Date Published: April 18, 2019
Publisher: Public Library of Science
Author(s): Qiao Li, Song Sun, Fang Zhang, Minxiao Wang, Mengna Li, Ismael Aaron Kimirei.
The Manila clam Ruditapes philippinarum has become a common and dominant macrobenthic species in coastal areas of the northwestern Pacific and temperate waters of Europe; it is also a major cultured shellfish, with annual worldwide production exceeding 3.3 million tonnes. This species faces greater risk of exposure to hypoxia as eutrophication worsens throughout its coastal habitats; however, its tolerance to hypoxia remains unclear, and the toxicological indicators including LC50 and LT50 have not yet been assessed. Previous studies on the effects of hypoxia on marine benthos have focused largely on functional responses, such as metabolism and gene expression, leaving potential structural damage to the mitochondria or the cells unknown. In this study we assessed the effects of hypoxia on Manila clam in terms of survival, behavior, metabolism and cellular damage, using a newly designed automated hypoxia simulation device that features exceptional accuracy and good stability. The clams exhibited strong tolerance to hypoxia as the 20-day LC50 for dissolved oxygen (DO) was estimated to be 0.57 mg L-1, and the LT50 at 0.5 mg L-1 DO was 422 hours. Adaptations included fewer buried clams and a depressed metabolism, while the unexpected rise in the activities of key enzymes involved in glycolysis may indicate a diverse strategy of shellfish under hypoxia. Cellular damage was observed as collapse of the mitochondrial cristae and both cellular and mitochondrial vacuolization. This multi-level study complements and updates our knowledge of the effects of hypoxia on marine benthos, by improving our understanding of the potential for marine ecological transformation under hypoxic conditions and providing useful information for Manila clam farming.
As the eutrophication of coastal waters intensifies, both the extent and strength of oxygen depletion (hypoxia) increase [1,2]. Though certain mobile species can flee hypoxic zones, less-tolerant organisms with poor mobility often suffer mass mortalities under severe hypoxia [2–4], often leading to further harmful ecological changes, such as jellyfish blooms [5,6] and other forms of ecosystem degradation [7,8].
To investigate long-term tolerance to hypoxia by the commercially important Manila clam, a series of laboratory experiments were carried out. This bivalve’s responses in terms of mortality, behavior and metabolism under a wide range of DO concentrations were determined in the context of the 20-day LC50, respiratory enzyme activities, and cellular damage. A novel hypoxia simulation device was used to ensure the accuracy and stability of the experimental environment. The implications of the results are as follows: (1) Moderate or short-term hypoxia does not threaten the survival of Manila clam; however, their behavioral response of emerging from the sediment to get more oxygen could increase their chance of being preyed upon by hypoxia-tolerant predators. (2) As a consequence of a strongly depressed metabolism under severe hypoxia, energy would be consumed first by basic life-sustaining activities, thus the animal’s growth and reproduction may be inhibited. (3) Manila clam may have a different strategy for glycolysis regulation, as the activities of PFK and PK in adductor muscle showed an unexpected rise after exposure to severe hypoxia, a finding that differs from determinations for many other hypoxia-tolerant species. Finally, (4) Manila clams that survive a hypoxic environment may subsequently suffer from further oxygen fluctuations, pathogens, or other environmental stresses due to collapse of the mitochondrial cristae and vacuolization in both the cells and the mitochondria.