ハゼ科の2近縁種 (アゴハゼとドロメ) の潮溜りにおける共存関係

抄録

Summary The sea gobiid fishes Chasmichthys dolichognathus (Hilgendorf) and C. gulosus (Guichenot), both common tide-pool species ranging from Japan to the southern Korea, were studied on the morphological and ecological specificities including spawning, growth, age, food preference, population density, rate of co-existence, “stay-in” habit, etc. Special attention was called to the problems of interspecies competition in the tide-pool. The studies were conducted selecting about 130 tide-pools located on the tidal zone at Kominato and Banda, Chiba Prefecture in 1966 and 1967, and nearly 2, 000 specimens were collected.
These two species of gobies, which have often been treated as a same species due to their closeness in external characters and habitat, were well dis., tinguished by the pectoral ray counts, number of vertebrae and gill-rakers (Table 2), and the color pattern both in the young (Fig. 1) and adult. The age of maturation (full one year), spawning season (January to May with peak in February and March), spawning habit, and eggs were not well specified by the two species, but the egg-mass of C. dolichognathus was found consisting of 300 to 600 ova against 800 to 1, 400 of C. gulosus. The age of these gobies, tested by size frequency (Fig. 2), but not by ill-defined circuli on the scales, was believed to be usually one year in C. dolichognathus, which grows to 75 mm in body length, but, with some reserves, two or more years in C. gulosus, of which the maximum size of 130 mm was noted. The floatup young of the two species were observed in the same waters of lower littoral and sub-littoral zones where the micro-habitat preference was believed working. The young of the two species enter into tide-pool and find their normal habitat there, but food preference developed at this stage. The food items taken by the adult of C. dolichognathus were found nearly identical with those of the young of C. gulosus, but such apparent food competition was thought compensated by the different age (or size) composition in the population of the two species. A significant difference of the two species was found in their food preference when they attained adult size, namely, C. dolichognathus heavily depends on amphipod crustaceans, whereas C. gulosus shows wide range of food items including macruran, anomuran and isopod crustaceans, sea-weed, etc. (Fig. 3). This fact, among others, was believed to be the most significant ecological factor to permit the co-existing of the two species in the same habitat.
The ecological specificity demonstrated by the two species was also found, but in lesser extent, in their preference of different tide-pools as the usual habitat. It was observed (Table 3) that the two species usually share the same pool water, but C. dolichognathus outnumbers the other species in the community, regardless of the types of pools which were categorized into four entries (Table 1). The significant fact was that C. dolichognathus preferred the tide-pools of types I and II (90 %) to those of the types III and IV (10%), whereas, C. gulosus inhabited in those of the types I and 11 (60%) and III and IV (40%) (Table 4). The test of“stay-in the pool” habit by marking and recapture of the fishes (Table 5) revealed that only 20% of C. dolichognathus remained in the same pools after the two months, and 60% of C. gulosus after the same period. It is, however, admitted that for both species the “stay-in” habit varied by the types of tide-pools, namely the higher the level of pools located the higher their extent to remain in the same pool. In conclusion, the two species of gobies, which developed in the limited surface of tidal zone, are able to co-exist successfully on rocky shore through ecological isolation particularly ascribed to food and habitat preference (Figs. 4 and 5).