The effects of shelter quality and prior residence on marmorkrebs (marbled crayfish).

Many animals fight over a limited valuable resource. In marmorkrebs (marbled crayfish), large animals usually defeat small opponents but they are frequently beaten by small opponents that are shelter owners. A prior residence effect of marbled crayfish was analysed quantitatively in the present study. More than 2 h of residency in a shelter was sufficient for small owners to defeat large intruders. Small animals that stayed in a shelter for 24 h still tended to win following removal of the shelter 10 min before pairing with large intruders, but 2 h residents were occasionally beaten by large intruders without the support of shelters during pairings. The prior residence effect thus developed depending on the duration of residency. To clarify whether the strength of the prior residence effect was affected by the quality of a shelter, large and small owners with different combinations of high- and low-quality shelters were paired. When both large and small owners possessed a high-quality shelter, the frequency of agonistic bouts was reduced. Even if agonistic bouts occurred, the win frequency of small owners was almost equal to that of large owners. Thus, the residence effect on small owners was sufficiently strong to overcome the physical disadvantage of small animals to large opponents. By contrast, small owners of low--quality shelters were frequently beaten by large owners with the shelters of same or better quality. We conclude that the outcome of fights over the resource shelter is highly dependent on both the perception of shelter quality and body size differences.

Enhancement of habituation during escape swimming in starved crayfish.

Feeding is important to supply the immediate energy needs of animals and starved animals must expend energy in attempting to acquire foods irrespective of the danger of predation risk. Crayfish escape from attack of predators by tailflipping and in response to rostral stimuli crayfish show backward escape swimming following an initial rapid flexion of the abdomen. Since the tailflip is an energetically costly behaviour, the occurrence of a tailflip diminishes if a stimulus is repeatedly applied through habituation. In this study, we have compared the process of this habituation between fed and starved crayfish. We found that in starved animals habituation was enhanced compared to fed animals. The presence of food in the experimental tanks further enhanced habituation of starved animals. Starved crayfish thus showed trade-offs between energy saving and predation risk.

Compact image scanner with large depth of field by compound eye system.

A compact image scanner is designed by using a compound eye system with plural optical units in which a ray path is folded by reflective optics. The optical units are aligned in two lines and take each image of a separated field of view (FOV), slightly overlapped. Since the optical units are telecentric in the object space and the magnification ratio is constant regardless of the object distance, the separated pieces of a total image are easily combined with each other even in the defocused position. Since the optical axes between adjacent optical units are crossed obliquely, object distance is derived from the parallax at each boundary position and an adequate deblurring process is achieved for the defocused image.

Reproductive isolation between 2 karyotypes in natural populations of the leaf beetle Chrysolina aurichalcea.

Japanese populations of the leaf beetle, Chrysolina aurichalcea, have 2 main karyotypes, Type 31 and Type 41, which greatly differ in chromosome number and form. We examined their geographical distribution and reproductive isolation in southern Hokkaido, northern Japan. Type 41 was widely distributed in the survey area, whereas Type 31 was restricted to a narrow portion. A transect across each karyotypic habitat revealed that both karyotypes co-occurred in a approximately 5 km wide zone, with few hybrids or hybrid derivatives. Assessment of the fine scale distribution showed that the 2 karyotypes do not occupy separate microhabitats within the sympatric zone. Karyotype combinations of copulating pairs in the sympatric zone showed no evidence of assortative mating according to karyotype. These findings suggest that postmating isolation between the 2 karyotypes is strong, whereas premating isolation is weak. It does not agree with results from previous studies, which reported a substantial degree of both post- and premating isolation between the karyotypes. Possible causes of the discrepancy in premating isolation are discussed.