Morphology and composition of the internal axis in two morphologically contrasting deep-water sea pens (Cnidaria: Octocorallia)

ABSTRACT

Umbellula encrinus Linnaeus, 1758 and Anthoptilum grandiflorum (Verrill, 1879) are two species of deep-water sea pens (Cnidaria: Octocorallia) commonly found in the Northwest Atlantic. These sea pens are frequently caught as fishing bycatch, but little is known about their biology. Here we characterise the internal skeleton (axis) of these two morphologically contrasting species, in terms of axis carbonate composition and proportion, morphology and metrics (i.e. cross-sectional area and weight) through gradients along their lengths. Using the conspicuous growth rings present in the axis of U. encrinus we also estimated intra-colonial variation in radial growth rates for this species. X-ray diffraction (XRD) analyses revealed that the carbonate portion of the axis is composed of magnesian calcite (Ca, Mg)CO₃ in both species. On average, the axis is composed of 71% carbonate material in U. encrinus, and 65% carbonate in A. grandiflorum. The carbonate proportion was greatly reduced from base (71%) to tip (33%) in the axis of U. encrinus, while in A. grandiflorum changes were less abrupt (~6% difference from base to tip). In U. encrinus, the axis is quadrangular in shape through its whole length, twisting longitudinally. Twisting can occur in both clockwise and anti-clockwise directions. The axis of A. grandiflorum varied from elliptical near the base to cylindrical through most of the remaining length, with no twisting observed. Axes’ cross-sectional areas also decreased with distance from the base in both species, being the largest near the base. Estimated radial growth rates in U. encrinus were variable within a single colony, ranging between 0.04 and 0.08 mm.yr⁻¹. This is the first study to describe changes along the axes of sea pens, and to characterise the axes in the studied species. It provides a baseline for studies considering sea pens’ skeletons (e.g. ocean acidification and palaeoceanography).

KEYWORDS:

• Pennatulacea
• Umbellula encrinus
• Anthoptilum grandiflorum
• internal axis
• deep-sea corals

Acknowledgements

We thank Glenn Piercey, Michael Shaffer and Brian Loveridge (CREAIT, Memorial University) for assistance with axis sectioning, and Wanda Aylward (CREAIT, Memorial University) for the XRD analysis. We also thank the Canadian Coast Guard captains and crew, and Fisheries and Oceans Canada (DFO), for the samples, and the Canadian Scientific Submersible Facility (CSSF) crew and ArcticNet for the in situ photos and video. This manuscript benefited from comments by three reviewers.

Disclosure statement

No potential conflict of interest was reported by the authors.

Geolocation information

Specimen 042 (point): 68.7300ºN, 65.6787ºW; Specimen 044 (point): 74.6590ºN, 75.0299ºW, Specimen 045 (point): 74.9368ºN, 75.0414ºW; Specimen 048 (point): 74.9368ºN, 75.0414ºW; Specimen 052 (point): 74.4941ºN, 74.4460ºW, Specimen 030H (point): 66.2648ºN, 61.1890ºW; Specimen 111A (point): 45.6750ºN, 57.1975ºW; Specimen 111B (point): 45.6750ºN, 57.1975ºW; Specimen 111C (point): 45.6750ºN, 57.1975 ºW; Specimen 125A (point): 46.0967ºN, 57.2967ºW; Specimen 126A (point): 45.4708ºN, 57.0683ºW; Specimen 126B (point): 45.4708ºN, 57.0683ºW; Specimen 126C (point): 45.4708ºN, 57.0683ºW; Specimen 039 (point): 63.6000ºN, 58.6833ºW.

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Supplemental material

Supplemental material for this article can be accessed here.

Additional information

Funding

This work was supported by the National Sciences and Engineering Research Council of Canada (doctoral scholarship to the first author); and Canadian Healthy Oceans Network (CHONe) I.