AB 1:99-108 (2007)  -  DOI: https://doi.org/10.3354/ab00011

ABSTRACT: As indices of protein synthetic activity, RNA:DNA ratios were determined for >80 copepod species caught from the epipelagic (0 to 500 m), mesopelagic (500 to 1000 m), upper bathypelagic (1000 to 2000 m), lower bathypelagic (2000 to 3000 m) and abyssopelagic (3000 to 5000 m) zones of the North Pacific Ocean. The copepods from the epipelagic zone exhibited higher RNA:DNA ratios compared to those from the mesopelagic through the abyssopelagic zones. Parameters (other than the depth zones) affecting the ratios were explored, by grouping the copepods into 4 developmental-stage/sex categories (C4, C5 and C6 females and males), 3 feeding types (carnivore, detritivore and suspension feeder), or 2 phylogeny-related reaction-speed groups in which detecting the presence or absence of myelinated sheath enveloping axons indicated fast or slow reacting species, respectively. Stage and sex data showed that RNA:DNA ratios were greater in C6 females than in other groups. Among the feeding-type data, carnivores exhibited the lowest RNA:DNA values. Reaction-speed data indicated that slow-reacting species are characterized by lower ratios than fast-reacting species. Taking into account all of these biological parameters as independent variables, stepwise multiple regression analysis revealed that the pattern of the reduction in RNA:DNA ratios toward greater depths was still valid. The typical interzonal copepods, Neocalanus spp., showed that non-feeding C6 females of N. plumchrus and N. flemingeri collected from the mesopelagic through the abyssopelagic zones had high RNA:DNA ratios comparable to those of the epipelagic-zone copepods. The present results, combined with recent reports on the depth-related decline in the metabolism and body nitrogen content of pelagic copepods downward, suggest overall slower life modes of deeper-living species. We think the ‘predation-mediated selection’ hypothesis explains this ‘slow’ life more effectively than limited food supply or colder temperatures in the deep sea.

KEY WORDS: Copepods · RNA · DNA · Abyssopelagic · North Pacific