Lower bathyal and abyssal distribution of coral in the axial volcanic ridge of the Mid-Atlantic Ridge at 45°N

Abstract

The deep-sea floor below 3000m occupies 50% of the surface of the planet and is composed mainly of fine sediments. Most studies of deep-sea benthic fauna have concentrated on soft sediments with little sampling in rocky areas and even less on non-vent mid-ocean ridges. To assess the distribution and abundance of coral between 2500m and 3500m depths, video footage from the ROV Isis taken during a cruise to the Axial Volcanic Ridge (AVR) of the Mid-Atlantic Ridge at approx 45°30′ N was analysed. Abundances per 100m were calculated and mapped using Arc GIS, with a maximum of 59 being observed. 20 putative species were identified. Scleractinia were absent from the observed area and the coral fauna was dominated by Octocorallia. The data were separated into four substratum types, sediment, sloped rock, flat rock and mixed substratum, with both abundance and community being compared. Sedimented and rocky areas had different coral communities with sediment having a higher occurrence of Pennatulidae and Chrysogorgidae than rock. Sloped rock had the highest abundance of corals. We suggest that this increase in abundance reflects higher food availability as well as the solid substratum on which coral larvae settle.