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Department of Zoology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605
* To whom correspondence should be addressed. E-mail: jvoight{at}fmnh.org
More than 100 specimens of the rare and enigmatic echinoderm Xyloplax, a deep-sea wood-fall specialist described as the new class Concentricycloidea on the basis of nine specimens from New Zealand (1), were recently collected from a depth of 2675 m in the North Pacific. These first submersible-collected specimens of Xyloplax from the Pacific contrast with the submersible-collected specimens of the Atlantic’s X. turnerae in developmental mode. Embryos of the Pacific specimens—including those from New Zealand—undergo extended development in the ovary; members of the Atlantic species apparently release small embryos or eggs. In what may be an associated difference, females appear to dominate the North Pacific population in numbers and exceed males in individual size. In addition, the marginal spines of North Pacific specimens show a nonlinear relationship to body diameter on an ln-ln plot; relatively rapid growth of marginal spines in small individuals may enhance dispersal on near-bottom currents. In the Atlantic species, the sexes are equally represented (although sexual size dimorphism exists), and marginal spines grow at a constant rate. The increased biomass of females among North Pacific specimens may reflect their increased investment in each offspring.
Animals specialized to live on deep-sea wood falls remain little known, both taxonomically and biologically. Nine specimens from five New Zealand wood falls collected at depths from 1057 to 1208 m were assigned to the genus Xyloplax, which has been interpreted as composing a new class of echinoderms, the Concentricycloidea (1) and, alternatively, as a highly derived genus of the Asteroidea (2 and references therein). Detailed anatomical study of the genus (3), and a brief report of population parameters (4) were based on more than 200 specimens collected from an experimental wood deployment at 2066 m off the Bahamas. The recent collection of more than 100 specimens of the genus from an experimental wood deployment in the North Pacific provides a contrast between this viviparous species and the Atlantic species with its shorter embryonic development. Despite distinct reproductive strategies, both species exploit the rare and unpredictable habitat provided by deep-sea wood falls.
This study reports the population composition and growth patterns of specimens of the undescribed North Pacific species. The specimens probably represent the original population accurately because the deployments were recovered inside a closed box on the Deep Submergence Vehicle Alvin. C. Mah, University of Illinois at Urbana-Champaign, is describing the new species represented by these specimens.
The wood from which the specimens were collected had been deployed on 28 July 2002. The deployment consisted of a mesh diver’s bag containing one 45.7-cm-long piece of machine-cut, bark-free, green 10.1-cm-square Douglas fir (Pseudotsuga sp.) and an identical piece of oak (Quercus sp.); cable ties secured the bag. The Remotely Operated Vehicle Tiburon (Monterey Bay Aquarium Research Institute) placed the bag at 2675 m near 42° 45.2'N 126° 42.5' W. The habitat was basalt talus within a few feet of widely scattered tubeworms of Ridgeia piscesae; the long, thin morphology of the tubeworms is typically associated with low concentrations of sulfide (5). Down-slope and to the south was the GR-14, or SeaCliff hydrothermal vent field, an off-axis vent field located on a west-facing slope (6) that, at least during the recovery dive, was characterized by exceptionally strong bottom currents.
On 31 August 2004, the DSV Alvin recovered the deployment by grasping the wood through the bag with its manipulator arm and placing the bag inside a lidded box on the submersible. The box lid was then closed and secured with an elasticized band. No elevated temperatures associated with hydrothermal activity could be found in the area. Five hours later, the submarine was recovered and the wood was removed from the collection box and placed into cold seawater.
The bag containing the wood was opened and animals visible to the naked eye (Fig. 1) were removed from the wood by hand. Because the partially anoxic wood had been inside the closed recovery box for more than 5 h, the animals’ positions on the wood could not be considered to accurately reflect normal life positions. The wood was rinsed in cold seawater and later sectioned with an electric saw. All water that had contact with the wood, including water from the recovery box on Alvin’s basket, water into which the wood was placed after recovery, and rinse water, was sieved with a 250-µm mesh. All material retained in the sieve was preserved in 95% ethanol and later sorted under a dissecting microscope. The specimens are catalogued at The Field Museum, Chicago, Illinois, as FMNH 12458, 12459, 12460, and 12461.
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The size range of the specimens was from 0.75 to 9 mm (Fig. 2); largest were 14 gravid females with diameters of from 6 to 9.5 mm and a total of 55 to 65 tube feet. Only four individuals, ranging in diameter from 3.75 to 5 mm, were considered to be males on the basis of their kidney-shaped gonads. Diameters of juveniles overlapped those of males. Males had 40 to 45 tube feet; juvenile specimens had between 25 and 45 tube feet.
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Close examination of the preserved specimens revealed that in many, the marginal spines were angled relative to the plane of the body, minimizing their apparent length. Rather than risk damaging the specimens by removing the spines, specimens with detectably angled spines were eliminated from consideration, leaving only 18 individuals. Even with this reduced sample, nonlinear growth of the marginal spines was clear (Fig. 3). The spines lengthened at a consistent rate as body diameter expanded to about 2.75 mm. Beyond that point, lengthening of the roughly 0.74-mm-long spines was limited to less than 0.1 mm, even though body diameter tripled.
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This report offers the first data from a Pacific population of the enigmatic genus Xyloplax. The other Pacific species, X. medusiformis, is known only from 9 specimens pulled from five pieces of wood collected at depths from 1057 to 1208 m around the islands of New Zealand. Judging from the notation that they were removed from molluscan borings (3), these animals were probably trawl-collected. More than 200 specimens of Xyloplax turnerae were collected on experimental wood deployments off Andros Island, Bahamas, at 2066 m. The deployments had been in place for 18 and 24 months when the recoveries were apparently performed by Alvin (possibly during dives 751–755, 800, 851–853, a conclusion based on the dates and position (3) and Alvin dive log: http://www.whoi.edu/marops/vehicles/alvin/alvin_dive_log.html). The morphology and aspects of the population biology of this species described in published reports (3, 4) contrast with those of the present species in that the Atlantic species is larger (Fig. 2; diameters of females of up 12 mm, and of males up to 7.2 mm), has more tube feet (up to 110 compared to 65), and has a stomach. The Atlantic species has a very nearly equal sex ratio, female-biased sexual size dimorphism, and linear growth of the marginal spines. Embryos contained in Atlantic females have all been smaller than 180 µm (3).
In the New Zealand X. medusiformis, embryos develop to a near-juvenile stage inside the ovary (3), as they appear to do in the North Pacific species. The North Pacific specimens contrast with Atlantic specimens in apparently having a strongly female-skewed sex ratio; however, the two species share a female-biased sexual size dimorphism, which is common in deep-sea animals (8). The increased female investment in each individual offspring may be associated with greater increase in female biomass in the North Pacific population.
Differences in the species’ dispersal strategies may be expressed in morphological differences. Xyloplax has been suggested to disperse by a parachute-like mode of locomotion, apparently in reference to passively floating (1) or via a more active, medusoid method of locomotion driven by pulsations of the oral surface (3). The relatively large spines of young North Pacific specimens of Xyloplax increase the animal’s surface area by nearly 50% while only minimally affecting weight. The discovery that the spines of most preserved specimens are angled supports the hypothesis that muscles control spine movements. The large, mobile marginal spines of small individuals (Fig. 1) would enhance either parachuting or medusoid locomotion. In the Atlantic X. turnerae, marginal spines show negatively allometric growth relative to body diameter through life, meaning that the spines are relatively longest in the smallest animals. They would therefore also enhance dispersal potential, despite being shorter than those of the Pacific species, in all but the very largest females (Fig. 1).
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This article has been cited by other articles:
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  J. R. Voight Experimental deep-sea deployments reveal diverse Northeast Pacific wood-boring bivalves of Xylophagainae (Myoida: Pholadidae) J. Mollus. Stud., November 1, 2007; 73(4): 377 - 391. [Abstract] [Full Text] [PDF]
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