Growth, Injury, and Population Dynamics in the Extant Cyrtocrinid Holopus mikihe (Crinoidea, Echinodermata) near Roatan, Honduras

The crinoid order Cyrtocrinida is known mainly from Mesozoic fossils; its few surviving members, all from bathyal environments, are among the most peculiar living crinoids. Cyrtocrinids attributed to Holopus mikihe Donovan and Pawson, 2008, have been observed in large numbers via submersible off the western coast of Roatán, Honduras, on vertical and overhanging walls at depths between 430 and 640 m. Observations in 2012, 2013, and 2014 have permitted the first estimates of population structure, growth, and regeneration. Two size modes were observed; the flat barnacle-like “juvenile” stage resembles confamilial and co-occurring Cyathidium pourtalesi Améziane, 1999, whereas the larger “adults” elevate the crown on a stumplike calyx. The 99th percentile growth rate was 0.19 cm yr ^–1 , giving a minimum predicted age of 16 yrs for the largest specimen and 8.7 yrs for the median specimen; the median growth rate was 0.04 cm yr ^–1 , corresponding to 72 and 39 yrs. However, the slower rate of growth in juvenile compared to adult specimens means that these ages are underestimates; actual median age may be closer to 50 yrs. Arm regeneration rate is estimated at 0.6 cm yr ^–1 , and 9.8% of adult individuals were visibly injured, giving an interval of about 1.4 yrs between arm loss events. No recruitment or mortality was observed, and aggregations of evenly-sized individuals were prevalent, consistent with sporadic local recruitment and mortality.