About Atagema spongiosa (Kelaart, 1858)
Atagema spongiosa (Kelaart, 1858) has an oval, flattened body. Its dorsal surface is covered in prominent, rounded tubercles that gradually decrease in size toward the mantle margin. A distinct longitudinal ridge runs centrally along the body midline, spanning the area between the rhinophores and the gill. On either side of this ridge, there is a series of depressions that also get smaller as they approach the mantle edge. Outside of these depressions, the entire dorsal surface is densely covered in caryophyllidia, which are specialized microscopic sensory organs located on the dorsal surface of the mantle. The branchial sheath is split into three large, clearly defined lobes. The gill is made up of five tripinnate branchial leaves, which are held in a horizontal position in living individuals. The rhinophoral sheaths are elevated, and contain elongated rhinophores that have 24 separate lamellae. Juvenile specimens have noticeably less prominent dorsal tubercles than adult specimens. Adult Atagema spongiosa have an opaque grayish-brown body, while the depressions range from dark brown to black. Juveniles are translucent gray. Both the rhinophores and branchial leaves match the color of the rest of the dorsum. This species was first described from Sri Lanka, and has been reported as widespread in the tropical Indo-West Pacific Ocean and off the coast of Australia.
