ABSTRACT
Primary productivity by plants and algae is the fundamental source of energy in virtually all food webs. Furthermore, photosynthetic organisms are the sole source for ω-3 and ω-6 essential fatty acids (EFA) to upper trophic levels. Because animals cannot synthesize EFA, these molecules may be useful as trophic markers for tracking sources of primary production through food webs if different primary producer groups have different EFA signatures. We tested the hypothesis that different marine macrophyte groups have distinct fatty acid (FA) signatures by conducting a phylogenetic survey of 40 marine macrophytes (seaweeds and seagrasses) representing 36 families, 21 orders, and four phyla in the San Juan Archipelago, WA, USA. We used multivariate statistics to show that FA composition differed significantly (P < 0.001) among phyla, orders, and families using 44 FA and a subset of seven EFA (P < 0.001). A second analysis of published EFA data of 123 additional macrophytes confirmed that this pattern was robust on a global scale (P < 0.001). This phylogenetic differentiation of macrophyte taxa shows a clear relationship between macrophyte phylogeny and FA content and strongly suggests that FA signature analyses can offer a viable approach to clarifying fundamental questions about the contribution of different basal resources to food webs. Moreover, these results imply that taxa with commercially valuable EFA signatures will likely share such characteristics with other closely related taxa that have not yet been evaluated for FA content.
ABSTRACT
This work was motivated by subtidal observations of red urchins (Strongylocentrotus franciscanus) moving their spines into streamlined positions as water current increased in the field. Trials in a flume across flow speeds from 0 to 65 cm s(-1) enabled us to observe the change in overall shape of the urchins and quantify the decrease in spine angle that occurred as flow speeds increased. The effect of this behaviour on drag and lift was measured with physical models made from urchin tests with spines in the `up' position (typical in stagnant and slow velocities) and in the `down' position (typical of posture in high velocities). Streamlining spines decreased the drag, but increased the lift experienced by urchin models at flow speeds between 10 and 40 cm s(-1), current velocities that are commonly encountered by these animals in the field in Washington, USA. Total force (combination of drag and lift) was similar for `up' and `down' models at all flow speeds, lift comprising the majority of the force for `down' models, and drag slightly higher for `up' models. Live urchins in the field routinely adopt a streamlined `down' posture in flow, suggesting that they may be better able to cope with lift than drag. This behaviour, although affecting hydrodynamic forces and enabling S. franciscanus to remain attached to the substratum in high currents, may lead to reduced capture of drift kelp, which is entrapped on upright spines and then eaten, delivery of which is positively related to flow speed. Urchins living in deep subtidal habitats rely on drift kelp capture but must stay attached to the substratum to be successful in a habitat. Therefore, this streamlining behaviour may be an important factor enabling S. franciscanus to persist in deep, high-current areas.
