Ecological characteristics of eucalanoid copepods of the eastern tropical North Pacific Ocean: Adaptations for life within a low oxygen system

The eastern tropical North Pacific Ocean (ETNP) is home to one of the largest and most severe mid-water oxygen minimum zones (OMZs). Members of the copepod family Eucalanidae are abundant in this region and display varied vertical distributions throughout the OMZ. This research assessed the diversity of ecological strategies used by these copepods to cope with the presence of the OMZ based on their biochemical, physiological, and behavioral characteristics. Five species of copepods (Eucalanus inermis, Pareucalanus attenuatus, Rhincalanus nasutus, Rhincalanus rostrifrons, and Subeucalanus subtenuis) were collected at the Costa Rica Dome (9°N, 90°W) and Tehuantepec Bowl (13°N, 105°W) during cruises in fall of 2007 and winter of 2008–2009. Adult females of all species were collected and analyzed for water, ash, carbon, nitrogen, hydrogen, phosphorus, protein and lipid content, lactate dehydrogenase (LDH) activity, and survivorship when exposed to prolonged low-oxygen conditions. Four distinct ecological strategies were observed for the five species based on genus. E. inermis, found throughout the OMZ and surface waters, had very high water content (94% of wet weight (WW)), leading to low organic content per unit WW. However, when corrected to ash-free dry weight (AFDW), protein and storage lipid contents of this species had intermediate (46 and 8% of AFDW, respectively) values compared to other species. E. inermis demonstrated high survivorship under low-oxygen conditions (>90%), and had detectable levels of LDH, indicating an ability to rely on anaerobic pathways. Rhincalanus spp., found primarily in the lower-oxygen subsurface waters, also had detectable LDH activity and high survivorship under low-oxygen conditions (>85%), but had much higher storage lipid levels (>37% of AFDW), very low protein levels (28–33% of AFDW), and low water content (87% of WW). S. subtenuis and P. attenuatus are both distributed primarily in surface waters, but showed distinct ecological strategies. S. subtenuis protein levels were very high (67% AFDW), storage lipid levels low (0%), and water content low (87% of WW). They also had very low survivorship under low-oxygen conditions (22%) and no detectable LDH activity. P. attenuatus also did not have detectable LDH activity or large lipid stores. P. attenuatus, however, had much lower protein content (30% AFDW) and higher water content (89% of WW), indicating a distinct ecological strategy. Comparisons of ETNP individuals with conspecifics or congeners from higher-oxygen environments suggest that low protein levels (indicative of lower overall activity) seen in some groups may be an adaptation for survival in lower-oxygen regions.