Comparative morphology of Southern Ocean Euphausia species: ecological significance of sexual dimorphic features

Species of the genus Euphausia dominate the euphausiid biomass of the Southern Ocean, the three largest being Euphausia superba, E. triacantha and E. crystallorophias. We measured a number of morphological features to identify differences between, and within, these species to obtain ecological insights. Interspecifically, the greatest difference was carapace size, with that of E. superba being by far the largest and most variable. This likely reflects its prolific spawning capacity compared with other euphausiid species. E. triacantha exhibited an extended sixth abdominal segment that could facilitate greater levels of thrust in the tail flip escape response. The pleopods, which provide propulsion in forward swimming, were more than 50% larger in E. superba, indicating a greater capacity for directional movement at high velocities. E. crystallorophias had eyes that were almost double the size of those in E. superba and E. triacantha, which may help retain visual resolution within its under-ice habitat. Intraspecifically, we found the above morphological features differed little between sexes and developmental stages in E. crystallorophias and E. triacantha, but differed significantly in E. superba. Compared to females and juveniles, male E. superba had significantly larger eyes and pleopods, whilst the carapace in males became shorter as a proportion of body length during growth. These features indicate a greater capacity for searching and swimming in males, which, we hypothesise, increases their ability to locate and fertilise females. This morphological specialisation in male E. superba is indicative of comparatively greater inter-male competition resulting from its tendency to form large, dense swarms

Relationship of morphometrics, total carotenoids, and total lipids with activity and sexual and spatial features in Euphausia superba

Morphological differences associated with sex or stage, together with total lipids and carotenoids, were studied in Euphausia superba as possible indicators of physiological condition. E. superba displays sexual dimorphism during growth. A group of mature males, called Males II herein, has a greater abdominal length, suggesting that they are faster swimmers, a feature implying higher metabolic rates and a higher demand for protecting pigments like carotenoids. Mature Males II have proportionally lower lipids but higher total lipid-soluble carotenoids, a counterintuitive finding. Males II also have bigger eyes. Significant regressions with carotenoids were found for wet weight, abdominal length, and eye diameter. On a spatial analysis, population composition reflects reproductive activity. Males II would be in search of females for fecundation and, thus, are dominant in some areas. The PCA analysis of 10 allometric and biochemical variables show a distinct Males II group differing in morphology, carotenoids, and lipid contents. The carotenoid:lipid ratio was highest for Males II, supporting the hypothesis of the role of carotenoids in the activity of the species. Mature males may experience physiological stress during reproduction and probably die shortly afterwards. A relationship between activity, morphometrics, and carotenoid content seems evident, deserving further investigation

²¹⁰Po/²¹⁰Pb dynamics in relation to zooplankton biomass and trophic conditions during an annual cycle in northwestern Mediterranean coastal waters

Monthly sampling in northwestern Mediterranean coastal waters was undertaken to better understand the relationship between zooplankton biomass and the cycling of the natural radionuclide 210Po/210Pb pair during a one-year period (October 1995-November 1996). In conjunction with mesozooplankton collections and 210Po/210Pb measurements in seawater, zooplankton and their fecal pellets, the biochemical composition of particulate organic matter (POM) was also examined at three depths (0, 20 and 50 m) as an indicator of trophic conditions. During May 1996, a strong zooplankton "bloom" was observed which was preceded by a prolonged increase in POM (protein + carbohydrates + lipids) starting at the end of March, and further demonstrated by a concomitant increase in the concentration of smaller particles, two features that are typical of mesotrophic waters. Simultaneous measurements of 210Po in sea water and zooplankton showed an inverse trend between these two parameters during the sampling period, with the two lowest 210Po concentrations in the dissolved phase of seawater coincident with the highest radionuclide concentrations in the zooplankton; however, this apparent relationship was not statistically significant over the entire year. Freshly excreted mesozooplankton and salp fecal pellets, which have been strongly implicated in the removal and downward transport of these radionuclides from the upper water column, contained 210Po and 210Pb levels ranging from 175 to 878 and 7.5-486 Bq kg-1 dry weight, respectively. Salp pellets contained 5 and 10 times more 210Po and 210Pb than in fecal pellets produced by mixed zooplankton, a finding most likely related to their different feeding strategies. During the zooplankton biomass peak observed in May, the 210Po concentration in zooplankton was at a minimum; however, in contrast to what has been reported to occur in some open sea oligotrophic waters, over the year no statistically significant inverse relationship was found between zooplankton biomass and 210Po concentration in zooplankton. This observation may have resulted from the general lack of very low biomass concentrations (<1 mg m-3) measured in these coastal waters, biomass levels which commonly occur in open ocean oligotrophic regions. © 2012 Elsevier Ltd