Called "fish" but not "fish": the humble lancelet is actually a "living fossil" that has lived for 500 million years

1. Overview

Although the word "fish" is in the name of lancelets, it is not a true fish. It belongs to the phylum Chordata and subphylum Cephalochordata. This subphylum has only one class and one family, namely the class Amphioxus and the family Branchiostoma, which have two genera, Amphioxus and Partial Amphioxus, with a total of 25 species. Amphioxus is between invertebrates and vertebrates, but is more like a vertebrate. It is a species that plays an important role in the study of evolutionary history.

They are widely distributed in tropical and temperate waters, especially in the area between 40 degrees north latitude and 40 degrees south latitude. They like bottom composed of smooth sand with good permeability and stable surface, and shallow seas with stable salinity and warmth. They often half bury their bodies in the sand with the front end exposed outside the sand, or lie on their left side on the sand. They can swim briefly by staggered contraction of the muscle segments on the sides of their body. They are filter feeders, and their main food includes Thalassiosira, Sievella, and Nitzschia.

Amphioxus

There are many legends about the origin of the name of lancelet. One is that during the Zhenyuan period, crocodiles in Chaozhou caused a disaster. Han Yu was ordered to eliminate crocodiles. One of them escaped to Xiamen. After the maggots on its body died, they turned into lancelets. Another is that during the Song Dynasty, Zhu Xi went to Xiamen and killed crocodiles with a "red pen". Many lancelets appeared in the sand, so lancelets also have the alias of "crocodile worm". Another is that during the Ming Dynasty, Zheng Chenggong led the navy to Xiamen, and the soldiers poured the leftover rice into the sea, forming lancelets. Another is that it was a gift from Emperor Wenchang to believers. Due to the appearance of lancelets with pointed ends and wide middle, some areas in Xiamen call them "pole fish" or "silver tuna". The word "Wenchang" may come from the pronunciation change of "silver tuna", which has a mythical color.

The body shape of lancelets is slightly similar to that of small fish, with a body length of 30-50 mm. It has no obvious head, is flattened on both sides, and is translucent. The myometrium under the skin and the lumpy reproductive glands on the ventral side can be vaguely seen. There is a funnel-shaped mouth cap on the ventral side of the front end of the body, with tentacles around the edge, which have sensory cells on them and play a role in screening food.

The translucent body of lancelet

Amphioxus mouth cap

The body of the amphioxus is supported by a notochord, an elastic rod-like structure that runs through the body. The back muscles are thick and the abdomen is thin. The muscle segments on the sides of the body are arranged in a "<" shape, and the body has obvious asymmetry. It does not have a brain in the strict sense, but there is a slight swelling at the front end of the dorsal neural tube; it has no heart, only a pulsating abdominal aorta; it has no paired fins, only dorsal fins and pelvic fins. The former refers to a fin formed by the epidermis protruding from the snout along the midline of the back of the amphioxus; the latter refers to the bulge formed from the back of the peribranchial cavity opening along the midline of the abdomen to the rear end.

Left side view of lancelet

(Photo from Zhang Shicui, "Evolutionary Biology of Amphioxus: Tracing the Origin of Vertebrates", slightly modified)

2. Reproductive Development

Amphioxus is dioecious, with 26 pairs of gonads arranged in segments in the body cavity on both sides of the peribranchial cavity wall. When sexually mature, the testes are white and the ovaries are light yellow. They lay eggs in June and July, and both spawning and fertilization take place in the evening. Mature sperm and eggs are released into the peribranchial cavity through the rupture of the gonad wall, and then discharged into the seawater, where the fertilization process is completed.

The development process of amphioxus goes through various stages such as fertilized eggs, morula, blastocyst, gastrula, and neurula. After about 20 hours, the embryonic development of amphioxus is basically completed. The larvae covered with cilia break through the egg membrane and move in the seawater. At this time, they have the habit of vertical migration, swimming to the seabed during the day and swimming to the surface at night. After about 3 months, the larvae sink to the seabed for metamorphosis, and the vestibule appears. The number of gill slits increases due to the occurrence of secondary gill bars, and the original opening directly outside the body is changed to pass into the peribranchial cavity. The body length of one-year-old amphioxus is about 40 mm, the gonads are mature, and it can reproduce in the same year. The life span of amphioxus is nearly 3 years, and it can reproduce 3 times in its lifetime.

3. Bridge

1. Origin of the brain

Darwin mentioned in "The Origin of Species" that the formation of each species must go through countless intermediate transition stages, which is very obvious in lancelets.

(II) Photosensitive structure

The photoreceptors on the body of lancelet include frontal eyes, lamellar bodies, Joseph cells and dorsal ocelli. Therefore, lancelet can not only respond quickly to light stimulation, but also have various light-dependent behaviors. For example, lancelet larvae tend to move on the seabed during the day, and as the light weakens after sunset, the larvae tend to move on the water surface.

The frontal eye is an important photosensitive structure in amphioxus. In molecular biology research, scientists have found that amphioxus not only has important transcription factors related to the development of vertebrate retinal neurons and retinal pigment epithelium, but they are also expressed in the frontal eye. Based on the gene expression pattern, the frontal eye of amphioxus has homology with the rods and cones of vertebrates, and it is likely to represent the photoreceptor cells of the common ancestor of vertebrates.

The lamellar bodies are homologous to the pineal organ of vertebrates. The other two photoreceptors, dorsal ocelli and Joseph cells, are composed of rod-shaped photoreceptor cells, which are typical visual photoreceptor cells of invertebrates.

(III) Myometrium

The myomeres of amphioxus are arranged neatly in a "<" shape from front to back, separated by myometrium formed by connective tissue, without any changes. The muscles of jawless vertebrates (such as lampreys) are still composed of myomeres with similar structures. Therefore, the myomerome characteristics of amphioxus are considered to be the original myomerome state of the ancestors of vertebrates.

Comparison of the myometrial structure between amphioxus and lamprey

(Photo from Zhang Shicui's "Evolutionary Biology of Amphioxus: Tracing the Origin of Vertebrates", slightly modified)

(IV) Appendages

Although amphioxus does not have paired fins or limbs, the prototypes of many factors and signal pathways that regulate limb formation can be found in amphioxus, and the locations where genes that guide forelimb formation are expressed are confined to positions similar to those of other vertebrates, indicating that the formation of limbs has begun at least in the amphioxus stage.

The origin and evolution of the pre-formation zone of the forelimbs in lancelets, lampreys and gnathostomes

(Photo from Zhang Shicui, "Evolutionary Biology of Amphioxus: Tracing the Origin of Vertebrates", slightly modified)

5. Bone structure

The body of amphioxus contains collagen similar to that of vertebrates. The tissue structure containing collagen, its gene expression pattern, and the gene regulation process of bone formation in amphioxus are very similar to those of vertebrate cartilage. Therefore, this connective tissue containing collagen is likely to represent the bones of primitive vertebrates.

4. Humans and Amphioxus

Amphioxus is very suitable as an experimental animal due to its small size, large egg-laying capacity, in vitro fertilization, and transparent embryos. In addition, amphioxus plays an important role in the study of the origin and evolution of vertebrates. Amphioxus has made outstanding contributions to the development of vertebrates and their tissue and organ systems, as well as to the study of evolutionary developmental biology, comparative functional genomics, and comparative immunology.

On the other hand, the content of unsaturated fatty acids and essential amino acids in amphioxus is high, it is nutritious, delicious, and has certain economic value. Xiamen used to be a very important amphioxus fishing ground, but with human activities and environmental pollution, the number of amphioxus has dropped sharply, and it is now a national second-class protected animal. Fortunately, artificial breeding of amphioxus has achieved great results, and in the future, amphioxus is likely to return to the table.

References:

Zhang Shicui, "Evolutionary Biology of Amphioxus: Tracing the Origin of Vertebrates"

David Attenborough: The Evolution of Life

Liu Lingyun's "General Zoology" 4th edition

If no source is indicated, the pictures are from the Internet

Copywriter/Editor: Liang Liuhong

Instructor: Chu Qingzhu

Producers: Aquatic Biology Museum of Guangdong Ocean University, Chinese Society of Oceanography