The Potent Respiratory System of Osedax mucofloris (Siboglinidae, Annelida) - A Prerequisite for the Origin of Bone-Eating Osedax?

Members of the conspicuous bone-eating genus, Osedax, are widely distributed on whale falls in the Pacific and Atlantic Oceans. These gutless annelids contain endosymbiotic heterotrophic bacteria in a branching root system embedded in the bones of vertebrates, whereas a trunk and anterior palps extend into the surrounding water. The unique life style within a bone environment is challenged by the high bacterial activity on, and within, the bone matrix possibly causing O2 depletion, and build-up of potentially toxic sulphide. We measured the O2 distribution around embedded Osedax and showed that the bone microenvironment is anoxic. Morphological studies showed that ventilation mechanisms in Osedax are restricted to the anterior palps, which are optimized for high O2 uptake by possessing a large surface area, large surface to volume ratio, and short diffusion distances. The blood vascular system comprises large vessels in the trunk, which facilitate an ample supply of oxygenated blood from the anterior crown to a highly vascularised root structure. Respirometry studies of O. mucofloris showed a high O2 consumption that exceeded the average O2 consumption of a broad line of resting annelids without endosymbionts. We regard this combination of features of the respiratory system of O. mucofloris as an adaptation to their unique nutrition strategy with roots embedded in anoxic bones and elevated O2 demand due to aerobic heterotrophic endosymbionts

Confocal laser scanning microscopy (CLSM) of the trunk of <i>Osedax mucofloris</i> females.

<p>A: Dorso-lateral view of a complete specimen, lateral ciliary bands occupies half the length of the trunk. B: Dorsal view of the anterior part of the trunk, elliptical shaped cilia bundles are directed anteriorly from the lateral part of the trunk. C: Depth coded z-stack, the elliptical shaped cilia bundles constituting the lateral ciliary band are formed by ciliary tufts. D: Close-up of transverse section of a trunk. E: Transverse section of a trunk, note the muscularized dorsal blood vessel. F: Single z-stack image of the trunk musculature, longitudinal muscles beneath circular and diagonal muscles. Abbreviations: ciliary tufts (ct), circular muscles (cm), diagonal muscles (dm), elliptical ciliary bundles (ecb), lateral ciliary band (lcb), longitudinal muscles (lm), muscular gap (mg), palp (p), root structure (r), torn ovisac (to), trunk (t), dorsal blood vessel (dbv).</p

O₂ concentration measured by micro sensors in a mucus tube wall and at the epidermis of palps of <i>Osedax mucofloris</i>.

<p>O₂ concentration measured by micro sensors in a mucus tube wall and at the epidermis of palps of <i>Osedax mucofloris</i>.</p

Weight specific O₂ consumption (MO₂) of <i>Osedax mucofloris</i>.

<p>MO₂ was determined in sea water at 100% atmospheric saturation and carried out on <i>O. mucofloris</i> inhabiting sectioned cow bone (B1–B3) and cow bone in cuvettes (C1–C2).</p>a<p>Standard deviation.</p>b<p>numbers of measuring sequences on which MO₂ is based.</p

Overview of bone types in experimental sampling device for recruitment of <i>Osedax</i>.

<p>The experimental sampling devices were placed at 125 m depth off the coast of Tjärnö, Sweden (58°52.976N; 11°05.715E) in close vicinity to a minke whale carcass sunk in October 2003 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035975#pone.0035975-Dahlgren1" target="_blank">[13]</a>.</p

DIC light micrographs of benzidine stained <i>Osedax mucofloris</i>.

<p>A: Sketch of the path of the longitudinal trunk vessels into the root structure, drawn from the light microscope with a camera lucida of a benzidine stained <i>O. mucofloris</i> female. Trunk twisted in midsection. B: DIC light micrograph of a benzidine stained <i>O. mucofloris</i> female. Lateral view, trunk twisted in midsection. Ventral and dorsal blood vessels continues, folded, into the anterior part of the ovisac/root system. C: close up of blood vessels near ovisac. D: close up of blood vessels supplying more distally placed capillaries. E: Capillaries supplying tissue and endosymbionts. Abbreviations: blood traces (b), blood vessel (bv), capillaries (cap), dorsal blood vessel (dbv), egg cluster (ec), oviduct (od), ovisac (os), palp (p), root structure (r), trunk (t), ventral blood vessel (vbv).</p

O₂ concentration measured by micro sensors at the bone or tissue surface of <i>Osedax mucofloris</i> inhabiting whale bone in two cuvettes, WB1 and WB2.

<p>O₂ concentration measured by micro sensors at the bone or tissue surface of <i>Osedax mucofloris</i> inhabiting whale bone in two cuvettes, WB1 and WB2.</p

Schematic illustration of O₂ distribution in the internal and external environments of <i>Osedax mucofloris</i>.

<p>A: <i>Osedax mucofloris</i> extend its palps and pinnules, with large respiratory surfaces, into the overlying O₂-rich water in order to uptake O₂. O₂ is then distributed to the buried root system through the extensive blood vascular system also supplying the heterotrophic endosymbionts. The O₂ distribution to the root system is crucial as local uptake is not possible in the anoxic bone environment. The anoxic environment is partly produced by intense bacterial processes (green arrows) utilizing O₂ at the bone surface. Hydrogen sulphide is produced by anoxic bacterial processes within the bone matrix during decomposition of organic content using sulphate. B: Schematic illustration of assumed blood flow in palp and pinnules, longitudinal section. Blue vessels carrying venous blood through afferent vessels, red vessels carrying arterial blood through efferent vessels. C: Schematic illustration of assumed blood flow in palp and pinnules, transverse section. Likewise blue vessels carries venous blood through afferent vessels, red vessels carries arterial blood through efferent vessels. Note that the palp blood vessels are created by an invagination of the basement membrane. Green indicates musculature.</p

Diagram of a transverse section of the basal part of palps (A), DIC light micrographs of benzidine stained palps (B–D) and transverse 1.2 µm sections of <i>O. mucofloris</i> palps stained with toluidine blue (E–G).

<p>A: Diagram of a transverse section at the basal part of the palp region. Circular musculature (continued green lines) encircles the longitudinal musculature (green broken lines) in a cylinder formation. Two gaps separate the longitudinal muscle bands. Black lines illustrate the motile lateral ciliary bands and two main palp nerves run along each palp as shown (blue dots). B: Pinnular loop filled with blood. C: Pinnular loops, broken lines and arrows indicate the assumed direction of blood flow. D: Midsection of palp, longitudinal blood vessels and pinnular loops visible. E: Transverse section of a pinnule, the pinnular loop enclosed by a membrane fusing in the centre. F: Transverse section of the distal part of a palp, arrow tips shows circular musculature. G: transverse section of the distal part of a palp, the two longitudinal blood vessels obvious. Abbreviations: circular muscles (cm), epidermis (ep), lateral ciliary band (lcb), left dorsal palp nerve (ldpn), left ventral palp nerve (lvpn), membrane fusion (mf), musculature (m), palp blood vessel (pbv), pinnule (pin), pinnular loop (pl), right dorsal palp nerve (rdpn), right ventral palp nerve (rvpn).</p

CLSM of the root system and pinnules of <i>Osedax mucofloris</i> females.

<p>A: Single z-stack image of the ‘trunk-root system’ connection, note the bundles of longitudinal muscles. B: Musculature located by the ovisac, assumed to be the posterior end of the longitudinal dorsal blood vessel. C: Single z-stack image of pinnules, circular musculature encircling the pinnular loop, note the distal perikaryon and sensory cilia. D: Single z-stack image showing a longitudinal section of the pinnule in C, note the internal nerve. E: Depth coded z-stack, pinnule nerves in <i>Osedax</i> ‘yellow-collar’. Abbreviations: cilia (ci), circular muscles (cm), lateral ciliary band (lcb), longitudinal muscles (lm), nerve (n), perikaryon (pe).</p