About the Cover
Cover
The cover photographs show dorsal and ventral (inset) views of a female of a new species of Xyloplax, a genus for which a new class of echinoderms, the Concentricycloidea, was erected on its discovery in 1986. Although intervening years have seen thoughtful discussion of the taxon's appropriate classification, little enlightenment on the biology of these rare animals or of their only known habitat, wood falls in the deep sea, has resulted. Xyloplax was previously known only from around New Zealand and off the Bahamas at depths from 1057 to 2066 m. This 9-mm-diameter female represents a population of 103 individuals collected from wood experimentally deployed at 2675 m in the North Pacific Ocean in July 2002 and recovered 25 months later by the Deep Submergence Vehicle Alvin. Voight (p. 77 this issue) examines aspects of the biology of this species.
The ventral view of the female shows that in this species, young are retained inside the female's body until they resemble miniature adults. This contrasts with reports of Xyloplax turnerae, the Atlantic species from off the Bahamas, in which young are apparently released at a much smaller size. Despite the differences in the development mode between these species, the size distribution of the populations is statistically the same.
Although this Pacific species would seem to have less opportunity for dispersal because young begin their lives at a larger size, this limitation may be overcome by an unusual growth pattern. The spines that extend beyond the edge of the body lengthen rapidly in small individuals to near their adult lengths of 0.75 mm. Their growth then nearly stops, increasing only slightly as the diameter of the body expands from 2.75 mm to over 9 mm in females, the larger sex. The spines of the Atlantic species, in contrast, lengthen uniformly as body diameter increases. In addition, the angling of the spines away from the plane of the disk-shaped body in some preserved Pacific specimens suggests that the marginal spines are under muscular control. The oversized, movable spines of the Pacific species may enhance dispersal by young individuals as they seek islands of their extremely rare and randomly distributed habitat-wood falls on the sea floor, miles away from the nearest land and 2 km beneath the ocean surface.
Credits: Photographs of Xyloplax n. sp. taken by Todd Haney aboard the R/V Atlantis. Cover design: Beth Liles (Marine Biological Laboratory).
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Copyright © 2005 by the Marine Biological Laboratory.
