A new piece in the puzzle for the riverine slugs of the Acochlidiidae (Panpulmonata: Acochlidia)

The slugs of the family Acochlidiidae are an outlier among otherwise marine, meiofaunal and therefore minute members of the Acochlidia: by comparison, they are giant (in centimeter-range), with adults living exclusively in coastal rivers and creeks of tropical islands in the Indo-West Pacific. Historically, the family is the longest known among the order, but only recently research has shown that Acochlidiidae are a derived clade that evolved an amphidromic lifestyle with a specialized larval stage and that adults predate on the calcareous egg-capsules of freshwater snails of the Neritidae.

Currently there are two known morphologically distinct taxa among acochlidiids: one lineage with the slender, uniformly reddish colored Strubellia (2 described species) which resembles the Pseudunelidae (the marine sister group of the Acochlidiidae), and a second lineage with seemingly more derived, flattened and pigmented, brown or greenish Acochlidium (3 sp.) and Palliohedyle (2 sp.).

A 2010 survey of freshwater gastropods on the Moluccan island of Amboina, Indonesia, discovered two specimens of a hitherto unknown acochlidiid that shows intermediate characters. Externally, it resembles a blue-green Strubellia, and 3-dimensional reconstruction based on semithin histological sections shows that digestive and excretory systems largely resemble that in Strubellia. Scanning electron microscopy and histology, however, shows that the species has evolved the complicated trap-like copulatory organ which resembles that of Acochlidium. A molecular phylogeny of acochlidiids using multilocus markers confirms that the species is indeed sister to a clade including Acochlidium and Palliohedyle. This suggests that a complex copulatory organ evolved earlier than the Acochlidium-like morphotype.