RNA concentration and the RNA to protein ratio in cephalopod tissues: sources of variation and relationship with growth rate

NA and protein concentrations, and the RNA to protein ratio, were measured in four species of cephalopods, to evaluate sources of variation and the potential for using RNA concentration and the RNA to protein ratio as growth indices. In field samples of Loligo forbesi and Eledone cirrhosa, RNA concentrations and the RNA to protein ratio were higher in immature animals than in mature animals. In Loligo forbesi, values were also higher in males than in females and higher in smaller individuals than in large individuals. Both these trends are consistent with expected differences in growth rate, i.e. RNA is higher in faster growing animals. Mature female Eledone cirrhosa, a species in which the female is larger and presumably grows faster, had higher RNA concentrations than mature males. However, no such difference between the sexes was seen for immature E. cirrhosa or Todarodes sagittatus. Methods for transport and maintenance of Loligo forbesi in captivity in the Azores are described. Many of the captive squid showed poor growth and survival but results from these animals nevertheless confirmed that RNA concentrations were higher in males than in females and higher in animals with smaller gonads than in animals with large gonads. Higher protein concentrations were found in males than in females, and protein concentration was also positively correlated with feeding rate and digestive gland indices. Octopus vulgaris held in captivity grew rapidly and consistently and RNA concentrations were lower in bigger animals than in smaller animals. Neither experiment provided direct support for the hypothesis that RNA concentration or the RNA to protein ratio is directly related to growth rate. Systematic variation in protein concentration, e.g. in relation to recent feeding, leads us to suggest that protein concentration (mg/g fresh body weight) is likely to provide a more reliable index than the RNA to protein ratio

Vertical structure, biomass and topographic association of deep-pelagic fishes in relation to a mid-ocean ridge system

The assemblage structure and vertical distribution of deep-pelagic fishes relative to a mid-ocean ridge system are described from an acoustic and discrete-depth trawling survey conducted as part of the international Census of Marine Life field project MAR-ECO (www.mar-eco.no). The 36-station, zig-zag survey along the northern Mid-Atlantic Ridge (MAR; Iceland to the Azores) covered the full depth range (0 to >3000 m), from the surface to near the bottom, using a combination of gear types to gain a more comprehensive understanding of the pelagic fauna. Abundance per volume of deep-pelagic fishes was highest in the epipelagic zone and within the benthic boundary layer (BBL; 0-200 m above the seafloor). Minimum fish abundance occurred at depths below 2300 m but above the BBL. Biomass per volume of deep-pelagic fishes over the MAR reached a maximum within the BBL, revealing a previously unknown topographic association of a bathypelagic fish assemblage with a mid-ocean ridge system. With the exception of the BBL, biomass per volume reached a water column maximum in the bathypelagic zone between 1500 and 2300 m. This stands in stark contrast to the general "open ocean" paradigm that biomass decreases exponentially from the surface downwards. As much of the summit of the MAR extends into this depth layer, a likely explanation for this mid-water maximum is ridge association. Multivariate statistical analyses suggest that the dominant component of deep-pelagic fish biomass over the northern MAR was a wide-ranging bathypelagic assemblage that was remarkably consistent along the length of the ridge from Iceland to the Azores. Integrating these results with those of previous studies in oceanic ecosystems, there appears to be adequate evidence to conclude that special hydrodynamic and biotic features of mid-ocean ridge systems cause changes in the ecological structure of deep-pelagic fish assemblages relative to those at the same depths over abyssal plains. Lacking terrigenous input of allochthonous organic carbon, increased demersal fish diversity and biomass over the MAR relative to the abyssal plains may be maintained by increased bathypelagic food resources. The aggregation of bathypelagic fishes with MAR topographic features was primarily a large adult phenomenon. Considering the immense areal extent of mid-ocean ridge systems globally, this strategy may have significant trophic transfer and reproductive benefits for deep-pelagic fish populations