Rapid plastic responses to chronic hypoxia in the bearded fireworm, Hermodice carunculata (Annelida: Amphinomidae)

Abstract

Hypoxia is a widespread and increasing phenomenon in marine environments, including coral reefs. The bearded fireworm (Hermodice carunculata) is a large corallivorous amphinomid polychaete, with a high tolerance of environmental stress, including temperature, salinity, and dissolved oxygen (DO). Currently, little is known about the response of H. carunculata to chronic (≥ 18 h) hypoxia, although this knowledge is crucial to understand its impact on coral reef health under hypoxia scenarios. We tested the hypothesis that the number of branchial filaments (previously used as a diagnostic character for species identification) increases in response to chronic hypoxia. We subjected wild-caught fireworms to two levels of reduced DO (Mid: 4.5 ± 0.25 mg O2 L−1 and Low: 2.5 ± 0.25 mg O2 L−1) to explore their morphological and physiological responses to seven days of chronic hypoxia. Hypoxia exposure resulted in a higher number of branchial filaments (low = 57.2 ± 5.3, mid = 57.4 ± 6.1, and normal = 47.4 ± 11.2) after seven days. Fireworms exposed to hypoxia further reduced their rate of regeneration, but returned to normal regenerative rates after fifteen weeks under normoxic conditions. There was no difference in regeneration rates between low and mid DO groups. Our results demonstrate the importance of considering multiple physiological and morphological endpoints as well as phenotypic plasticity in species delimitations. Indeed, the results suggest that morphological variation can be indicative of environmental conditions.