Toshio Inoda

Effect of osmolality on the initiation of sperm motility in Xenopus laevis

1. Seminal plasma of the South African clawed toad Xenopus laevis exhibited osmolality around 250 mosmol/kg isotonic to blood plasma. 2. Spermatozoa remained immotile when the semen was diluted in solutions of 100 mM NaCl, 100 mM KCl or 200 mM glucose containing 20 mM Hepes-NaOH buffer which exhibited almost the same osmolalities (approximately 240 mosmol/kg) as seminal plasma. 3. The spermatozoa became motile in these three solutions if the osmolalities were decreased. 4. These suggest that motility of Xenopus sperm is suppressed by seminal osmolality in the reproductive organ and initiated by a decrease of osmolality when they are spawned into hypotonic fresh water.

Asymmetric mandibles of water-scavenger larvae improve feeding effectiveness on right-handed snails.

Asymmetric morphology and function are commonly observed in bilateral animals (Palmer 1996). Recent studies also revealed how the asymmetric morphology of animals is developed (Wood 1997). However, the adaptive significance of directional asymmetries (i.e., the advantage to being asymmetric in a particular direction) is rarely known. Well-known cases are the interspecies interaction between crustaceans and snails. In the case of molluscivorous reef crabs Calappa, they use asymmetric claws to manipulate right-handed snails: a robust left claw to peel the shells and the other smaller one to cut the flesh into pieces (Shoup 1968; Ng and Tan 1985; Smith and Palmer 1994; Seed and Hughes 1995). In another case, hermit crabs develop asymmetric bodies at least in part because of the right-handed shells into which they grow (Harvey 1998). A practical restriction of study in this field is that we cannot easily make antiasymmetric or symmetric animals on which to execute an ideal experiment. Therefore, it is usually difficult to obtain direct evidence indicating how any particular asymmetric structure has any ecological significance or how the morphological chirality is related to the interspecies interaction with other animals. However, some of the practical problems could be solved, although not completely, by comparing the performance of a pred-

New Open Aquarium System to Breed Larvae of Water Beetles (Coleoptera: Dytiscidae)

Abstract For conservation purposes and to supply rare insects for laboratory use, a system for artificial breeding is crucial. However, in the case of carnivorous freshwater insects such as diving beetles, constant conditions in aquariums are difficult to maintain due to their high rate of food consumption. Furthermore, surface rippling caused by the pumping system for water circulation hinders the respiration of small larvae. We developed a new open aquarium system without water circulation that was successfully applied to the rearing of larvae of diving beetles, Dytiscus sharpi (Wehncke) (Coleoptera: Dytiscidae). In comparison to conventional methods, a high proportion of larvae developed into adult insects. The size of reared adults was almost the same as those of field-collected adults. The new method could be applied to the conservation and breeding of other rare species, such as water beetles and water bugs.

Mating and Reproduction of Predaceous Diving Beetles, Dytiscus sharpi, Observed Under Artificial Breeding Conditions

Abstract Mating season and embryonic development of the predaceous diving beetles, Dytiscus sharpi, (Coleoptera; Dytiscidae) were observed under artificial breeding conditions. Female and male adult insects started mating from November to March and gave first instar larvae mainly in April. When the mating was artificially delayed until February, first instar larvae appeared from the end of March to the middle of May. I also investigated the effects of temperature on larval development. Apparent hatchability of eggs was not affected by high temperature, however, their normal development after hatching was significantly interfered. Most of the first instar larvae kept at 20–25°C from before hatching died within one day after hatching. By contrast, juveniles kept outdoors (7.0–20.9°C) could develop at least until second instar larvae. Temperature >23°C after hatching had no effects on larval development. From these observations, it was concluded that the reproduction strategy of Dytiscus sharpi, i.e. mating in late autumn and hatching in early spring would be the reasonable results of adaptation to the warm habitats where they are collected.

Dietary Program for Rearing the Larvae of a Diving Beetle, Dytiscus sharpi (Wehncke), in the Laboratory (Coleoptera: Dytiscidae)

Abstract For the conservation of the diving beetle Dytiscus sharpi (Wehncke) (Coleoptera: Dytiscidae), which is included on the Red Data List of Japan, it is critical to understand its ecological background. In the present study, we focused on its feeding behavior and nutritional needs under laboratory breeding conditions. First, we made a list of the possible candidates of prey in the same habitats where we caught D. sharpi. We found that the tadpoles of Rana ornativentris (Werner) were the major species present from March to April, when the beetle larvae appeared. Second, under our laboratory conditions, we investigated the size preference of beetle larvae preying on R. ornativentris tadpoles. We found a significant positive correlation between the developing stage of the larvae and the preferred prey size, i.e., the first and third instars preferred smaller and larger prey, respectively, but second instars did not show any size preference. The size of full–grown adult beetles was almost the same as that of wild insects found in the field, indicating that R. ornativentris tadpoles provide almost complete nutrition for larval growth. Finally, we investigated how the size and number of R. ornativentris tadpoles were correlated with the developing stage of beetle larvae. We suggest that it is crucial for Dytiscus larvae to have access to tadpoles of the proper size and amounts, depending on their growth stage.

Temperature-Dependent Regulation of Reproduction in the Diving Beetle Dytiscus sharpi (Coleoptera: Dytiscidae)

Abstract The effects of temperature on the mating behavior, gonad development, germ cell maturation, and egg spawning of the predaceous diving beetle Dytiscus sharpi (Coleoptera; Dytiscidae), were investigated. By field observations, we found that mating behavior started in October and occurred more frequently from November to December. Under our laboratory breeding conditions, we observed almost the same seasonal variation in mating behavior. We found that temperatures lower than 20°C were required to trigger mating behavior. We also found the same temperature threshold triggered gonadogenesis as well as spermatogenesis. Furthermore, for females, exposure to lower temperatures (<8°C) during the winter was required for egg maturation and spawning in spring; that is, there was a second threshold for successful female reproduction. We conclude that the termination of summer reproductive diapause of D. sharpi is regulated in a temperature-dependent manner, thus effecting the adaptation of D. sharpi to southern warm habitats.

Predaceous Diving Beetle, Dytiscus sharpi sharpi (Coleoptera: Dytiscidae) Larvae Avoid Cannibalism by Recognizing Prey

Larvae of diving beetles such as the various Dytiscus species (Coleoptera: Dytiscidae) are carnivorous and usually prey on other aquatic animals. Cannibalism among larvae of Dytiscus sharpi sharpi (Wehncke) was observed to begin when they were starved for more than two days under artificial breeding conditions. However, the 2-day starved larvae did not show cannibalism in the presence of intact, motionless, frozen tadpoles, or frozen shrimps. The beetle larvae attacked and captured intact tadpoles faster (15 sec) than other motionless and frozen tadpoles (120 sec), indicating that prey movement was an important factor in stimulating feeding behavior in larvae. Prey density does not have an effect on larval cannibalism. In cases in which preys are present at lower densities than that of larvae, a group of beetle larvae frequently fed on single prey. This feeding behavior, therefore, provides direct evidence of self-other recognition at the species level. Using two traps in one aquarium that allows the larvae to detect only prey smell, one containing tadpoles and another empty, the beetle larvae were attracted to the trap with tadpoles at high frequency, but not to the empty trap. In another experiment, the beetle larvae were not attracted to the trap containing a beetle larva. These results suggest that the larvae of D. sharpi sharpi are capable of recognizing prey scent, which enables the promotion of foraging behavior and the prevention of cannibalism.

The Inheritance of Intrasexual Dimorphism in Female Diving Beetles (Coleoptera: Dytiscidae)

Many species of Dytiscus diving beetles exhibit intrasexual dimorphism, e.g., the elytra is smooth in some females and grooved in others. However, the expression of the grooves and whether they are a product of heredity or the environment remain unknown. One Japanese species, Dytiscus sharpi sharpi Wehncke, 1875, also shows female dimorphism, with grooved and smooth morphs, while D. sharpi validus Régimbart, 1899, only has a single morph (the grooved type). A hybrid of the two species should therefore provide a means of sorting out how the grooves are inherited. We found two independent wetlands of D. sharpi sharpi in Chiba Prefecture, Japan. One was a place where a high proportion of grooved females lived, and the others had high proportions of smooth females. After five to eight generations of beetles from two populations with different proportions of grooved females were reared under aquarium conditions constituting a common garden design, i.e., water temperature, water depth, and presence of a plant for oviposition, the differences remained. We mated smooth virgin females of D. sharpi sharpi with males of D. sharpi validus to obtain hybrid offspring. The elytral traits of the hybrid females produced only grooved forms. These results suggested that the female dimorphism is determined by genetics, and that the grooved morph was dominant over the smooth one, independent of environmental factors. In addition, the hybrid insects did not differ from the two subspecies insects in larval survivorship, pupation success, or sex ratio. They also showed neither morphological abnormality nor reduced survival.

Choice of Prey Body Parts for Effective Feeding by Predaceous Diving Beetle Larvae, Dytiscus sharpi sharpi (Wehncke) (Coleoptera: Dytiscidae)

Diving beetle larvae use their mandibles in two ways: capturing prey and sucking their body fluid. Catching and consuming the prey’s most nutritious body part leads to the highest feeding efficiency. To test this, Dytiscus sharpi sharpi larvae were given tadpoles (Rana ornativentris) as food and their feeding behaviors were observed. Dytiscus larvae preferred to catch tadpoles by the abdomens rather than by other parts. Tadpoles soon became immobilized and in most cases the beetle larvae started eating abdomens first. Beetle larvae tried to change biting site to tadpole’s abdomen when the tadpole was initially caught by the head or tail. More food was absorbed from the abdomen than the head or tail suggesting that the feeding behavior of beetle larva is optimized to obtain nutrition efficiently from the tadpole abdomen.