Development and sexual dimorphism of branchiae in Streblospio benedicti

The Spionidae is one of the largest and most studied annelid families, but to date, the development and differentiation of post-metamorphic anatomy have not been documented. This study used scanning electron microscopy to examine the development of the branchiae, presumed respiratory organs, in Streblospio benedicti. Branchiae in this species are prominent, paired head structures and first appear around the time of metamorphosis, but do not complete their development until the worm reaches the older juvenile or adult stages. We observed that as the branchiae grew, their overall morphology changed through four different shapes: small bud, tubular, tapered, and, finally, bilimbate. In addition, the abfrontal and frontal surfaces each possessed a unique set of cilia patterns, which we named, and these arose in a particular sequence between the 8- and 35-chaetiger stages. This detailed examination of every stage of branchial development led us to discover that branchia in Streblospio benedicti was a sexually dimorphic organ. Streblospio benedicti is one of approximately eight Spionidae in which there is any type of structural sexual dimorphism, and it is the only species in which sexually dimorphic branchiae are found. The male's frontal surface had four unique cilia patterns, and we hypothesize that those located around the medial protrusion capture and control the release of the spermatophores. This first documentation of a spionid's branchial developmental sequence revealed that not only is this respiratory organ involved in reproduction, but it significantly differentiates after metamorphosis through adulthood.