A QUANTITATIVE PHOTOGRAPHIC SURVEY OF SPOKE-BURROW TYPE LEBENSSPUREN ON THE CAPE-VERDE ABYSSAL-PLAIN

A photographic survey of the seafloor was conducted during RRS Discovery cruise 204 in September/October 1993 at the oligotrophic site of the French EUMELI programme at c. 21 degrees N:31 degrees W (4650 m) on the Cape Verde Abyssal Plain. Two systems were used, a wide angle system (WASP) obtaining usable photographs covering 10 to 30 m(2), and an epibenthic sledge mounted system obtaining photographs of about 2 m(2). A total of more than 3000 usable frames were obtained, covering about 64 000 m(2) of seafloor. The photographs indicated an epibenthic invertebrate megafaunal density of 5.2 hectare(-1) and a demersal fish density of 1.8 hectare(-1). In contrast, the photographs were dominated by lebensspuren, particularly of the 'spoke-burrow' type usually attributed to echiurans. Spoke-burrow density, where related burrows and other features such as sediment mounds are considered unitary, is estimated at 871 hectare(-1). We describe and quantify the five principal types of spoke burrow encountered in this study and, on the basis of these observations and some morphometric studies of the lebensspuren, discuss the possible behaviour of their producers. We have no direct knowledge of the production rate of, or longevity of, lebensspuren in the study area. However, based on a number of assumptions and limited data from other areas we suggest that trace producer (mainly echiurans) density is on the order of tens of individuals per hectare. These trace producers probably greatly outnumber the rest of the invertebrate megabenthos, and we can only guess at the role this potential large population of hidden benthos might have in the overall structure and function of the deep-sea floor ecosystem.