Genome of the enigmatic watering-pot shell reveals adaptations of a living suction anchor

Abstract

The watering pot shells have been rightly called "the weirdest bivalves" for their highly modified body plan and fused tubular shell that resemble the spout of a watering can. This adventitious tube has defied rational explanation as an adaptive pathway and yet arose twice convergently. We present the first genome for the clade Anomalodesmata: an annotated chromosome-scale genome of Verpa penis (Linnaeus, 1758). The first watering pot shell ever described ranges across southeast Asia but is nonetheless extremely rare and has never previously been sequenced. The assembly length of 507 Mb, with a contig N50 of 5.33 Mb, has 96.5% of sequences anchored onto 19 pseudochromosomes. Contrary to expectations from such a highly modified body plan, there is no evidence of chromosome reduction compared to the ancestral condition of heterodont bivalves (1N=19). A new hypothesis based on analysis of the living animals and literature explains the adaptive significance of this body form: it is structurally optimised for vertical stability in soft sediments, with parallels to engineering principles of a suction anchor. Our study offers new insights to a long-standing mystery in molluscan body forms, and provides genomic resources that are relevant to understanding molluscan evolution, biomineralisation, and biomimetic design.