Do male rotifer copulatory organs have systematic value? An integrative analysis of Asplanchnopus aff. multiceps

Female monogonont rotifers are among the smallest animals, with many species being less than 100 mm in length. Male rotifers are enigmatic; relatively few species have been identified; they are infrequently seen; and they are usually smaller than their female counterparts, often lacking a complete gut. Consequently, knowledge of their morphology is scarce, particularly with respect to the fine details of their copulatory apparatus. In this study, we apply confocal laser scanning microscopy and transmission electron microscopy to study the copulatory (penis) organ of Asplanchnopus aff. multiceps, for which no data is currently available. Our studies reveal the copulatory organ to be a complex structure with a circumferential ciliated band, a microvillar tip with an actin rich terminal web, and an intricate musculature that functions in protraction and retraction. Two prostate glands and two accessory glands supply the male apparatus. The overall organization of the organ fits into a “type” that characterizes the family Asplanchnidae, with the main exception being the possession of a microvillar tip. The complexity of the organ, which functions purely through attachment to the female and not insertion, is perplexing considering its size (~ 10 mm) and the apparent simplicity of its function. We hypothesize that other male organs are probably equally complex, but their size has limited our knowledge of their anatomy, which might otherwise provide valuable details for both taxonomic studies and a better understanding about the evolution of reproductive structures and behaviors throughout the phylum.