Min-Li Tsai

Bursting firing of action potentials in central snail neurons elicited by d-amphetamine: Role of the intracellular calcium ions☆

Abstract The role of the intracellular calcium ions on the bursting firing of action potentials in central snail neurons elicited by d-amphetamine was studied in the identified RP4 neuron of the African snail, Achatina fulica Ferussac. Bursting activity of the potentials was elicited by d-amphetamine in central RP4 neuron of snail if amphetamine was applied either extracellularly or intracellularly. The bursting firing of action potentials elicited by amphetamine was decreased following intracellular injection with either EGTA or magnesium ions or extracellular application of lanthanum ion. Intracellular injection of calcium ions elicited bursting activity of potentials in the RP4 neuron, however, intracellular injection of calcium ions did not prolong the duration of the bursting firing of action potentials elicited by amphetamine. W7 (N-(6-aminohexyl)-5-chloronaphtha-lenesulfonamide hydrochloride), a calmodulin antagonist, did not decrease the bursting firing of action potentials elicited by amphetamine, although W7 did decrease the bursting firing of action potentials elicited by pentylenetetrazol. It is concluded that the bursting activity of potentials elicited by amphetamine is associated with intracellular calcium ions.

Why selection favors protandrous sex change for the parasitic isopod, Ichthyoxenus fushanensis (Isopoda: Cymothoidae)

A flesh burrowing parasitic isopod, Ichthyoxenus fushanensis, was found infecting the body cavity of a freshwater fish, Varicorhinus bacbatulus, in pairs. The marked sexual size dimorphism, with much larger females than males, and the presence of penes vestige on mature females suggest a protandrous sex change in I. fushanensis. Here we investigate the question of why selection favors protandrous sex change for I. fushanensis, by analyzing the interactions among clutch size, female size, male size, and their host size. The number of manca, the first free-living juvenile stage released, per brood was closely related to the size of the female. Excluding the effects of interaction among causal variables, the negative correlation of male size alone on clutch size suggests that a small male did not limit an individuals mating and fertilization success. When the effect of host size is removed statistically, there exists a significant negative relationship between the sizes of paired males and females. This indicates that the resources available from host fish are limited, and that competition exists between paired male and female resulting in a trade-off of body size. Due to the very low success rate of hunting for a host of mancas, a female with larger body size and higher fecundity has a fitness advantage. To augment the clutch size, a productive combination is a smaller male and a larger female in a host. The constraints of the limited resources and the trade-off between the sizes of paired male and female may favor I. fushanensis to adopt the reproductive strategy of protandrous sex change resulting in a larger female and hence more mancas. The pattern of the interactions among male, female, and the number of mancas, may be considered as a selective force for I. fushanensis protandrous sex change, where the available resources are constrained by the size of the host.

Desiccation resistance of two semiterrestrial isopods, Ligia exotica and Ligia taiwanensis (Crustacea) in Taiwan.

The ability to resist desiccation stress was examined in two semiterrestrial Ligia species, Ligia exotica Roux and L. taiwanensis Lee, in Taiwan, under a certain desiccation condition. L. exotica exhibited the longer survival time, lower weight-specific rates of water loss, and the slightly higher ability of tolerance to water loss, compared to L. taiwanensis. In each species, the animal size displays a positive correlation to the survival time and total ability to resist desiccation, yet this size effects on the weight-specific water loss rate is negative. Neither water content nor maximum tolerance to water loss shows the association with the animal size in both species. The path ways and magnitudes of the interactions between these traits of desiccation resistance are analyzed and diagrammed using a stepwise regression model. In this model, the body sizes of animal can explain the most part of the variations in the survival time. The body size has a direct effect and an indirect effect, through the effect on water loss rate, on the time that the experimental animals can survival under this desiccated condition. These results suggest that L. exotica attains larger size than does L. taiwanensis, a lower transpiration rate and, consequently, a greater ability in desiccation resistance. The performances of these interactions in the desiccated resistance are more advantageous for L. exotica to migrate and colonize in variable land habitats within a certain limit, and as a result that L. exotica shows a broader distribution pattern than did L. taiwanensis in Taiwan.

CANNIBALISM WITHIN MATING PAIRS OF THE PARASITIC ISOPOD, ICHTHYOXENUS FUSHANENSIS

Abstract Two types of sexual cannibalism, differing in the sex of the victim, were found among heterosexual pairs of the parasitic isopod Ichthyoxenus fushanensis Tsai and Dai, 1999, residing in the body cavity of the freshwater fish Varicorhinus bacbatulus (Pellegrin, 1908). In one type, categorized as sexual cannibalism, the male was consumed by the female before or after mating. In the other, reversed type, the female was eaten by her mate during or after breeding. Both types of cannibalism occurred during the breeding season from April to November, with female-on-male cannibalism occurring earlier on, and the reversed type later. Both types of cannibalism occurred in pairs inhabiting smaller hosts (<10 cm in body length), which suggests that resource limitation is an important factor triggering the cannibalism. Cannibalism may lead to rapid growth or sex change of the cannibal. The availability of free-living mancas during the breeding season enables a mating pair to be re-formed after cannibalism has occurred. Both types of cannibalism occurred in mating pairs with a low size ratio (below 1.6, female to male), and both resulted in a greater discrepancy between female and male sizes in the re-formed pairs. This increase in size ratio between paired individuals may ultimately lead to an increase in clutch size. Because an individual of I. fushanensis undergoes protandrous sex change, the cannibalistic behavior could not have evolved in response to selection on either the male or female sexuality. Rather, both types of cannibalism may be regarded as the result of competition between paired individuals, which appears to be a by-product in the evolution of a reproductive strategy rather than a consequence of sexual selection.

Responses of Two Semiterrestrial Isopods, Ligia exotica and Ligia taiwanensis (Crustacea) to Osmotic Stress

When immersed in fresh water, Ligia taiwanensis is a poorer osmoregulator than Ligia exotica ,a s judged by a lower LD50 at 96 hr and by the osmolalities of haemolymph. Animals appear to osmoregulate more efficiently in air. On immersion, both species displayed hyper- and hypo-osmoregulatory ability. Both species subjected less osmotic selection pressure during their inland colonization. The results suggest that a route of terrestrial colonization not involving transitional freshwater stresses had been taken by L. exotica and L. taiwa- nensis. comp biochem physiol 118A;1:141-146, 1997.

Is Large Body Size always Advantageous for Terrestrial Adaptation

Sesarmops intermedium de Haan (1835) is a semi-terrestrial crab inhabiting the areas around a forest creek at Heng-chun Peninsula in southern Taiwan. Small and large individuals excavate tunnels near the waterline, while those individuals with medium body size are more distant from the water. This unusual distribution pattern might reflect the habitat preference related to physiological requirement of crabs with different body sizes. In natural conditions, the animals keep their haemolymph osmolality at a relatively low level near those of fully-rehydrated individuals. In dehydrated conditions, the haemolymph osmolality of the animal was much higher and was negatively related to the body size. Both the rates of water loss and uptake decrease with increasing body size. While, both the potential time exposed in air and the rehydration time increase with increasing body size. The low water loss rate of large individuals may prolong the potential time exposed in air, but this advantage may be offset by their low efficiency in water uptake simultaneously. Small crabs, with a high water uptake rate and a small amount of water requirement, may have a shorter time needed for rehydration. However, those small crabs, smaller than about 2 g in living weight, due to their extremely fast loss of body water, have to access water more frequently and thus have a lower potential for terrestrial activity also. For medium sized crabs, the advantages of high water uptake rate, less water requirement and better ability to utilize interstitial water may allow them to emancipate from the water body. Due to the constraints of performance in water balance, the optimum size for extending terrestrial activity of S. intermedium is below 14 g in weight (=28.5 mm in carapace width) at which the crabs may have a higher potential for terrestrial activity. In the terrestrial adaptation regarding the water balance of S. intermedium, how to increase water uptake efficiency and the capability to use various interstitial water on land seem to be more important than how to reduce the water loss rate. Large body size and hence the low water loss rate may not be always advantageous for the terrestrial adaptation of S. intermedium and possibly other crustaceans.