Mesozooplankton size structure in the Canary Current System.

Changes in plankton composition influences the dynamics of marine food webs and carbon sinking rates. Understanding the core structure and function of the plankton distribution is of paramount importance to know their role in trophic transfer and efficiency. Here, we studied the zooplankton distribution, abundance, composition, and size spectra for the characterization of the community under different oceanographic conditions in the Canaries-African Transition Zone (C-ATZ). This region is a transition zone between the coastal upwelling and the open ocean showing a high variability because of the physical, chemical, and biological changes between eutrophic and oligotrophic conditions through the annual cycle. During the late winter bloom (LWB), chlorophyll a and primary production were higher compared to that of the stratified season (SS), especially in the upwelling influenced area. Abundance distribution analysis clustered stations into two main groups according to the season (productive versus stratified season), and one group sampled in the upwelling influenced area. Size-spectra analysis showed steeper slopes during daytime in the SS, suggesting a less structured community and a higher trophic efficiency during the LWB due to the favorable oceanographic conditions. We also observed a significant difference between day and nighttime size spectra due to community change during diel vertical migration. Cladocera were the key taxa differentiating an Upwelling-group, from a LWB- and SS-group. These two latter groups were differentiated by Salpidae and Appendicularia mainly. Data obtained in this study suggested that abundance composition might be useful when describing community taxonomic changes, while size-spectra gives an idea of the ecosystem structure, predatory interactions with higher trophic levels and shifts in size structure.

Energetic Implications of Morphological Changes between Fish Larval and Juvenile Stages Using Geometric Morphometrics of Body Shape.

The fish body shape is a key factor that influences multiple traits such as swimming, foraging, mating, migrations, and predator avoidance. The present study describes the body morphological changes and the growth trajectories during the transformation from 24 to 54 days post-hatching in the golden grey mullet, Chelon auratus, using geometric morphometric analysis (GMA). The results revealed a decrease in morphological variability (i.e., morphological disparity) with the somatic growth. The main changes affected head size, elongation, and widening of the body. Given that this variability could affect the metabolism, some individuals with different morphologies and in different ontogenetic developmental stages were selected to estimate their potential respiration rate using the Electron Transport System (ETS) analysis. Differences were detected depending on the developmental stage, and being significantly smaller after 54 days post-hatching. Finally, a multivariate linear regression indicated that the specific ETS activity was partially related to the fish length and body shape. Thus, our findings emphasized the relevance of larval morphological variability for understanding the physiological processes that occur during the development.

First zoeal stage of Plesionika crosnieri Chan Yu, 1991, P. ortmanni Doflein, 1902, and P. semilaevis Bate, 1888, with remarks on the early larvae of Plesionika Bate, 1888 (Crustacea, Decapoda).

The morphology of first stage zoea larvae of three pandalid shrimps Plesionika crosnieri Chan Yu, 1991, P. ortmanni Doflein, 1902, and P. semilaevis Bate, 1888 are described for the first time from larvae obtained in laboratory culture. The zoea I is compared amongst those species of the genus Plesionika with larval morphology known. It is found that the length of the rostrum, the number of anteroventral denticles on the carapace, presence of exopodal setae in the maxillule, and setation pattern of the peduncle of antennule, endopod of antenna, and coxa/basis of maxillipeds are useful characters in distinguishing these larvae.

Morphological and morphometric variability in the zoea I larvae of Pugettia quadridens (De Haan, 1839): looking for reliable characters for taxonomic studies on the genus Pugettia Dana, 1851 (Majoidea: Epialtidae).

The larval morphology of Pugettia quadridens (De Haan, 1839) has been described several times, but the lack of voucher specimens (both larvae and spent females), makes it impossible to verify whether the remarkable heterogeneity, which has been described by the different authors, are due to intra-specific variability. We re-described the morphological and morphometric features of the first zoeal stage of the species, depositing voucher specimens for future examination by the scientific community. We used four females that were collected in the same locality in Hakodate Bay, Japan, and found significant morphological and morphometric intra-specific variability, both within and among clutches. One important variation was the setal count on the endopod of maxilla (3-4, rarely 5), which have been considered in previous studies to be a key character to distinguish species within genus Pugettia Dana, 1851. The size differences were rather striking, and the mean carapace length of the largest clutch (0.73 ± 0.04 SD mm) was up to 0.1 mm larger than the smallest one (0.63 ± 0.02 mm). We establish herein a solid taxonomic background for further larval descriptions of species of Pugettia with reliable characters and recommend the use of different clutches to prevent insufficient morphological descriptions due to intra-specific variability.

Redescription of the early larval stages of the pandalid shrimp Chlorotocus crassicornis (Decapoda: Caridea: Pandalidae).

The first four larval stages of the pandalid shrimp Chlorotocus crassicornis (A. Costa, 1871) are described and illustrated from laboratory-reared material obtained from ovigerous females collected in the southwestern Spain and south Taiwan. The second to fourth larval stages of this species are reported for the first time to science. Detailed examination of the first larval stages reveals that previous description misidentified some key larval characters which have prevented its identification in plankton samples. It is found that the zoeal morphology of Chlorotocus is not very different from other pandalid larvae, and in fact closely resembles Plesionika and Heterocarpus.

Phylogenetics reveals the crustacean order Amphionidacea to be larval shrimps (Decapoda: Caridea).

We present evidence that the single representative of the crustacean order Amphionidacea is a decapod shrimp and not a distinct order. After reviewing available morphological evidence, it is concluded that Amphionides is a larval form, but with an as yet unknown parentage. Although the most likely adult form is in the family Pandalidae, the limited molecular data available cannot fully resolve its affinity. We therefore propose to treat Amphionides reynaudii as incertae sedis within Caridea, rather than a separate family. In view of the large scale, tropical and subtropical distribution of the taxon, the possibility is discussed that Amphionides is more likely to be a composite taxon at generic level, rather than larvae of a single shrimp species.

Biophysical interactions control the size and abundance of large phytoplankton chains at the Ushant tidal front.

Phytoplankton blooms are usually dominated by chain-forming diatom species that can alter food pathways from primary producers to predators by reducing the interactions between intermediate trophic levels. The food-web modifications are determined by the length of the chains; however, the estimation is biased because traditional sampling strategies damage the chains and, therefore, change the phytoplankton size structure. Sedimentological studies around oceanic fronts have shown high concentrations of giant diatom mats (>1 cm in length), suggesting that the size of diatom chains is underestimated in the pelagic realm. Here, we investigate the variability in size and abundance of phytoplankton chains at the Ushant tidal front (NW France) using the Video Fluorescence Analyzer (VFA), a novel and non-invasive system. CTD and Scanfish profiling characterized a strong temperature and chlorophyll front, separating mixed coastal waters from the oceanic-stratified domain. In order to elucidate spring-neap variations in the front, vertical microstructure profiler was used to estimate the turbulence and vertical nitrate flux. Key findings were: (1) the VFA system recorded large diatom chains up to 10.7 mm in length; (2) chains were mainly distributed in the frontal region, with maximum values above the pycnocline in coincidence with the maximum chlorophyll; (3) the diapycnal fluxes of nitrate enabled the maintenance of the bloom in the frontal area throughout the spring-neap tidal cycle; (4) from spring to neap tide the chains length was significantly reduced; (5) during neap tide, the less intense vertical diffusion of nutrients, as well as the lower turbulence around the chains, intensified nutrient-depleted conditions and, thus, very large chains became disadvantageous. To explain this pattern, we suggest that size plasticity is an important ecological trait driving phytoplankton species competition. Although this plasticity behavior is well known from experiments in the laboratory, it has never been reported from observations in the field.