Cephalochordate
Cephalochordates, also known as lancelets or amphioxi, are small, segmented marine animals belonging to the subphylum Cephalochordata within the phylum Chordata. These eel-like creatures are typically found buried in sandy substrates of tropical and temperate ocean waters and are notable for their distinctive anatomical features, including a notochord, a hollow dorsal nerve cord, and numerous pharyngeal slits used for filter feeding. Although they can grow to only 1 to 3 inches in size, cephalochordates play a significant role in understanding the evolutionary lineage of vertebrates, as they share many fundamental characteristics with these more complex animals.
Fossil records, notably from sites like the Burgess Shale in Canada, provide insight into the ancient history of cephalochordates, which are often referred to as living fossils due to their unchanged form over millions of years. Their lifecycle includes external fertilization and the development of free-swimming larvae after spawning, which typically occurs in the warmer months. Despite lacking certain advanced features found in true vertebrates, such as a major heart and kidneys, cephalochordates possess a well-developed circulatory system. They also contribute to the marine food web, serving as a food source for bottom-feeding fish and humans, particularly in some regions of Asia where they are harvested commercially. The study of cephalochordate anatomy and physiology continues to provide valuable insights into the evolution of more complex chordates, including humans.
Cephalochordate
Cephalochordates are segmented marine animals that are members of the chordate subphylum called Cephalochordata. Known as either lancelets or amphioxi, they are small eel-like creatures that spend most of their lives buried in sand. Cephalochordates have a notochord, which is a skeletal rod made of cartilage that supports the body; a hollow dorsal nerve cord that runs along the back; and pharyngeal slits or pouches, which are filter-feeding organs. Because of their small size and lack of any hard parts, it is difficult for researchers to find cephalochordate fossils, though some have been found in very old rocks predating vertebrates. Cephalochordates share several features with vertebrates, including a post-anal tail. However, they lack features found in most true vertebrates mainly due to their small poorly developed brain and sensory organs and the absence of any true vertebrae.
Background
Cephalochordates are often referred to as living fossils because they have retained the same form over millions of years and have few or no living relatives. Although their lack of hard parts makes fossilization difficult, the Middle Cambrian Burgess Shale—a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada—has yielded some cephalochordate fossils. The discovery and study of cephalochordates dates back to the 1700s, though they continue to be an important part of the study of vertebrates into the twenty-first century. Cephalochordates share many of the typical features found in chordates. Their dorsal nerve cord is supported by a muscular rod known as a notochord. The body is divided up into V-shaped blocks known as myomeres, and their pharynx is perforated by more than one hundred pharyngeal slits or gill slits, which are used to filter food particles out of the water.
Cephalochordates eat by taking water in through their mouths, aided by a wheel organ, which is a set of ridges inside the mouth with beating cilia that pull the water in. Cirri—thin strands that hang in front of the mouth, acting as sensory devices—filter the water first before it passes through the pharyngeal slits. The slits are enclosed by the metapleural folds of the cephalochordate's body wall, which forms a cavity called the atrium. Food particles in the water are trapped in the atrium while water passes through the gill slits and out of the atrium through the atriopore, or the opening of the atrium.
Cephalochordates lack blood cells, a major heart, and a kidney, though they do have a well-developed circulatory system and simple digestive and excretory systems. The cephalochordate's brain is very small and poorly developed. Cephalochordates have separate sexes, with males and females having multiple paired gonads. Spawning occurs around or after sunset, usually in the spring and summer seasons, at which time eggs and sperm are released into the water out of the atrium. Eggs are then fertilized externally and develop into free-swimming larvae. Cephalochordates generally have a lifespan of two to five years depending on the species. They swim in a spring-like motion, and they are small, usually only 1 to 3 inches in size.
Overview
Although they may seem like prehistoric creatures, cephalochordates can still be found in warm waters around the world. They generally live in shallow sandy environments in tropical and temperate oceans. In some areas, their population can be quite large, with several thousand cephalochordates per square meter, which is approximately 10 square feet. Because of their history, the twenty-five to thirty species of cephalochordates have proved crucial in understanding the evolution of chordates, including vertebrates. Cephalochordates are members of the phylum Chordata, and they have a notochord, which they retain throughout their life. Unlike the spine of a vertebrate, a cephalochordate's notochord extends to its head, which gives the subphylum the prefix Cephalo, which means "relating to the head" in Greek. Branchiostoma and Asymmetron are the only two genera of recognized cephalochordates.
Because they share many of the characteristics of chordates, scientists look to cephalochordates to gain a better understanding of the evolution of vertebrates, including humans. Fossils found in the Middle Cambrian Burgess Shale in British Columbia, Canada, and those found dating back to the Early Cambrian period in southern China, show the chordate lineage appeared very early in the known history of the animal kingdom. These fossils strengthen the case that vertebrates could have evolved from a cephalochordate-like ancestor.
Debate in the scientific community over cephalochordates evolutionary role continues into the twenty-first century. In a study published in early 2017, researchers in Spain examined cephalochordate brains, making a detailed map of the brain's regions. The study was done in the hope that a better understanding of the cephalochordate brain would lead to a better understanding of the brains of vertebrate ancestors. The researchers concluded that the brains of cephalochordates and the brains of vertebrates are divided into two main anterior and posterior regions. In cephalochordates, the anterior region is split into two domains, while in vertebrates it is split into many more portions.
In addition to playing a crucial role in the study of vertebrate evolution, cephalochordates serve a purpose in the food chain, both for animals and humans. Since they are found in the sand in shallow waters, cephalochordates are an important food item for bottom-feeding fish and small crustaceans. They are also a food source for humans and domesticated animals, and they are harvested commercially in China. They can be eaten fresh or dehydrated, and they are sometimes used as an additive in other dishes.
Bibliography
Carroll, Robert L. Patterns and Processes of Vertebrate Evolution. Cambridge UP, 1997.
"Chordates and the Evolution of Vertebrates." Boundless Biology, 26 May 2016, www.boundless.com/biology/textbooks/boundless-biology-textbook/vertebrates-29/chordates-171/chordates-and-the-evolution-of-vertebrates-664-11885/. Accessed 26 June 2017.
Ramel, Gordon. "The Cephalochordata." Earth Life, www.earthlife.net/inverts/cephalochordata.html. Accessed 23 June 2017.
Sanders, Robert. "Lancelet Genome Shows How Genes Quadrupled during Vertebrate Evolution." UC Berkeley, 18 June 2008, www.berkeley.edu/news/media/releases/2008/06/18‗lancelet.shtml. Accessed 23 June 2017.
Shapiro, Leo. "Cephalochordata: Comprehensive Description." Encyclopedia of Life, 2011, eol.org/pages/1585/overview. Accessed 23 June 2017.
"Study Revises the Development, Evolutionary Origin of the Vertebrate Brain." Science Daily, 28 April, 2017, www.sciencedaily.com/releases/2017/04/170428102054.htm. Accessed 23 June 2017.
"Subphylum Cephalochordata." University of Edinburgh, www.nhc.ed.ac.uk/index.php?page=24.25.312.327. Accessed 23 June 2017.
Waggoner, Ben. "Introduction to the Cephalochordata." UCMP Berkley, 1996, www.ucmp.berkeley.edu/chordata/cephalo.html. Accessed 23 June 2017.