The main difference between lancelets and tunicates is that lancelets belong to the subphylum Cephalochordata, which contains a notochord extending from the head region whereas tunicates belong to the subphylum Urochordata, whose adult forms do not possess a hollow, dorsal notochord. Furthermore, lancelets do not develop a bony skeleton while tunicates develop a tunic made up of cellulose-like carbohydrate material, covering their body from the outside. In addition to these, lancelets live buried in the sand at the bottom of warm temperate and tropical seas while tunicates are sessile in a dark or shaded location.
Lancelets and tunicates are the two groups of invertebrates found in the phylum Chordata. Generally, they possess four distinctive features of chordates at some point during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. However, they never develop a bony backbone.
Key Areas Covered
1. Lancelets
– Taxonomy, Development, Characteristics
2. Tunicates
– Taxonomy, Development, Characteristics
3. What are the Similarities Between Lancelets and Tunicates
– Outline of Common Features
4. What is the Difference Between Lancelets and Tunicates
– Comparison of Key Differences
Key Terms
Cephalochordates, Chordates, Lancelets, Tunicates, Urochordates
Lancelets – Taxonomy, Development, Characteristics
Lancelets are one of the two groups of invertebrate chordates that make up the subphylum Cephalochordata. Moreover, they are marine organisms possessing all features of chordates, including a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage. However, they lack a bony backbone. The main characteristic feature of lancelet is the presence of a notochord which extends into their head.
Generally, lancelets are also known as amphioxi, and they include around 35 species classified into the order Amphioxiformes, the modern representatives of the subphylum Cephalochordata. Besides, lancelets are the closest living relatives to vertebrates.
Figure 1: The Lancelet
Feature-wise, lancelets have a translucent, blade-shaped body with a poorly developed tail fin. Moreover, they can grow up to 2.5-8 cm. Also, they are mostly benthic, living half-buried in the shallow, subtidal sand flats in the subtropical, tropical, and temperate seas. Furthermore, lancelets are gonochoric animals with two sexes and reproduce during their spawning season.
Tunicates – Taxonomy, Development, Characteristics
Tunicates or sea squirts are the second groups of invertebrate chordates that make up the subphylum Urochordata. Only larval tunicates have all the four distinctive features of a chordate. However, adults retain pharyngeal slits. Generally, larval tunicates undergo metamorphosis in order to become their sessile adult forms that attach to the surfaces of the bottom of the sea. Furthermore, the name for tunicates is given due to the presence of an outer covering called the tunic made up of a cellulose-like carbohydrate.
Figure 2: Tunicates (a) Tunicate Colony, (b) Larva, (c) Adult
Moreover, around 3000 species of tunicates live in shallow water throughout the world. Both tunicates and lancelets are suspension feeders capable of capturing planktonic particles filtering out from the seawater. Adult tunicates form colonies and zooids refer to the individual organisms in a colony, integrating one another.
Similarities Between Lancelets and Tunicates
- Lancelets and tunicates are the two groups of invertebrates that belong to the phylum Chordata.
- They possess the four distinctive features of chordates at some point during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
- However, they never develop a bony backbone.
- They make up the informal taxonomic group of animals called protochordates along with the phylum Hemichordata.
- They are deuterostomes with bilateral symmetry.
- Both are suspension feeders and benthos that live attached to the surfaces of the bottom of the ocean.
- They have a complete digestive system with a mouth and anus.
- Moreover, their circulatory systems do not contain blood cells and respiratory pigments such as hemoglobin.
- They undergo external fertilization.
Difference Between Lancelets and Tunicates
Definition
Lancelets refer to the small elongated marine invertebrates that resemble a fish but, lack jaws and obvious sense organs, while tunicates refer to marine invertebrates that have a rubbery or hard outer coat and two siphons to draw water into and out of the body.
Taxonomy
While lancelets belong to the subphylum Cephalochordata, tunicates belong to the subphylum Urochordata.
Habitat
Lancelets live buried in the sand at the bottom of warm temperate and tropical seas, while tunicates are sessile in a dark or shaded location.
Type of Living
Moreover, lancelets are solitary animals, while adult tunicates are colonial.
Body Shape
Lancelets are blade-shaped animals, while tunicates are barrel-shaped animals.
Organization
Furthermore, lancelets are the closest living relatives of vertebrates. But, tunicates are a somewhat primitive form of chordates, more related to echinoderms.
Distinctive Features of Chordates
Lancelets have a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage, while the larval form of tunicates has all four distinctive features of chordates but, adults have pharyngeal slits.
Position of the Notochord
The notochord of lancelets extends from the head region while adult tunicates lack a notochord.
Skeleton
While lancelets do not develop a bony skeleton, tunicates develop a tunic made up of cellulose-like carbohydrate material, covering their body from the outside.
Coelom
Lancelets have a true coelom, while tunicates do not develop a very clear coelom.
Myomeric Segmentation
In addition to the above differences, lancelets have a segmented body while tunicates are the only chordates lacking myomeric segmentation.
Circulatory System
Moreover, lancelets have a simple circulatory system, which does not include a heart, while tunicates have a well-developed circulatory system with a heart.
Significance of the Circulatory System
The circulatory system of lancelets is responsible for transporting food throughout the body while the circulatory system of tunicates consists of a double U-shaped heart, blood vessels with simple connective tissue, and pale green color blood with many corpuscles.
Excretion
The excretory system of lancelets contains segmented kidneys with protonephridia while tunicates lack kidney-like metanephridial organs and therefore, they rely on the diffusion of ammonia across their tissues.
Sexual Reproduction
Whereas lancelets are dioecious, tunicates are hermaphroditic.
Asexual Reproduction
Lanceletes do not undergo asexual reproduction while tunicates undergo asexual reproduction by budding.
Conclusion
In short, Lancelets are one of the two groups of invertebrate chordates making up the subphylum Cephalochordata. Their main significant feature is the presence of a notochord, extending from the head. In addition, they have a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage. Lancelets are solitary animals that live buried in the sand. In comparison, tunicates are the second group of invertebrate chordates making up the subphylum Urochordata. Their adults retain one of the four distinctive features of chordates: the pharyngeal slits. However, the other three features occur in their larval stage. In addition, tunicates develop an outer covering called the tunic. Therefore, the main difference between lancelets and tunicates is their retaining of the distinctive features of chordates and the mode of living.
References:
1. “Chordates | Boundless Biology.” Lumen, Available Here.
Image Courtesy:
1. “Urochordates” By OpenStax CNX (CC BY 3.0) via OpenStax College.
2. “Figure_29_01_04” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia
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