Evidence of sperm removal behaviour in an externally fertilizing species and compensatory behaviour for the risk of self-sperm removal.

The removal of rival sperm from a female's sperm storage organ acts as a strong sperm competition avoidance mechanism, which has been reported only in internally fertilizing species and not at all in externally fertilizing species. This study demonstrated for the first time that nest-holding males of Bathygobius fuscus, an externally fertilizing marine fish, remove the sperm of rival sneaker males from the spawning nest by exhibiting tail-fanning behaviour within the nest. Males showed tail-fanning behaviour when semen was artificially injected into the nest but not when seawater was injected, and in open nests this behaviour resulted in higher paternity rates for the focal male. The sperm removal behaviour entails the risk of removing their own sperm; therefore, additional sperm release behaviour is likely necessary to benefit from the sperm removal effect. Consistent with this, males increased post-fanning sperm release behaviour more in the semen than in the seawater injection treatment. Moreover, males who had removed sperm for a longer time spent more time releasing sperm after the removal, suggesting that the additional sperm release behaviour compensated for the loss of their own sperm. These results suggest that sperm removal behaviour is not restricted to internally fertilizing organisms and deserves further investigation in this and other species.

Practically scheduling hormone therapy for prostate cancer using a mathematical model.

Hormone therapy is one of the popular therapeutic methods for prostate cancer. Intermittent androgen suppression (IAS) is the method which stops and resumes hormone therapy repeatedly. The efficacy of IAS differs depending on patients; both the cases have been reported where the relapse of cancer happened and did not happen, for the patients who had undergone IAS. For the patients who cannot avoid the relapse of cancer by IAS, we should delay the relapse of cancer as later as possible. Here we compared some practical methods of determining when to stop and restart hormone therapy for IAS using an existing mathematical model of prostate cancer. The method we suggest is to determine the ratio of on-treatment period and off-treatment period sparsely for each cycle, namely the "sparse search." We also compared the performance of the sparse search with the exhaustive search and the model predictive control. We found that the sparse search can find a good treatment schedule without failure, and the computational cost is not so high compared to the exhaustive method. In addition, we focus on the model predictive control (MPC) method which has been applied to the scheduling of IAS in some existing studies. The MPC is computationary efficient, although it does not always find an optimal schedule in the numerical experiments here. We believe that the MPC method might be also promising because of its reasonable computational costs and its possibility of expanding of the model.

Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench.

It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15-18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.