(11) Classification/Diagnostic characteristics Lancelets are cephalochordates, a type of chordate. Chordates are deuterostomes, meaning they have a pattern of embryonic development where the mouth develops at the opposite end of the embryo from the blastopore, which develops into the anus. Additionally, at some point in their development all chordates have a dorsal hollow nerve chord, a talk that extends beyond the anus, and a dorsal supporting rod called the notochord. The notochord is composed of large cells with turgid fluid-filled vacuoles, and is the most distinctive derived trait of chordates. However, in vertebrate species, it is replaced during later development by vertebrae that provide more support. In cephalochordates, the notochord is present their entire lives and extends the entire length of their body. (12) (6) Relationship to humans Both lancelets and humans are chordates, meaning at some stage in their development they possess a dorsal hollow nerve chord, a tail that extends beyond the anus, and notochord. However, humans lose their tail and notochord in later embryonic development, and instead develop vertebrae. Additionally, both species are types of deuterostomes, meaning the mouth develops at the opposite end of the embryo from the anus. (12) Due to the fact that, unlike vertebrates, lancelets have retained the primitive characteristics of chordates over millions of years, they are often used as an outgroup when evolutionary biologists draw phylogenetic trees of vertebrates. (5)
Habitat and niche
Lancelets bury themselves with their head protruding in the sediment of shallow marine and brackish waters. They spend most of their lives buried in the sand, but they are capable of swimming. (12) They are found most commonly in shallow subtidal tropical, subtropical, and temperate sand flats. (1)
Predator avoidance
Because Lancelets spend the majority of their time buried in the sand, they are able to avoid being detected by predators. However, if the are noticed by a predator, they are very fast swimmers and can swim both forward and backward. When disturbed, they leave their burrows and swim quickly for a short distance, then rapidly burrow into the sediment again. (2)
Nutrient acquisition Lancelets are filter feeders, and filter food particles from the water using their mouth and pharyngeal basket, which is actually a greatly enlarged pharynx. (12) The feeding system has many structures that prevent large particles from entering the mouth of the lancelet. (1) Water enters through the mouth, where the tentacle-like cirri strain out food particles that are too large. Water the carries particles that are small enough to the pharyngeal basket, where they are trapped in mucus that lines the gill-like slits. Once moved through the pharyngeal basket, the food particles are digested when they travel to a pouch lined with phagocytes that engulf the food particles (9).
Reproduction and life cycle
Lancelets have separate sexes and undergo sexual reproduction during the spring and summer. Fertilization occurs externally after eggs and sperm are released into the water. The embryos develop rapidly into free-swimming larvae, which then swim upward in the water column where they will remain as plankton for several weeks. The duration of this stage varies with the water temperature. (1).
Growth and development Fertilized eggs develop into free-swimming larvae quite rapidly. These larvae remain in the water column as plankton for several weeks before they start to develop into juvenile lancelets. The transition from the larval stage to the juvenile stage is generally a gradual one. Lancelets have average life spans of 2 to 5 years, depending on the species. (1) Lancelets develop their iconic notochords in early embryonic development, and it remains present their entire lives, extending the whole length of their body. (12)
Integument
Lancelets have an epidermis (or outermost) layer composed of column-like epithelial cells and randomly assorted unicellular glands. A thin layer of soft connective tissue makes up the dermis (or inner) layer of lancelets. These organisms have one of the simplest chordate integuments (3).
Movement Lancelets are capable of swimming quickly to avoid predators, but spend most of their time sessile, buried in the sediment of shallow waters. As larvae, however, they are free swimming. When lancelet larvae feed, they swim upward in the sink passively with the mouth facing downward, consuming plankton and other material that flows into its mouth. (1)
Sensing the environment Lancelets are able to sense the environment using the small tentacle-like cirri hanging around the mouth area. Cirri are their sole method for sensing their environment. The cirri are also used to filter food particles out of water when feeding. (8)
Gas exchange
Lancelets take up oxygen and eliminate carbon dioxide using their pharyngeal slits and pharyngeal basket. Gas exchange occurs as the water passes over the the gills of the organism and oxygen and carbon dioxide diffuse into the bloodstream, similar to the process of gas exchange in fish (10)
Waste removal Lancelets move their waste out through their pharyngeal gill slits into the surrounding atrium, and then their waste proceeds to exit the body through the ventral atriopore, an opening in the atrium. (1)
Environmental physiology
Since Lancelets are small, fish-like creatures, they thrive within water. Lancelets have adapted pharyngeal slits so they are able exchange oxygen and carbon dioxide with the water. (10) They are also able to swim quickly through the water when disturbed. (1)
Internal circulation
Lancelets have a closed circulatory system with no central heart. There have branchial hearts, which are muscular veins concentrated in the gills. Branchial hearts pump blood oxygenated blood to the aorta, which branches off into vessels that bring oxygenated blood to all the organs. Lancelet blood also lacks hemoglobin, a molecule that helps red blood cells carry oxygen. (4)
Chemical control There is little information available regarding chemical control in lancelets. However, a closely related cephalochordate called amphioxus has simple endocrine organs similar to those in vertebrates. (7) Based on evolution, lancelets most likely have similar systems for their own chemical control. The thyroid hormone triiodothyronine induces metamorphosis in amphioxus, so lancelets probably use this hormone during the transition from the larval stage to the juvenile stage. (7)
Review Questions:
1. How are humans similar to lancelets?
2. What happens when a predator detects a Lancelet?
Classification/Diagnostic characteristics
Lancelets are cephalochordates, a type of chordate. Chordates are deuterostomes, meaning they have a pattern of embryonic development where the mouth develops at the opposite end of the embryo from the blastopore, which develops into the anus. Additionally, at some point in their development all chordates have a dorsal hollow nerve chord, a talk that extends beyond the anus, and a dorsal supporting rod called the notochord. The notochord is composed of large cells with turgid fluid-filled vacuoles, and is the most distinctive derived trait of chordates. However, in vertebrate species, it is replaced during later development by vertebrae that provide more support. In cephalochordates, the notochord is present their entire lives and extends the entire length of their body. (12)
Relationship to humans
Both lancelets and humans are chordates, meaning at some stage in their development they possess a dorsal hollow nerve chord, a tail that extends beyond the anus, and notochord. However, humans lose their tail and notochord in later embryonic development, and instead develop vertebrae. Additionally, both species are types of deuterostomes, meaning the mouth develops at the opposite end of the embryo from the anus. (12)
Due to the fact that, unlike vertebrates, lancelets have retained the primitive characteristics of chordates over millions of years, they are often used as an outgroup when evolutionary biologists draw phylogenetic trees of vertebrates. (5)
Habitat and niche
Lancelets bury themselves with their head protruding in the sediment of shallow marine and brackish waters. They spend most of their lives buried in the sand, but they are capable of swimming. (12) They are found most commonly in shallow subtidal tropical, subtropical, and temperate sand flats. (1)
Predator avoidance
Because Lancelets spend the majority of their time buried in the sand, they are able to avoid being detected by predators. However, if the are noticed by a predator, they are very fast swimmers and can swim both forward and backward. When disturbed, they leave their burrows and swim quickly for a short distance, then rapidly burrow into the sediment again. (2)
Nutrient acquisition
Lancelets are filter feeders, and filter food particles from the water using their mouth and pharyngeal basket, which is actually a greatly enlarged pharynx. (12) The feeding system has many structures that prevent large particles from entering the mouth of the lancelet. (1) Water enters through the mouth, where the tentacle-like cirri strain out food particles that are too large. Water the carries particles that are small enough to the pharyngeal basket, where they are trapped in mucus that lines the gill-like slits. Once moved through the pharyngeal basket, the food particles are digested when they travel to a pouch lined with phagocytes that engulf the food particles (9).
Reproduction and life cycle
Lancelets have separate sexes and undergo sexual reproduction during the spring and summer. Fertilization occurs externally after eggs and sperm are released into the water. The embryos develop rapidly into free-swimming larvae, which then swim upward in the water column where they will remain as plankton for several weeks. The duration of this stage varies with the water temperature. (1).
Growth and development
Fertilized eggs develop into free-swimming larvae quite rapidly. These larvae remain in the water column as plankton for several weeks before they start to develop into juvenile lancelets. The transition from the larval stage to the juvenile stage is generally a gradual one. Lancelets have average life spans of 2 to 5 years, depending on the species. (1)
Lancelets develop their iconic notochords in early embryonic development, and it remains present their entire lives, extending the whole length of their body. (12)
Integument
Lancelets have an epidermis (or outermost) layer composed of column-like epithelial cells and randomly assorted unicellular glands. A thin layer of soft connective tissue makes up the dermis (or inner) layer of lancelets. These organisms have one of the simplest chordate integuments (3).
Movement
Lancelets are capable of swimming quickly to avoid predators, but spend most of their time sessile, buried in the sediment of shallow waters. As larvae, however, they are free swimming. When lancelet larvae feed, they swim upward in the sink passively with the mouth facing downward, consuming plankton and other material that flows into its mouth. (1)
Sensing the environment
Lancelets are able to sense the environment using the small tentacle-like cirri hanging around the mouth area. Cirri are their sole method for sensing their environment. The cirri are also used to filter food particles out of water when feeding. (8)
Gas exchange
Lancelets take up oxygen and eliminate carbon dioxide using their pharyngeal slits and pharyngeal basket. Gas exchange occurs as the water passes over the the gills of the organism and oxygen and carbon dioxide diffuse into the bloodstream, similar to the process of gas exchange in fish (10)
Waste removal
Lancelets move their waste out through their pharyngeal gill slits into the surrounding atrium, and then their waste proceeds to exit the body through the ventral atriopore, an opening in the atrium. (1)
Environmental physiology
Since Lancelets are small, fish-like creatures, they thrive within water. Lancelets have adapted pharyngeal slits so they are able exchange oxygen and carbon dioxide with the water. (10) They are also able to swim quickly through the water when disturbed. (1)
Internal circulation
Lancelets have a closed circulatory system with no central heart. There have branchial hearts, which are muscular veins concentrated in the gills. Branchial hearts pump blood oxygenated blood to the aorta, which branches off into vessels that bring oxygenated blood to all the organs. Lancelet blood also lacks hemoglobin, a molecule that helps red blood cells carry oxygen. (4)
Chemical control
There is little information available regarding chemical control in lancelets. However, a closely related cephalochordate called amphioxus has simple endocrine organs similar to those in vertebrates. (7) Based on evolution, lancelets most likely have similar systems for their own chemical control. The thyroid hormone triiodothyronine induces metamorphosis in amphioxus, so lancelets probably use this hormone during the transition from the larval stage to the juvenile stage. (7)
Review Questions:
1. How are humans similar to lancelets?
2. What happens when a predator detects a Lancelet?
References:
1. http://eol.org/pages/1585/overview
2. http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html
3. http://www.britannica.com/EBchecked/topic/289723/integument/33054/Dermal-derivatives
4. http://www.earthlife.net/inverts/cephalochordata.html
5. http://www.ijbs.com/v02p0030.htm (Int'l Journal of Bio Sciences)
6.http://bio1151.nicerweb.com/bio1152/Locked/media/ch34/lancelet.html
7. http://sicb.org/meetings/2010/symposia/chordates.php
8. http://www.absoluteastronomy.com/topics/Lancelet
9.http://www.realmonstrosities.com/2011/01/lancelet.html
10. http://biology.unm.edu/ccouncil/Biology_203/Summaries/Deuterostomes.htm
11. http://faculty.ycp.edu/~kkleiner/fieldnaturalhistory/fnhimages/l19images/lancelet.jpg
12. BSCS Biology. a Molecular Approach. New York: Glencoe/McGraw-Hill, 2006. Print.