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 Discove ry cruise 204 in September/October 1993 at the oligotrophic site of th e 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 syst em (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 abou t 64 000 m(2) of seafloor. The photographs indicated an epibenthic inv ertebrate megafaunal density of 5.2 hectare(-1) and a demersal fish de nsity of 1.8 hectare(-1). In contrast, the photographs were dominated by lebensspuren, particularly of the 'spoke-burrow' type usually attri buted to echiurans. Spoke-burrow density, where related burrows and ot her features such as sediment mounds are considered unitary, is estima ted at 871 hectare(-1). We describe and quantify the five principal ty pes of spoke burrow encountered in this study and, on the basis of the se observations and some morphometric studies of the lebensspuren, dis cuss the possible behaviour of their producers. We have no direct know ledge 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) de nsity is on the order of tens of individuals per hectare. These trace producers probably greatly outnumber the rest of the invertebrate mega benthos, and we can only guess at the role this potential large popula tion of hidden benthos might have in the overall structure and functio n of the deep-sea floor ecosystem.