Emersion behavior of the semi-terrestrial crab Neohelice granulata during hypoxic conditions: Lactate as a trigger

Climate changes affecting aquatic environments are increasing, and the resultant environmental challenges require animals to adopt alternative compensatory behavioral and physiological strategies. In particular, low levels of dissolved O₂ are a regular problem for estuarine animals, leading to activation of a series of behavioral and physiological responses. This study on the semi-terrestrial crab Neohelice granulata examined patterns of emersion behavior under different levels of dissolved O₂ availability and the role of lactate in this behavior. Emersion behavior was recorded for 4.5 h for crabs in water at four different levels of dissolved O₂ (6, 3, 2, and 1 mg O₂/L) and with free access to air. Oxygen consumption and hemolymphatic lactate levels were measured using the same experimental design. Emersion behavior was also recorded for 70 min in normoxic water after lactate or saline injections. Crabs increased their emersion behavior only in severe hypoxia (1 mg O₂/L), and O₂ consumption decreased under more severe hypoxic conditions. Despite the increase in emersion behavior, which leads to higher O₂ availability, an increase in hemolymphatic lactate levels indicates that the animals still need to resort to anaerobic pathways to fulfill their metabolic demand. Furthermore, animals injected with lactate showed higher emersion behaviors than animals injected with a saline solution even in normoxia. These results suggest that the increase in hemolymphatic lactate can act directly or indirectly as a trigger for the increase in emersion behavior in the semi-terrestrial crab N. granulata.