Evidence that the rotifer Brachionus plicatilis is not an osmoconformer

The rotifer Brachionus plicatilis is euryhaline ( growing between 2 and 97 ppt) and has previously been considered an osmoconformer. We suggest that B. plicatilis is an osmoregulator, exhibiting a pattern of Na+/ K+ ATPase activity in response to salinity consistent with that of other osmoregulating euryhaline invertebrates. To examine salinity tolerance, growth rates between 5 and 60 ppt were determined. The activity of Na+/ K+ ATPase was examined, over the same range of salinities, by measuring ATPase activity in rotifer homogenates in the presence and absence of a Na+/ K+ ATPase inhibitor. Maximum specific growth rate (0.95 day(-1)) occurred at 16 ppt, highest mean amictic eggs per female (1.41) occurred at 20 ppt, and both parameters decreased rapidly as salinity increased. Egg development time was constant with salinity at 0.92 days. The activity of Na+/ K+ ATPase per milligram protein increased from 3.9 mu mol h(-1) at 5 ppt to 6.8 mu mol h(-1) at 50 ppt and accounted for 15 and 30% of total ATPase activity, respectively. We suggest that these observations are consistent with increasing stress at high salinities and the occurrence of a hypo-osmoregulatory response. Given the high ATP consumption of Na+/ K+ ATPase at high salinities, it is possible that a proportion of the corresponding decreases in growth rate and egg production are a direct cost of regulation.