Gas bubble disease (supersaturation) in a population of lion’s mane sea slugs Melibe leonina

Two supersaturation events caused gas bubble disease (GBD) in captive Melibe leonina (lion's mane sea slugs). For the first event (a), supersaturation spikes (114-127% total dissolved gas [TDG]) occurred on Days 1a, 4a, and 23a. On Day 5a, nearly all 77 animals developed grossly visible gas bubbles within vasculature. Two recompression treatments were performed on 2 cohorts: (1) 7 animals were placed at 8.5 m depth (103 kPa) and unmonitored for 5 d, after which they had resolution of gas bubbles; and (2) 12 animals were placed in a hyperbaric chamber (152 kPa) for 5 d and had resolution of gas bubbles within 2 h of treatment initiation. Untreated animals had slow, consistent improvement with complete resolution at Day 12a. Starting on Day 23a (date of a supersaturation spike) through Day 79a, 0-4 animals died per day, totaling 60 deaths (78% mortality), after which no further mortalities occurred. After the first supersaturation event, 17 animals remained, and 16 new animals were added to the tank. Two months later, during the second event (b), supersaturation spikes (106-110% TDG) occurred on Days 1b, 2b, and 6b-15b. On Day 15b, all M. leonina developed grossly visible bubbles and were placed in a hyperbaric chamber for 14 d; gas bubbles resolved, and no mortalities occurred in the second event. Histologically, gas emboli were in the hemolymphatic sinuses and caused soft tissue edema. The cause for GBD was unknown for the first event and, for the second event, was due to cracked plumbing valves.