Relationship Between Male Size and Newborn Size in the Seaweed Pipefish, Syngnathus Schlegeli

Abstract

Relationships between male size, number, and weight of ‘newborn’ were determined in the seaweed pipefish, Syngnathus schlegeli, with paternal care of embryos in the brood pouch. While the number of newborn increased linearly with the increase in male size, the pouch volume increased exponentially with the male size. This resulted in embryo density in the pouch negatively correlating with the male size. Newborn dry weight was negatively correlated with the embryo density in the pouch. Thus, larger males were considered to brood embryos at lower densities and give ‘birth’ to heavier newborn. Neither paternal size nor embryo density seemed to have an effect upon developmental stage and total length of the newborn. Substantial weight loss of embryos during incubation indicated that paternal nutritional contribution to embryos is insignificant; therefore, the tendency of larger males to produce heavier newborn may not relate to allocation of paternal nourishment. Because egg weight increased with the female size, larger males are considered to receive the eggs of larger females and give birth to heavier newborn that developed from heavier eggs. Lower embryo density in larger males is attributable to the larger size of the heavier eggs. Assuming the pouch to be a cylindrical tube and the egg to be a sphere, geometrical calculations revealed that the relationship between the volumetric fraction of the pouch used by eggs (i.e. functional volume) and the male size was negatively quadric and had a peak. Increase in egg diameter increased the male size at which the functional volume of the pouch is maximized. Thus it appears that the size relationship of mates in pipefish is related to effective use of the pouch space.