Brooding season, sex ratio, and brood pouch development in the seaweed pipefish, Syngnathus schlegeli, in Otsuchi Bay, Japan |
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Authors: | Satoshi Watanabe Yoshiro Watanabe |
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Institution: | (1) Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan (e-mail: swatanab@ori.u-tokyo.ac.jp), JP |
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Abstract: | Males of the seaweed pipefish, Syngnathus schlegeli, take care of their eggs in the brood pouch. These pipefish were periodically collected from the shallow seagrass beds in
Otsuchi Bay on the Pacific coast of northern Honshu, Japan, from spring to autumn to investigate the basic reproductive ecology.
Appearance of the pipefish in the coastal seagrass beds coincided with the initiation of reproduction. The reproductive season
was from May to at least October, with its peak in July. A rearing experiment revealed that the brooding period of the male
had a negative correlation with water temperature, and it was estimated to last about 1 month in the bay. Almost all males
were brooding during the peak of the reproductive season. Although, the brood pouch of most males was either full or devoid
of eggs, 6.2% of the males had a partially filled (20%–90%) brood pouch, and multiple clutches were identified in the brood
pouch of some males, indicating that the mating system of the pipefish is polygamous, perhaps polygynous. Sex ratio fluctuated
among months, and the overall sex ratio tended to be biased to male. Body size of males with an immature brood pouch had a
wide range, from 133 to 215 mm standard length (SL). The smallest brooding male was 134 mm SL. Mean SL of brooding males was
significantly larger than that of nonbrooding mature males. The number of males with an immature brood pouch was greater at
the beginning than later in the reproductive season. The results seem to collectively indicate that the occurrence of a larger
proportion of immature males at the onset of the reproductive season may be ascribed to both new recruitment and larger body
size at maturation, resulting from the males trading the reproductive effort to somatic growth, perhaps to increase future
reproductive success.
Received: April 4, 2000 / Revised: September 21, 2000 / Accepted: January 16, 2001 |
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Keywords: | Reproductive season Sex ratio Seagrass bed Brood pouch Pipefish |
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