I heard that the "earthworm" actually "rules" the coast? In fact, its true appearance is...

This creature that looks a bit like an earthworm is actually a sandworm. Sandworms have a strong ability to adapt to the environment. During the adaptation process, their head, trunk and papillae will cause corresponding morphological changes, and they have evolved a variety of environmental adaptation traits including segmentation, regeneration, reproduction mode and lifestyle.

Symbiotic relationship

The ring-lipped nerei is a temperate cold-water species distributed on both the east and west coasts of the North Pacific. The external appearance of the ring-lipped nerei varies in different states (as shown in the picture). It inhabits soft mud and the bottom of the Western Sands. It is often found in the shells of large hermit crabs and forms a symbiotic relationship with them. The ring-lipped nerei and hermit crabs coexist in order to obtain food resources and effective protection. The body of the ring-lipped nerei is soft and vulnerable to attack, while the shell can provide it with protection. The host hermit crab also provides the ring-lipped nerei with opportunities to move and feed during its activities and predation on the seabed.

According to the experiment, the behavior of the annular linnet worms drilling into the snail shells mainly occurs when they feel the physical vibrations generated by the activities of the giant hermit crabs. When the annular linnet worms are in the empty snail shells, the probability of actively drilling out to find the giant hermit crabs is low, while when they are in the bottom, the probability of drilling into the shells of giant hermit crabs is high, and they actively search for the giant hermit crabs that are active around them. The giant hermit crabs have no selectivity for the annular linnet worms. When choosing a snail shell, the giant hermit crabs only consider the snail shell factor, and do not show obvious reactions to whether the annular linnet worms are inside the shell.

Dorsal whiskers specialized into gills

The feather-barbed gills of the Nereidia, which are uniquely distributed in the Indo-Pacific region, have their dorsal whiskers specialized into gills, forming a unique morphological trait that is adaptable to the environment compared to other Nereididae species. These special body structures may have evolved to adapt to the living environment.

Have reproductive behaviors such as group floating and nuptial dance

The dicynodontia, also known as sea centipede, sea leech, sea worm, sandworm, etc., is an important economic marine invertebrate with high nutritional value and is known as the "cordyceps sinensis" of the sea [3]. In addition, its unique amino acid composition has a strong attractant effect on fish, shrimp, and crabs. It is usually used as a parent breeding for marine fish, shrimp, and crabs and as bait for fishing, so it is also called "universal bait". With the rapid development of marine aquaculture, the dicynodontia has become one of the most exported species of nereids in my country.

They have special reproductive behaviors such as group floating and nuptial dance. During the breeding season, on clear nights with a full moon, male and female individuals will crawl out of their caves and float to the surface of the sea. Male individuals will surround one or more female individuals to release sperm and eggs to complete mating. In addition, the moon phase is also an important factor in the nuptial dance of lugworms. Some lugworms prefer to complete the nuptial dance under a full moon (photophilic reproduction), while others prefer to do it under a new moon (photophobic reproduction).

Green sandworms are smarter and use their own renewable advantages to reproduce, thus greatly reducing the damage they may suffer during the group floating process. For example, when the green sandworms in Samoa, Pacific Ocean, are floating in groups, female individuals do not go there in person, but cut off the rear part of their sexually mature bodies and let this part float to the surface for mating.

References

[1] Zhang Linlin. Research progress on adaptive evolution and diversity of marine animals[J]. Oceans and Lakes, 2021. 03

[2] Yu Haicheng. Study on the symbiotic behavior of the ring-lipped sandworm and giant hermit crab[J]. 2020.

[3] Chen Shenxue. Research progress and prospects of Bi-toothed Perinegaster[J]. Journal of Aquaculture, 2019.02.016

[4] XU Xing-hong. Reproductive biology of Echinops monocylindrica[J]. Journal of Fisheries of China, 2020, 44(8): 1275−1285