INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM POGONOPHORA (POGONOPHORA)

The phylum Pogonophora comprises fascinating marine creatures which, unlike the overwhelming majority of free-living Multicellular animals, completely lose their Digestive System in the adult stage and instead rely on symbiotic sulfur- and methane-oxidizing Bacteria for Nutrition. The first pogonophores were discovered in 1914 in the Indian Ocean during deep-sea trawling and were initially described as new polychaetes; it was not until 1944 that they were recognized as a distinct phylum. For a long time, pogonophores were thought to be closely related to deuterostomes. However, molecular genetic studies have validated the earlier hypothesis linking pogonophores to polychaetes. These animals have now been found at various depths across nearly all seas and oceans with normal salinity. Over 150 species have been described to date.

Pogonophores inhabit the seabed inside open-ended tubes of various structures—ranging from delicate, parchment-like and elastic to robust and rigid. The tube components (protein compounds and Chitin) are secreted by numerous multicellular glands opening via ducts on the body surface. Throughout their lives, pogonophores never leave their tubes, though they can move freely up and down within them. The length of the tubes sometimes exceeds the body length of the pogonophore twofold.

Most known species are colorless or whitish and translucent. Blood Vessels show clearly through the body wall, especially in the tentacles, giving the latter a bright red color.

The body of pogonophores is thread- or cord-like, cylindrical, and exceeds its thickness by 100-500 times. It consists of four distinct regions (Fig. 190); the first region bears tentacles equipped with rows of numerous appendages known as pinnules (Fig. 191), which gives the phylum its name—pogonophores, meaning "beard-bearers" (from Greek pogon – beard, phoros – bearing). Pogonophores possess a cutaneous-muscular sac covered externally by a cuticle. The body cavity is a tetrameric coelom. The digestive system is absent. Pogonophores feed via METABOLISM/21.html">Chemosynthesis through Symbiosis with methane- and sulfur-oxidizing bacteria, as well as by absorbing dissolved Amino Acids directly from seawater. Excretory Organs, when present, are represented by a single pair of coelomoducts. The Circulatory system is well-developed, closed, with directed blood flow and a specialized Heart. Special respiratory organs are lacking. The Nervous system lies within the epidermal layer and consists of a Brain and a longitudinal nerve cord. Pogonophores are dioecious. Reproduction and embryonic development have been studied exclusively in Representatives of the Class Frenulata.

Fig. 190. Female of the frenulate pogonophoran Siboglinum: a – without tube; b – detached posterior end of the body (from Gruner):

1 – tentacle (partially severed); 2 – first body region; 3 – frenulum; 4 – second body region; 5 – pre-annular region of the trunk; 6 – ciliary band; 7 – attachment papillae with chitinous plates; 8 – blood vessel;

9 – glandular papillae; 10 – annuli; 11 – post-annular body region; 12 – pseudo-segments; 13 – setae

Fig. 191. Cross-section of three adjacent tentacles of Lamellisabella (from Ivanov):

1 – tentacular nerve; 2 – efferent vessel of the tentacle; 3 – efferent blood capillary of the pinnule;

4 – afferent capillary; 5 – pinnule; 6 – its Nucleus; 7 – ciliated Cell; 8 – afferent vessel of the tentacle;

9 – coelomic canal of the tentacle; 10 – longitudinal Muscle fibers; 11 – epidermis; 12 – cuticle

Pogonophores represent a rather ancient group of animals; their tubes, originally described under the name Sabelitida, are known from the Late Precambrian, dating back 350 million years.

Regarding the systematics of pogonophores, varying viewpoints exist, but most specialists recognize two classes within the phylum. Research on pogonophores is still ongoing.