While many intertidal animals exhibit circatidal rhythms, the nature of the underlying endogenous clocks that control these rhythms has been controversial. In this study American horseshoe crabs, Limulus polyphemus, were used to test the circalunidian hypothesis by exposing them to four different tidal regimes. Overall, the results obtained support the circalunidian hypothesis: each of the twice-daily rhythms of activity appears to be controlled by a separate clock, each with an endogenous period of approximately 24.8h. First, spontaneous “skipping” of one of the daily bouts was observed under several different conditions. Second, the presence of two bouts of activity/day, with different periods, was observed. Lastly, we were able to separately synchronize bouts of activity to two artificial tidal regimes with different periods. These results, taken together, argue in favor of two separate circalunidian clocks in Limulus, each of which controls one of the two bouts of their daily tidal activity rhythms.

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Introduced species influence the dynamics and structure of freshwater systems; understanding the variables that shape introduced species’ distributions can aid in anticipating their spread. We examined multiple factors that may influence the distribution of northern crayfish (Orconectes virilis (Hagen, 1870)), an introduced species, in New Hampshire, USA. Sampling occurred July to August 2010 in 20 lakes. We tested catch per unit effort (CPUE) and body size of crayfish against lake trophic status, size, depth, and shoreline development, as well as substrate type. We also compared CPUE and body size in the presence or absence of known predators, smallmouth bass (Micropterus dolomieu Lacepède, 1802) and largemouth bass (Micropterus salmoides (Lacepède, 1802)). Crayfish body size was not strongly associated with any tested variables, nor were there significant correlations between lake-level parameters and CPUE. CPUE increased with rocky substrates and decreased with macrophyte cover. We also found significantly lower CPUE in lakes with bass predators; this could be due to consumptive effects directly lowering crayfish abundance, nonconsumptive effects of bass on crayfish behavior, or both. Our work provides a baseline for future surveys examining northern crayfish or bass expansion in New Hampshire and highlights a variable that could be important as this crayfish colonizes additional locations outside its native range.

By 2050, mean temperature in the state of Maine, located in the Northeastern USA, is expected to increase nearly 1°C, which could directly affect native coldwater salmonid behaviour and increase competition with warmwater smallmouth bass. We conducted a microcosm experiment to examine the feeding and agonistic behaviour of endangered juvenile Atlantic Salmon (Salmo salar) at two temperatures (18 and 21°C) in the presence and absence of non‐native Smallmouth Bass (Micropterus dolomieu). By visually reviewing footage of fish competition in our tanks, we quantified feeding and agonistic interactions. We predicted salmon would exhibit lower feeding activity than bass at 21°C and antagonistic interactions between the two species would increase with warming. We found salmon feeding activity was reduced by smallmouth bass presence and this effect was stronger at 21°C. We also found smallmouth bass aggression was strongest at 21°C when salmon were present. Lastly, feeding activity and aggression in both species changed with food availability. These findings illustrate the potential for invasive warmwater species to outcompete native salmonids for resources, especially under the warmer conditions predicted by climate change scenarios.