Repeatability of Hypoxia Tolerance of Individual Juvenile Striped Bass Morone saxatilis and Effects of Social Status.

Chesapeake Bay is the primary nursery for striped bass (Morone saxatilis), which are increasingly being exposed to hypoxic waters. Tolerance to hypoxia in fish is generally determined by a single exposure of an isolated individual or by exposing large groups of conspecifics to hypoxia without regard to social status. The importance of social context in determining physiological responses to stressors is being increasingly recognized. To determine whether social interactions influence hypoxia tolerance (HT) in striped bass, loss of equilibrium HT was assessed in the same fish while manipulating the social environment around it. Small group settings were used to be more representative of the normal sociality experienced by this species than the paired encounters typically used. After establishing the dominance hierarchy within a group of fish, HT was determined collectively for the individuals in that group, and then new groups were constructed from the same pool of fish. Individuals could then be followed across multiple settings for both repeatability of HT and hierarchy position ( X¯=4.2±0.91 SD groups per individual). HT increased with repeated exposures to hypoxia ( P<0.001 ), with a significant increase by a third exposure ( P=0.004 ). Despite this changing HT, rank order of HT was significantly repeatable across trials for 6 mo ( P=0.012 ). Social status was significantly repeatable across trials of different group composition ( P=0.02 ) and unrelated to growth rate but affected HT weakly in a complex interaction with size. Final HT was significantly correlated with blood [hemoglobin] and hematocrit. The repeatability and large intraspecific variance of HT in juvenile striped bass suggest that HT is potentially an important determinant of Darwinian fitness in an increasingly hypoxic Chesapeake Bay.

Daily and seasonal changes in the stickiness of viscous capture threads in Argiope aurantia and Argiope trifasciata orb-webs.

The spirally arrayed viscous capture threads of spider orb-webs are formed of small, regularly spaced adhesive droplets and are responsible for retaining insects that strike the web, giving a spider more time to subdue these prey. These sticky threads are deposited from the perimeter of the web inward. We tested the hypothesis that depletion of silk reserves during web construction affects the properties of capture threads spun by adult female Argiope aurantia and Argiope trifasciata. In both species the droplet volume (DV) per millimeter thread length was the same in outer and inner capture threads and in early and late season webs. However, the outer threads of both species were stickier than their inner threads and, consequently, had a greater stickiness per DV. Thus, dwindling silk reserves during web construction appeared to reduce the stickiness of both species' threads by changing the composition rather than the volume of their viscous droplets. In A. trifasciata, which we studied later in the fall than A. aurantia, there were also seasonal declines in both thread stickiness and stickiness per DV, which may result from either the depletion of silk reserves or the reallocation of these resources. Early season webs of A. trifasciata also had greater stickiness per square centimeter of capture area than late season webs, better equipping these early webs to retain insects.