Dwarf males of the seven-arm octopus, Haliphron atlanticus: morphology and adaptations

Abstract

The Argonautoidea is a monophyletic superfamily of pelagic incirrate octopods primarily distinguished by an unusual means of copulation where a sexually modified arm, or hectocotylus, is detached and transferred from dwarf males to the female. Males of the seven-arm octopus Haliphron atlanticus are scarcely observed and little is known about their reproductive strategy. A detailed description of H. atlanticus digestive system, hectocotylus morphology, histology and functioning was carried out with fresh and preserved material from two males captured in the Bay of Biscay (North Atlantic). It is the first time that a hydrostatic sac/swimbladder in the anterior region of the intestine is described in an argonautoid male. Investing in reproductive rather than somatic growth is evident in the dwarf males of H. atlanticus, which lack the Needham sac and the terminal organ. Functionally, these organs have been replaced by two external modifications at the distal end of the detachable hectocotylus: a spermatophore reservoir with a single long spermatophore and a muscular penis, both essential to ensure fertilization once the hectocotylus is detached from the dwarf males. Haliphron has been considered a monospecific genus, with a single species distributed around the world. However, mitochondrial genetic analysis (cytochrome c oxidase subunit I and 16S ribosomal RNA) supports the existence of at least two species of Haliphron, one in the northern Atlantic and another in the southern Atlantic/Pacific. Further genetic and morphological studies are needed to unravel the diversity of this oceanic octopod family.