Coordination between arm and leg movements during locomotion

To evaluate the contrasting dynamical and biomechanical interpretations of the 2:1 frequency coordination between arm and leg movements that occurs at low walking velocities and the 1:1 frequency coordination that occurs at higher walking velocities, the authors conducted an experiment in which they quantified the effect of walking velocity on the stability of the frequency and phase coordination between the individual limb movements. Spectral analyses revealed the presence of 2:1 frequency coordination as a consistent feature of the data in only 3 out of 8 participants at walking velocities ranging from 1.0 to 2.0 km/h, in spite of the fact that the eigenfrequencies of the arms were rather similar across participants. The degree of interlimb coupling, as indexed by weighted coherence and variability of relative phase, was lower for the arm movements and for ipsilateral and diagonal combinations of arm and leg movements than for the leg movements. Furthermore, the coupling between all pairs of limb movements was found to increase with walking velocity, whereas no clear signs were observed that the switches from 2:1 to 1:1 frequency coordination and vice versa were preceded by loss of stability. Therefore, neither a purely biomechanical nor a purely dynamical model is optimally suited to explain these results. Instead, an integrative model involving elements of both approaches seems to be required

Periodicity of propagule expulsion and settlement in the competing native and invasive brown seaweeds, Cystoseira humilis and Sargassum muticum (Phaeophyta)

Dense stands of the invasive species Sargassum muticum (Yendo) Fensholt develop in tidal pools close to its southern distributional limit in Europe, the southwest coast of Portugal. Along this coast, sheltered tidal pools form a specific habitat in which colonization occurs. Invaded pools are originally inhabited by Cystoseira humilis Kützing. Differences in gamete release between the competing native and alien species might be important for the initial settlement and further spread of the invader. Therefore, we tested whether egg expulsion and embryo settlement in both species had the same timing with respect to lunar and tidal cycles. For more than 2 months during the reproductive season egg expulsion and embryo release were monitored daily for each species. Egg expulsion in S. muticum showed a broadly semilunar periodicity peaking around full and new moon (spring tides), when low tides take place in the morning/evening. In contrast, C. humilis egg expulsion showed an asymmetric semilunar-to-lunar periodicity peaking around waning quarter moon, when low tides occur around midday. Embryo settlement detected in pools was low for both species and less periodic. Phase differences in expulsion events between the two species with respect to the semilunar cycle suggest that cues other than the moon are involved in their timing. Our observations suggest that variations in physiological mechanisms and/or environmental conditions result in different patterns of egg expulsion between the two species. This might have consequences for fertilization success, gamete dispersal and survival. It was further found that peaks in egg expulsion and embryo release (i.e. settlement) in S. muticum were much more synchronous at a site in northern Portugal compared with a site close to the current southern distributional limit in south-west Portugal, possibly as a consequence of thermal stress experienced in the south