Novel use of burrow casting as a research tool in deep-sea ecology

Koji Seike Robert G. Jenkins Hiromi Watanabe Hidetaka Nomaki Kei Sato Articles on similar topics can be found in the following collections: ecology (794 articles) environmental science (161 articles) palaeontology (98 articles) Receive free email alerts when new articles cite this article - sign up in the box at the top right-hand corner of the article or click here - Subject collections Email alerting service To subscribe to Biol. Lett., go to: http://rsbl.royalsocietypublishing.org/subscriptions Biol. Lett. (2012) 8, 648651 doi:10.1098/rsbl.2011.1111 Published online 1 February 2012 Novel use of burrow casting as a research tool in deep-sea ecology Koji Seike1,*, Robert G. Jenkins2,3, Hiromi Watanabe4, Hidetaka Nomaki4 and Kei Sato5 1Coastal and Estuarine Sediment Dynamics Group, Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka 239-0826, Japan 2Graduate School of Environment and Information Science, Yokohama National University, 79-7 Tokiwadai, Yokohama 240-8501, Japan 3Bayerische Staatssammlung fur Palaontologie und Geologie, 80333 Munchen, Germany 4Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 5Department of Earth and Planetary Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan *Author for correspondence (). Although the deep sea is the largest ecosystem on Earth, its infaunal ecology remains poorly understood because of the logistical challenges. Here we report the morphology of relatively large burrows obtained by in situ burrow casting at a hydrocarbon-seep site and a non-seep site at water depths of 1173 and 1455 m, respectively. Deep and complex burrows are abundant at both sites, indicating that the burrows introduce oxygen-rich sea water into the deep reducing substrate, thereby influencing benthic metabolism and nutrient fluxes, and providing an oxic microhabitat for small organisms. Burrow castings reveal that the solemyid bivalve Acharax johnsoni mines sulphide from the sediment, as documented for related shallow-water species. To our knowledge, this is the first study to examine in situ burrow morphology in the deep sea by means of burrow casting, providing detailed information on burrow structure which will aid the interpretation of seabed processes in the deep sea. 1. INTRODUCTION Burrowing organisms are particularly important in seafloor environments, because they mix sediments, disrupt microstratigraphy, influence the biogeochemistry of sea-floor sediment, and produce burrows that harbour other organisms and microbes [1 3]. The deep sea is the largest single marine ecosystem on Earth and contains abundant benthic fauna living on and in the sea floor sediment: understanding their subsurface ecology is therefore important. Faunal activities in sediments of the deep-sea environment have been observed in the fossil record and in modern sediment retrieved using core samplers. In the former case, it is easy to observe deep-sea trace fossils in onland exposures: however, trace fossils are Electronic supplementary material is available at http://dx.doi.org/ 10.1098/rsbl.2011.1111 or via http://rsbl.royalsocietypublishing.org. commonly flattened during diagenesis [4], and are generally overprinted by subsequent burrowing activities. In addition, trace fossils rarely provide clues to the nature of their producers, even though such information is an essential component of deep-sea biology. Hence, many researchers have tried to observe modern deep-sea burrows. The trails of such organisms on the sea floor and their burrows beneath the sea floor have been documented in core samples since the 1970s [5]. X-ray radiography has been used to observe sub-fossilized (sediment-filled) and active (open) burrows in deep-sea sediment cores [6]. In addition, axial tomodensitometry (computed tomography scanning) has been used to determine the three-dimensional morphology of burrows in marine sediment core samples [7]. However, burrows are commonly unobservable in core samples, because the soupy mud of the deepwater sea floor surface is deformed or disrupted during coring [8]. In addition, it is generally not possible to retrieve cores from unconsolidated sandy and gravelly sediments. Large organisms in the sediment often show escape movements, downward or upward, during coring, thereby disrupting the original burrow structures or life positions (H.N., personal observation). Furthermore, in the case of X-ray radiography and axial tomodensitometry, the observable volume is limited because of the size of the core and the device used for analysis; indeed, it is rarely possible to observe the entire structure of large-scale burrows in core samples. We overcame these problems by in situ burrow castingan extremely useful approach to understanding the autoecology of burrowers, the influence of burrows on the geochemistry of the host sediment [2], the nature of interspecific and conspecific biological interaction that occurs in burrows, and the palaeoecology of the organism that made the burrow [3]. Here we report the first burrow casting in the deep sea, to reveal the morphology of burrow structures in the substrate. Castings were made at a hydrocarbon seep and in an area of normal sea floor, away from any seep, both on the soft muddy sea floor in Sagami Bay, central Japan. In situ burrow casting, as described in this study, is expected to provide new insights into deep-sea ecology. 2. MATERIAL AND METHODS We observed the burrow morphology by employing an in situ burrowcasting device (named Anagatchinger) controlled by the remotely operated vehicle (ROV) Hyper-Dolphin (JAMSTEC). This method enables observations of the three-dimensional morphology of millimetredecimetre-scale burrows in the deep-sea environment (figure 1). In situ burrow castings were made at two sites in Sagami Bay, central Japan: a hydrocarbon-seep (figure 2a: the Off Hatsushima seep, water depth 1173 m) and an area of normal sea floor away from any seep (figure 2b: OBB2 Station, water depth 1455 m). All of the field observations and burrow castings were made during cruise NT10-19 of the R/V Natsushima and cruise KY11-01 of the R/V Kaiyo. Polyester resin (Rigolacw 2004WM-2, Showa Denko K. K., Japan) and hardener (methyl ethyl ketone peroxide; Kayaku Akuzo Co. Ltd, Japan) were used to create casts of deep-sea burrows at a ratio of hardener to resin of 5 per cent by weight. The resin density is greater than that of water at the sea floor, meaning that it flows down burrows and creates a cast of the entire burrow lumen structure, sometimes capturing the burrow producer within the cast. The casting procedure (see the electronic supplementary material, video S1) was as follows. A plastic cylindrical frame (15 cm high 10.5 cm in diameter, covering an area of per 86 cm2) was partially buried in the sediment, surrounding several burrow openings. The resin was then mixed with harden (...truncated)