations to other groups
The relationship between pogonophoras and other invertebrate groups has been a massive source of controversy and debate in the marine and biological circles. The absence of a mouth and a developed alimentary canal makes these organisms challenging to classify. This meant marine biologists and astrobiologists have always juggled these organisms between several classes as they attempt to classify them concisely in their family. The various families that these organisms have been closely related are hemichordates, Annelida, and other various classes which show a close association with. The prostomial affinity was confirmed by the developmental studies of vestimentiferans and perviates due to the ventral nerve trunk within the organisms’ larval stages. The affinity to the hemichordates was suggested due to the development of the organism but was discarded with the discovery that hind end grew over time. The debate has raged on the relationship between the zoologists on the inclusion of these organisms in the phylum Annelida. However, Ivanov maintained that Pogonophora is closely related to the Annelida in anatomical features as a result of convergence and not descent.