These GEITP pages often discuss evolution, because the changing environment affects the genome of all living organisms; adverse stress (diet, climate, predators) results in fixation of mutations that ultimately result in “survival of the fittest” species. Bivalve molluscs are fascinating because they first arose in the early-Cambrian Period [>500 million years ago (MYA)] when bilaterians (animals that have two-sided symmetry) first appeared. Bivalves are well adapted to benthic life (living at, or near, bottom of a body of water) as sessile (immobile), semi-sessile, or free-living filter feeders and play critical roles in benthic ecology. Many bivalves are important fishery and aquaculture species providing significant economic benefits to humans (i.e. they are yummy to eat). Scallops have unique characteristics — making them good models to study development, adaptation to a changing environment, and early animal evolution.
Scallops have a large adductor muscle — undoubtedly as an adaptation to swimming by clapping valves to avoid predation (getting eaten) and to seeking favorable habitats. They are rare among lophotrochozoans (clade of protostome animals that includes segmented worms, molluscs, lophophorates, and several smaller phyla) in having numerous image-forming eyes along the edges of their mantles that perform vital functions in detecting predators and guiding swimming. Scallops can attach to objects (e.g. rocks) as juveniles by rapidly producing adhesive and strong silky fibers that can be either retained or lost in adults. As filter feeders that are able to feed on toxic dinoflagellates, scallops can accumulate and tolerate high levels of neurotoxins such as paralytic shellfish toxins (PSTs) — among the most potent natural toxins for humans.
The Zhikong scallop Chlamys farreri (also known as Chinese scallop) is a subtropical Western Pacific bivalve, naturally distributed along the coasts of Northern China, Korea, Japan, and Eastern Russia. It has an outstanding ability to accumulate PSTs (up to 40,241 μg saxitoxin eq. per 100 g, compared to the 80 μg STX eq. per 100 g safety level for human) and therefore is widely used for studying PST accumulation and detoxication. C. farreri is among the best genetically
characterized bivalve species with available linkage, physical and cytogenetic maps, fosmid and bacterial artificial chromosome (BAC) libraries, and a large number of expressed sequence tags (ESTs) — making it a good candidate for whole-genome sequencing (WGS).
Authors [see attached article] investigated the genome, various transcriptomes, and proteomes of this Chinese scallop. Its large striated muscle is energy-dynamic and not fully differentiated from smooth muscle; its eyes are supported by highly diverse, intronless opsin genes (expanded by retroposition for broadened spectral sensitivity). Rapid byssal (silky fibrous) secretion is enabled by a specialized foot and multiple proteins including expanded tyrosinases. Its hepatopancreas is used to accumulate neurotoxins and kidney to detoxify highly-toxic neurotoxins through expanded sulfotransferases (probably as deterrence against predation), while it achieves neurotoxin resistance through point mutations in sodium channel genes. These data show that expansion of gene groups, and mutations in genes within those groups, appear to have profound effects on the scallop’s phenotype and adaptation to its environment. 😊
Nat Commun Nov 2017; 8: 1721