Burrow forms, growth rates and feeding rates of wood-boring Xylophagaidae bivalves revealed by micro-computed tomography

Diva J Amon,Farah Ahmed,Jonathan T Copley,Adrian G Glover,Paul A Tyler,Craig M Young,Kirsty M Kemp,Daniel Sykes

doi:10.3389/fmars.2015.00010

Abstract

Wood sinking into the deep sea is often colonized by species of the bivalve subfamily Xylophagaidae; specialist organisms that bore into it and digest cellulose with the aid of symbiotic bacteria. Very little is known about the nature of Xylophagaidae borings, Xylophagaidae abundances and population size structures, their rates of growth and their consumption rates of wood. To investigate this, several sets of experimental wood packages were deployed and retrieved: two sets from two seamount sites on the Southwest Indian Ridge (732-750 m), one from the Mid-Cayman Spreading Centre in the Caribbean (4773 m), and three sets from 500 m in the Tongue of the Ocean, Bahamas. The wood samples were scanned using X-ray micro-computed tomography (micro-CT). The wood at each deployment site was colonized by a different species of xylophagaid. Making novel use of micro-CT images, the morphology of intact xylophagaid borings were shown to resemble Prince Rupert’s Drops with ‘drop lengths’ varying between species. Mean sizes of Xylophagaidae and mean minimum growth rates (2.55 to 8.76 mm yr-1) varied among species also. Rates of wood degradation were up to 60 cm3 per year per 100 individuals but in reality, this may have been an underestimate. This analysis has given insight into the importance of the subfamily Xylophagaidae with regard to wood remineralization in the deep sea.

Highlights

With the exception of sites of autochthonous primary production such as hydrothermal vents, the food webs of the deep-sea floor are sustained by organic matter from the upper ocean
The wood package deployed at the Mid-Cayman Spreading Centre (MCSC) was within 0.5 km of the active Beebe Hydrothermal Vent Field (Connelly et al, 2012) at 4773 m, and consisted of a block of three planks of pine wood at 20 m off the seafloor on a mooring that had oceanographic instruments and whale bones attached
One boring was located on the MCSC deployment, which was sawn free from the rest of the wood, photographed and preserved in 100% ethanol

Summary

Introduction

With the exception of sites of autochthonous primary production such as hydrothermal vents, the food webs of the deep-sea floor are sustained by organic matter from the upper ocean. Upon reaching the deep-sea floor, wood creates ephemeral patchy habitats that host distinct assemblages of fauna (Turner, 1973). These fauna colonize and congregate around the organic enrichment caused by the wood, using it as a food source, substratum and shelter (Turner, 1973, 1977; Wolff, 1979). The significance of wood falls to the overall energy budget of the deep-sea environment and carbon mineralization on the seafloor is still unquantified (Gage, 2003)

Methods

Results

Discussion

Conclusion