Effects of hypoxia and organic enrichment on growth of the brittle stars Amphiura filiformis (O.F. Müller) and Amphiura chiajei Forbes

Abstract

Effects of oxygen concentration (normoxia: >8.0 mg O 2 l −1 and hypoxia: 1.8–2.2 mg O 2 l −1) and organic enrichment (control: 0 g C m −2, medium: 27 g C m −2 and high: 55 g C m −2) were studied on arm regeneration rate in two infaunal brittle star species, Amphiura filiformis and Amphiura chiajei, during a two-month period in a flow-through aquaria system. In both species arm regeneration rates were significantly higher in organic enriched aquaria compared to control aquaria. Hypoxia significantly decreased the arm regeneration rate in Amphiura filiformis but did not affect the rate in Amphiura chiajei. The results suggest that Amphiura filiformis is more sensitive than Amphiura chiajei to hypoxia. Analyses on growth and energy allocation in Amphiura filiformis were carried out on the main factors oxygen concentration, organic enrichment, and amount of arm loss that simulated sublethal predation. The effect of sublethal predation was measured as growth of one or three arms that were amputated at the start of the experiment. Significant interactions between sublethal predation and organic enrichment, and oxygen concentration and organic enrichment, were observed for arm regeneration in Amphiura filiformis. Arm regeneration rate in specimens with one arm amputated differed significantly between the three treatments of organic enrichment, but no difference was found in specimens with three arms amputated. In normoxia, arm regeneration rates increased in both medium and high organic load compared to control. However, in hypoxia, a positive growth response was only observed in the treatment medium organic load. At the termination of the experiment, dry mass of disks was significantly higher in specimens with one regenerated arm compared with specimens with three regenerated arms. The response of Amphiura filiformis to sublethal predation indicates also an ability to allocate energy between disk growth (reproductive investment) and arm regeneration (somatic growth).