Lancelet (fish-like chordate)

Lancelet


external image Lancelet.JPG



Classification/Diagnostic characteristics
A lancelet is a chordate. It is most notable for having a notochord, which is a unique characteristic of chordates. It is composed of large cells with fluid [[#|filled]] vacuoles that make it rigid yet flexible. In lancelets, the notochord provides body support and extends the entire length of their body. In addition, all lancelets have a dorsal hollow nerve cord, a tail that extends beyond the anus, and a dorsal supporting [[#|rod]] called the notochord.

lancelet.gif
A [[#|complete]] display of the anatomy of a lancelet. The organisms has relatively few exterior features that help characterize it.

Unknown.jpeg (Evan Kates)

Lancelets are up to 5 cm long, and they live with their posterior end [[#|buried in the sand]] and the anterior end exposed for feeding. (2)
LanceletAnatomy.gif

Lancelets have a fish like appearance, but are invertebrates. They have a notochord and nerve cord. They do not have bones, a brain, and eyes.
http://www.angelfire.com/mo2/animals1/phylum/chordata.html

Lancelets, though possessing cartilage-like material stiffening the gill slits, mouth, and tail, they have no true skeleton.
https://en.wikipedia.org/wiki/Lancelet
*Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 18–21.


Relationship to humans
In parts of China people eat lancelet. Lancelet is most important to humans for evolutionary purposes however, and is studied worldwide.http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html

In 2008, a [[#|study at University]] of California, Berkeley showed that humans and lancelets have a common ancestor that diverged just over 550 million years ago. This discovery supports the theory that vertebrates developed in a manner of a four-fold duplication process of less complex animals (Prashant).
http://berkeley.edu/news/media/releases/2008/06/18_lancelet.shtml

Habitat and niche
Lancelets live in shallow and brackish water around the world. Most of the time they are lie in the sand with their hear above it.


Lancelets live in all the oceans of the world in warm and cool waters. However, most live in the temperate regions, such as on the shores of Florida or Southern China. Lancelets live in sandy bottoms near the shore at depths down to about 28-30 m. They move by wriggling back and forth through the water, but they are more apt to bury themselves in the sand with only their mouth ends sticking up out of the sand. In coarse sand, where water is rich in oxygen and food particles circulate freely, they buries themselves entirely. In fine sand, only half of the body is buried and half of it emerges from the ground. Lancelets live near the coastlines because they prefer sand mixed with shells. Also, they hate the mud because they are not adapted to penetrate ground with small particles. In muddy ground, they don't bury themselves at all because they can neither feed nor breathe there. Unfortunately, muddy areas have been increasing during the last decade due to intensive dumping of dead [[#|organic matter]]. So, the Lancelet population has been declining.

Taken from the website:
http://www.grid.unep.ch/bsein/redbook/txt/amphioxu.htm?%20ACRANIA


Predator avoidance
Lancelets aviod predators by burrowing in sandy [[#|gravel]] and keeping just their head above. Lancelets also swim very quickly and are able to escape predators. Lancelets also live in large groups of almost 9,000 lancelets per square yard.
http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html

Lancelet.jpgThe Lancelet using a protection method by burrowing in the sand- http://y11evolution.wikispaces.com/file/view/Lancelet.jpg/30729455/Lancelet.jpg

Nutrient acquisition
Lancelets swim and [[#|filter]] in prey from water using their pharyngeal basket.
Lancelets are suspension feeders, feeding by trapping tiny particles on mucous nets secreted across the pharyngeal slits. Ciliary pumping creates a flow of water with suspended food particles into the mouth and gill slits. (2)
http://www.course-notes.org/Biology/Outlines/Chapter_34_Vertebrates

external image lancelet.jpg
Lancelet feeding on plankton. (Colin Gray)
http://www.livebinders.com/play/present?tab_view=side&id=239741

The Lancelet's different feeding positions are dependent on the type of sand in the given environment For example, with fine sand, it will lie on the bottom versus with coarser sand, the lancelet they bury most of their body (except the head). (http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html)

Reproduction and life cycle
Many chordates reproduce through asexual budding, although lancelets have distinct sexes. There is an equal ratio between males and females in any natural population of lancelets. Lancelets can reproduce sexually by releasing their eggs and sperm into the water where they live, and the gametes unite and develop into larvae. When the larvae have grown large enough, usually when twelve to fifteen pairs of gill slits have formed, the larvae sink to the bottom of their aqueous habitat and mature into adult lancelets. Embryos will obtain o nutrients from yolk with the egg while quickly developing into larvae that will then live amongst and feed off plankton for the next few weeks of their development. Additional gill slits, muscle segments throughout the body, and reproductive organs grow to full maturity over time, and then the lancelets are able to reproduce again. (3) [GW] http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html

Growth and development
They start out as a tadpole like larvae. After swimming in plankton, they will settle down and transform into sessile adults. Commonly chordates will loss their pharyngeal slits in adulthood. They will develop a pharyngeal basket. They will rarely exceed 5 cm in length. The life span of chordates is 2-5 years depending on the species. (GC) http://eol.org/pages/1585/overview

Integument
Lancelets have a tough protein outer laying. Their skin serves many functions. some of its functions include producing mucus, creating skin pigment, and storing glycogen. (http://link.springer.com/article/10.1007%2FBF00384200?LI=true)

Movement
Bands of muscles surround the notochord provide it with support for the body.

The Lancelet is an avid swimmer, whose mechanism resembles that of fishes through the coordinated contraction of serial muscle blocks.
Contraction of chevron-shaped muscles flexes the notochord and produces lateral undulations that thrust the body forward.
(2)
http://www.course-notes.org/Biology/Outlines/Chapter_34_Vertebrates

Sensing the environment
Lancelets have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices. (http://www.absoluteastronomy.com/topics/Lancelet)

Gas exchange
They uptake oxygen through the pharynx (develops around the pharyngeal slits), as well as, get rid of carbon dioxide and water using it.

Waste Removal
They remove waste through their anus. Lancelets also have a type of kidney, but it is vastly different from vertebrate kidneys. The Lancelet kindeys use protonephria rather than vertebrate nephrons.
[http://encyclopedia2.thefreedictionary.com/Protonephridium]

Water, drawn in through the by beating of cilia, is filtered by slender projections around the mouth (oral cirri) and passed through to the gill slits, where food particles in the water are trapped by mucus and the water is filtered out of the atrium. A pouch (hepatic caecum) secretes digestive enzymes, and digestion occurs in a specific region of the intestines (iliocolonic ring). The wastes are then expelled through the anus.
http://www.ucmp.berkeley.edu/chordata/cephalo.html (BHu)

Environmental Physiology
They live in the water.



Internal circulation

Some chordates have a week circulotary system.

Along with its digestive role in trapping incoming food particles, the wall of the pharynx, or the part of the food canal immediately behind the mouth, is the main respiratory surface for the lancelet. The pharynx contains diagonal openings, referred to as the gill slits, which are structurally supported by the thin gill bars. When blood passes through the gill bars, it is oxygenated by the water that moves across the surface. Since the lancelet has no heart, the blood vessels themselves are contractile, meaning that they contract to propel the blood throughout the body. The space between the pharynx and the rest of the body is the atrial cavity, where blood primarily collects, that leads to the atriopore, through which water exists. A flap of tissue called the metapleural folds extends down from the sides of the body. When the transverse muscles between the metapleural folds contract, they force water in the atrial cavity out of the atriopore. (Alexander Soloviev)

http://salinella.bio.uottawa.ca/digitalzoology/ChorCepl/default.php?Cepl_Lab_Bran.htm??E?Md2ChapterMcp1?Md2Ch1SectionMcp1??1


Chemical control


Review Questions:
1. Using this certain basket-like structure the lancelet is able to filter the water for nutrients.
2. Describe how lancelets differ from other chordates when it comes to reproduction.
3. What structure do lancelets utilize in order to exchange gases with their environment?
They have a closed [[#|circulatory system]] and they do not have a true heart. The walls of the blood vessels contract to circulate the blood through the body.

4. What unique characteristic does the hydra have that characterizes it as a chordate? In what ways does having this quality help the hydra?


Miller, Kenneth R., Joseph S. Levine. Biology. Upper Saddle River, NJ: Prentice Hall, 2004. Print.
http://www.course-notes.org/Biology/Outlines/Chapter_34_Vertebrates