Axial rod growth and age estimation of the sea pen, Halipteris willemoesi Kölliker

Abstract

Halipteria willemoesi is a large octocoral commonly found in the Bering Sea. It is a member of a ubiquitous group of benthic cnidarians called sea pens (Octocorallia: Pennatulacea). Sea pens have a skeletal structure, the axial rod, that in cross section exhibits growth rings. Pairs of adjacent rings, or ring couplets, were assumed to be annuli and were used to estimate the age and growth of H. willemoesi. Twelve axial rods, extracted from H. willemoesi collected in the Bering Sea, were selected to represent small (25-29 cm total length), medium (97-130 cm TL) and large (152-167 cm TL) colonies. Each rod resembled a tapered dowel; the thickest part (0.90-6.75 mm in diameter) was at about 5-10% of total length from the base tip, the distal part was more gradually tapered than was the base. The number of ring couplets increased with rod size indicating their utility in estimating age and growth. Estimated age among rods was based on couplet counts at the thickest part of each rod: the average estimated age (±SE) was 7.1 ±0.7, 19.3 ±0.5, and 44.3 ±2.0 yr for small, medium and large-size rods, respectively. Based on these estimated ages, average growth rate in total length was 3.9 ± 0.2, 6.1 ± 0.3, and 3 6 ± 0.I cm yr -1 for small, medium, and large-size colonies. The average annual increase in maximum rod diameter among all colonies was 0.145 ± 0.003 SE mm yr -1 ; therefore, age prediction from maximum rod diameter was calculated (estimated age (yr) = 7.0 * (maximum rod diameter, mm) -0.2; R 2 = 0.99). At maximize diameter, the average couplet width was relatively constant among the three colony sizes (0.072 ± 0.05 mm). X-ray diffraction and electron microprobe analyses revealed that the inorganic portion of the rod is composed of a high-magnesium calcite. Radiometric validation of these age and growth rate estimates was attempted, but high amounts of exogenous 210 Pb precluded using the disequilibria of 210 Pb: 226 Ra. Instead, 210 Pb activities were measured in a series of cores extracted along the axial rod. These activities ranged from 0.691 ± 0.036 (SE) to 2.76 ± 0.13 dpm g -1 , but there was no pattern of decay along the length of the rod; therefore, the growth rates and corresponding ages could not be validated. Based on estimated age from ring couplet counts, growth in total rod length is slow at first, fastest at medium size, and slows toward maximum size, with an estimated longevity approaching 50 yr.