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Keywords:

  • deep-sea comatulid crinoid;
  • Pentametrocrinus tuberculatus;
  • nearest neighbour distance;
  • aggregation;
  • patch formation;
  • regular distribution

Abstract

Spatial distribution of a deep-sea crinoid Pentametrocrinus tuberculatus was investigated in the Izu-Ogasawara Arc, western Pacific. Photographic and video recordings coupled with direct visual observations were conducted from the manned submersible Shinkai 2000 between the depths of 1340 and 1840 m on the south-western slope of the An'ei Seamount (29° 15′N, 138° 37′E). Close to the seabed, temperature, salinity and dissolved oxygen concentration were in the range of 2.2–3.1 °C, 34.38–34.52 and 1.40–2.15 ml l−1, respectively, and the bottom substratum consisted mainly of flat-surfaced lava rocks covered by thin manganese oxide crusts. Pentametrocrinus tuberculatus individuals were attached to the surface of lava rocks with their five arms extended horizontally and showed no recognizable movement. Incidence of arm loss (which may indicate mechanical damage or predation pressure) was very low. The highest density of P. tuberculatus occurred in an area termed ‘terrace’ where the slope was reduced above a relatively steep hillside. Analysis of nearest neighbour distances showed that P. tuberculatus individuals generally demonstrate regular spacing with either no or partial overlap of their ambits (= circular area defined by the extended arms), due to a combination of aggregative tendency (to form patches) and behavioural spacing among individuals within a patch. The observed pattern of distribution was clearly different from the pattern expected from random occurrence of individuals in space. Thus, on spatial scales of 101 to 103 m, P. tuberculatus showed a contagious or aggregated distribution, while on smaller scales and in areas where crinoids were concentrated they showed a combination of patch formation and regular distribution of individuals.