Brooding season, sex ratio, and brood pouch development in the seaweed pipefish, Syngnathus schlegeli, in Otsuchi Bay, Japan

Males of the seaweed pipefish, Syngnathus schlegeli, take care of their egg s in the brood pouch. These pipefish were periodically collected from the s hallow seagrass beds in Otsuchi Bay on the Pacific coast of northern Honshu , Japan, from spring to autumn to investigate the basic reproductive ecolog y. Appearance of the pipefish in the coastal seagrass beds coincided with t he initiation of reproduction. The reproductive season was from May to at l east October, with its peak in July. A rearing experiment revealed that the brooding period of the male had a negative correlation with water temperat ure, and it was estimated to last about 1 month in the bay. Almost all male s were brooding during the peak of the reproductive season. Although. the b rood pouch of most males was either full or devoid of eggs, 6.2% of the mal es had a partially filled (20%-90%) brood pouch, and multiple clutches were identified in the brood pouch of some males, indicating that the mating sy stem of the pipefish is polygamous, perhaps polygynous. Sex ratio fluctuate d 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 2 15 mm standard length (SL). The smallest brooding male was 134 mm SL. Mean SL of brooding males was significantly larger than that of nonbrooding matu re males. The number of males with an immature brood pouch was greater at t he beginning than later in the reproductive season. The results seem to col lectively indicate that the occurrence of a larger proportion of immature m ales at the onset of the reproductive season may be ascribed to both new re cruitment and larger body size at maturation, resulting from the males trad ing the reproductive effort to somatic growth, perhaps to increase future r eproductive success.