Growth rates and ages of the small bamboo coral Acanella arbuscula in the Northwest Atlantic

Acanella arbuscula is a common deep-water bamboo coral in the Northwest Atlantic and is considered a Vulnerable Marine Ecosystems indicator. Growth rates and ages of A. arbuscula have not been confirmed, despite the importance of these metrics to understand the species recovery potential when disturbed. One-hundred and fifteen A. arbuscula samples were analyzed to examine the relationship between age, location, and depth, and how environmental conditions influence growth rates. Colonies were collected during scientific trawl or remotely operated vehicle surveys from the SW Grand Banks, Northern Labrador shelf, and SE Baffin shelf from depths of 178–1354 m. Skeletons were cross-sectioned at the proteinaceous nodes and examined under reflected light and fluorescence microscopy to quantify growth rings and determine age and growth rates. The species exhibited both major and minor growth rings. Major growth rings were confirmed to represent annual growth by comparing size metrics and age from a previously bomb- 14 C dated specimen of A. arbuscula versus specimens aged by growth rings. Minor (sub-annual) growth rings were not consistently observed across specimens and the cause of their formation is unknown. Ages ranged from 8 to 29 years, with radial growth rates of 0.025–0.160 mm/year and axial growth rates of 1.87–16.1 mm/year. Longevity for A. arbuscula is lower than the bamboo coral Keratoisis grayi in the NW Atlantic, but growth rates were consistent with other documented bamboo corals. Multiple regressions revealed that age influenced radial and axial growth rates, with no significant influence of bottom temperature or chlorophyll-a observed for radial growth rates. This study confirms frequency of ring formation in A. arbuscula and that growth rates for the species do not vary significantly based on geographic or bathymetric location. • Acanella arbuscula is a slow-growing small bamboo coral with short longevity. • Laser induced fluorescence improved clarity of growth rings within organic nodes. • Major growth rings represent annual growth for Acanella arbuscula . • Age influenced the growth rate variability observed across the NW Atlantic.