When it comes to shellfish, everyone feels familiar. Scallops, conchs, snails and even nautiluses are all very common shellfish representatives. Slightly more specialized readers also know that octopuses, squid, sea hares, slugs, and shellless animals also belong to shellfish, which evolved from their shelled ancestors. However, shellfish are actually much more than just these species mentioned above. Some small taxa, such as horned shells, stone turtles, plateless species, etc., presumably most readers do not know much.
That's right, these are shellfish
Upper row from left to right: cephalopods (squid), bivalves (clams), gastropods (snails), diggers (horned shells), lower rows from left to right: plateless (Sulenegastres), plateless (Caudafoveata), multi-plate (stone turtle), veneers (new disc shells) (Image source: Wikipedia, researchers at the Institute of Oceanography, Chinese Academy of Sciences)
Compared with other animals, the biggest feature of shellfish is that their body structure changes greatly. However, biologists have long known that shellfish follow a basically consistent pattern of body structure (otherwise they would not be classified as a single discipline). The difference between living shellfish is so great, mainly because different shellfish organs are like wild horses that have lost their reins, riding on the road of evolution, producing countless specialized forms. In the case of shells, for example, the shells of gastropods are spiral-shaped (there are also cap-shaped or even nearly tubular), bivalves are two shells (with various shapes), and polyplates have eight shells, not to mention octopus, sea hares and other species without shells. Why are shellfish specialized in so many ways? Whether there is a common mechanism driving this evolutionary process, or whether different shellfish have adopted different evolutionary strategies, is a very important question.
The anatomy of different shellfish can correspond to each other (image source http://www.huodongxing.com/event/2348278267200?layout=EN)
The latest research results of Liu Baozhong's research team at the Institute of Oceanology of the Chinese Academy of Sciences provide an important reference for answering this question. Their research suggests that the Hox gene is likely to be an important driver of this diversity of body patterns. This work is currently published in the academic journal PNAS.
For biology students, the Hox gene is familiar. They are a set of genes that are expressed sequentially from front to back along the body, regulating the development of organs in different parts. Researchers can mutate Hox to make the legs appear on the head of the fruit fly, or to grow an extra pair of wings in the position of the original long balance bar.
Hox gene. Left: Hox gene expression patterns along the body front and back; right: Hox gene mutant fruit flies (Image source: Wellik, 2007; Lewis, 1978; http://www.basfeijen.nl)
However, unlike animals such as fruit flies, the researchers found that shellfish's Hox gene does not follow the classic rules of expression along the body in anterior and posterior order. Their expression is difficult to find an obvious pattern. It has been suggested that the Hox gene of shellfish has already had new functions in different species. There is some truth to this statement, but there are also problems when you think about it. The Hox gene is, after all, a basic tool for animal development, and even though shellfish have evolved a very diverse body structure, their early development is actually very similar (most shellfish have a larval stage called a carrier larval). It's hard to imagine that the basic genes like Hox, which regulate animal development, have evolved completely different functions in different shellfish—without affecting their early developmental processes.
Several representative shellfish are the larvae of the steinle, from left to right: bivalve, gastropod, digotes, and polyplates. (Image source: Wanninger, 2001; Silberfeld, 2006; Researcher, Institute of Oceanography, Chinese Academy of Sciences)
By comparing a large amount of data, researchers at the Institute of Oceanography of the Chinese Academy of Sciences found that the expression of Hox genes in different shellfish seems to be very different, but in fact it implies a deep common law. Regardless of the period, the expression of the Hox gene can always be divided into two parts, associated with the shell on the back and the foot on the abdomen. Dorsal expression is associated with the unique shell pattern of each taxon, such as circular shells with circular shells, repeated stripe expressions for multiple shells, and abdominal expressions with conservative collinear patterns at specific stages of development, and then beginning to become different, and finally concentrated in the foot.
Kasabei and stone turtle Hox expression patterns
Based on this conclusion, it can be found that the expression of the Hox gene is actually associated with the two most prominent organs of shellfish: shells and feet. What is the significance of this correlation? When researchers look at different shellfish with this question in mind, it becomes clear that the most obvious difference between shellfish in different taxa is the shell and foot.
The most obvious difference between shellfish is the shell and foot (from left to right: gastropods, bivalves, diggers, cephalopods, multiplycae)
Based on this, a story that has been playing out for nearly 600 million years is presented in the minds of researchers: because of some unknown mechanism, the Hox expression on the back and abdomen of the ancestors of mollusks became separated. Hox's powerful ability to regulate development gave the back and abdominal organs (mainly shells and feet) the potential to evolve various forms. Over the course of a long evolution, morphologies that can adapt to specific environments are chosen. Some of the shell examples given above are actually the same for the feet, such as the feet of gastropods, monoplatelets and multiplates are generally suction cup-shaped, the feet of the bivalve order generally evolve into axes suitable for digging sediment (bivalve are also called axe feet), the feet of the digpods are cylindrical, and the feet of cephalopods are the most special, evolving into multiple wrists (eight octopus, ten squid, hundreds of nautilus). Shells and feet of different specialized types are combined with each other to form an extremely diverse shellfish body structure.
The hypothesis of the evolutionary mechanism of mollusk body diversity