Sunken woods are abundant in deep oceanic environments, housing a huge faunal diversity. Studies on that substrate firstly focused on the associated organisms, but since a few years, identification of wood is a further aim. The purpose is to appreciate its degradation state, diversity, geographical origin and to identify specific associations between colonizing organisms and substrates. The first determinations were made on sunken woods from Taiwan/Philippines, the Vanuatu Archipelago, and the Mediterranean Sea. Samples’ identification was based on histological studies. Different preparation techniques were used, depending on their degradation state. Detailed anatomy descriptions were made and compared to the native flora and the introduced species. Wood samples were well preserved. Diversified species were encountered, seemingly originating from local floras.
Les bois coulés sont abondants dans le milieu marin profound et sont colonisés par une faune très diversifiée. Les recherches ont d’abord été axées sur les organismes associés, mais aujourd’hui, l’étude du substrat est une nouvelle approche, permettant d’apprécier l’état de dégradation des échantillons et d’effectuer leur identification. La diversité des essences présentes dans ces écosystèmes et leur origine géographique seront ainsi évaluées. D’éventuelles associations spécifiques entre organismes et bois seront mises en évidence. Les premiers bois coulés faisant l’objet d’une identification proviennent de Taiwan/Philippines, de Vanuatu et de la Méditerranée. Les techniques de préparation des échantillons ont été adaptées à leur état de dégradation. Leur détermination est basée sur une étude anatomique comparative avec la flore locale ou non native. La structure des bois étudiés était bien conservée. Diverses espèces ont été identifiées, originaires des flores locales. Des expériences in situ permettront l’étude approfondie des processus de dégradation et de colonisation des bois.
Sunken woods are debris (branches, trunks) that come from terrestrial woody plants. They are carried along rivers to the ocean, then they drift with marine currents, become saturated with water, and sink. They are present at all depths
However, sunken wood ecosystems remain poorly known compared with these other deep-sea ecosystems
Since 2003 (beginning of BOA program,
Nine sunken wood samples were recovered during several cruises: three samples recovered during MUSORSTOM 3 (June 1985) off the coast of the Philippines (195-m depth, CP 101, 14°00′N, 120°19′E), kept in alcohol and then in water; one sample recovered during TAIWAN 2002 (May 2002), off the southwestern coast of Taiwan (0–400-m depth) kept dry; three samples recovered during BOA 0 (November 2004) off the coast of the Vanuatu archipelago (one in Big Bay, CP 2313, 15°05′S, 166°55′E, 450-m depth, kept in glutaraldehyde; two near Epi, C2304, 16°37′S, 167°59′E, 570-m depth, kept in alcohol); two samples recovered in the Mediterranean Sea, off the coast of Banyuls-sur-Mer in October 2005 (27-m depth, 42°29′N, 03°08′E, kept in alcohol) and January 2006 (57-m depth, 42°29′N, 03°10′E, kept in alcohol).
Anatomical description and identification of each sample are based on the study of three classic sections: transverse, tangential longitudinal, and radial longitudinal. Four cellular types are observed: vessel elements, axial parenchyma, rays, and fibres.
Several techniques of sectioning are used, depending on the wood degradation state and expected observations. Soft samples are embedded in a polyester resin, cut with a diamond saw, dressed and pasted on a glass slide, and then abraded to have thin sections (thickness of 40 μm and less). Well-preserved samples are cut according to the three classic sections with a razor blade or a Reichert sliding microtome (thickness of 30 μm for transverse section and about 17.5 μm for longitudinal sections). Sections are commonly bleached, stained with iodine green, and mounted in gelatinous glycerine with phenol; some are dressed without staining. A third technique is used, based on the preparation of samples for electronic transmission microscopy. Samples are dehydrated, embedded in an Epon resin, sectioned with an ultramicrotome (thickness of 0.8 μm), stained with methylene blue, and mounted in Eukitt.
Every section was observed under a light microscope (Olympus BX 40). Each cellular type was studied (distribution, number and size of cells, thickness of walls etc.). Photos of the most significant wood features were taken with Pégase Pro software (3.0 version, 2I System, 2000) and with Nikon Coolpix 990 digital camera, installed and adapted on the microscope.
A comparative anatomical study was done with wood reference collections (xylarium, atlases, websites, and publications). Botanical features were compared first to the native flora, then to the introduced species.
It is a hardwood without growth rings
The wood sample is light beige (
The wood sample measures 83 cm in length and from 5.3 to 8 cm in diameter (
These features can be related to the sub-family Mimosoideae Kunth. (Fabaceae Lindley). Among the genera of Mimosoideae encountered in the Vanuatu
It is a ring porous hardwood with large growth rings (
In tranverse section, growth rings are just marked by rectangular fibres with thicker walls (
The wood is bored with many small tunnels (bivalves) (
The sample exhibits small tunnels bored by bivalves (
Very dense primary fibro-vascular bundles are observed (
It is a diffuse porous hardwood (
This sample can be related to the genus
Despite long underwater immersion, identification of sunken woods is still possible. Wood samples had a well-preserved structure, which allowed a classical preparation (sectioning with a razor blade or a sliding microtome). Samples were accurately determined. These identifications show a diversity of taxa for each sampling area (except for the Taiwan area, for which only one wood sample was studied). Samples are seemingly native to local floras, but the
The successful identification of sunken wood allows further studies on this particular ecosystem. Botanical diversity of sunken wood linked with faunal inventory will be studied to identify specific associations between colonising organisms and substrates. More samples will be observed, recovered during the BOA cruises around Vanuatu.
We are grateful to Dr C. Orliac (Museum national d’Histoire naturelle (MNHN), Paris) who gave us access to the Pacific Islands wood collections, Mrs G. Frébourg and N. Salel, who assisted us with histological work, and Mr M. Lemoine who performed the polyester resin technique of preparation. This work was partly funded by the European GDRE-CNRS DIWOOD program.
Transverse sections of identified wood samples.
Fig. 1 Sections transversales des échantillons identifiés.