Lancelet vs Tunicate: Difference and Comparison

Key Takeaways

1. Lancelets, also known as amphioxus, have long, slender body with a notochord capable of limited swimming movements. They filter feed using gill slits and respire through these slits as well.
2. Tunicates, or sea squirts, have a sac-like body with a tough outer covering called the tunic. They are mostly sessile and filter feed using a unique feeding structure called the branchial basket. They respire through pharyngeal gill slits.
3. Lancelets are more closely related to vertebrates than tunicates and have a more developed nervous system. They reproduce sexually with external fertilization, while tunicates have a complex life cycle involving both sexual and asexual reproduction.

What is Lancelet?

Lancelets are chordates that belong to the subphylum Cephalochordata, as their notochord extends from the anterior body to their tail. Lancelets are eel-like filter feeders with both ends pointed; hence, they are also called amphioxus. The anterior end is swollen to form a snout, while the posterior region is long and pointed.

These small-sized, i.e., one to eight-centimeter-long organisms live in saline water. Their body color varies from white to creamy yellow with a pink tint. Lancelets also have a pearly sheen due to the mucus secreted by their outer cover.

Lancelet’s organ system isn’t well developed. These fish-like organisms do have eyes, brains, and hearts. But, they have a nerve cord and blood vessels that run along the top of their notochord. The body has several sensory cells, especially near the snout region. Moreover, their respiratory system has over 200-gill slits that empty into a chamber in the body.

Lancelet’s reproductive system has two sexes, i.e., either male or female. Hence, they are also called gonochoric animals. These organisms have external reproduction that takes place through fecundation. These tiny organisms are seasonal breeders whose spawning seasons vary among Lancelet species.

What is Tunicate?

Tunicates are another chordate that falls under the subphylum Urochordata. They are the earliest chordates that live in shallow or deep marine or saline waters. These organisms are sessile, i.e., they can’t move from one place to another. Their body’s posterior region is firmly attached to fixed objects like rocks, boats, or docks.

The term tunicates were derived from the Latin term ‘tunicare,’ which means ‘clothe with a tunic.’ They have a flexible outer covering called a tunic that protects themselves from predators. These organisms shoot water at the predators as their defense; they are also called sea squirts.

Tunicates have a few developed organ systems, including a circulatory system with a fully developed heart. Moreover, they have a partially developed excretory system that lacks kidneys. The brain is absent in tunicates, but the cerebral ganglion helps in nervous coordination. Tunicates have notochord in their larval stage that they lack in their adult stage.

Tunicate’s feeding mechanism involves two siphons or openings that engulf water. The consumed water passes through a sieve-like structure called a branchial basket, where the food and oxygen get trapped. The remaining water is then excreted out through an atrial siphon. 

Difference Between Lancelet and Tunicate

1. Lancelets are tiny, fish-shaped organisms with pointed ends. On the other hand, tunicates are small with the barrel-shaped body.
2. Lancelet’s body has myomeric segmentation, i.e., it contains blocks of sequentially arranged skeletal tissues. But, the tunicates lack myomeric segments on their body.
3. Lancelets have a simple circulatory system as they lack the heart; hence, the blood vessels move along the notochord. While in Tunicates, a fully developed circulatory system is present as they contain a heart.
4. Lancelets contain a notochord throughout their lifespan that extends from their anterior head to their posterior tail. On the other hand, Tunicates have notochord in their larval stage that they lose in their adult stage.
5. Lancelets have protonephridia, the excretory tubules that help in excretion. While the tunicates excrete through diffusion as they lack metanephridia structures.

Comparison Between Lancelet and Tunicate

Parameter Of Comparison	Lancelet	Tunicate
Meaning	Also called amphioxus, which means ‘both ends pointed.’	Tunicate is derived from ‘Tunicare,’ which means ‘clothe with a tunic.’
Subphylum	Cephalochordata	Urochordata
Ability To Move	Motile Animals	Sessile Animals
Tunica	Tunica is absent.	Tunicates contain tunica.
Sexual Organs	Gonochoric animals, i.e., either male or female sex organ, is present.	Hermaphroditic animals, i.e., have both male and female sex organs.

References

1. https://www.nature.com/articles/nature06967
2. https://books.google.com/books?hl=en&lr=&id=3MgBMvh8MgYC&oi=fnd&pg=PA114&dq=DIFFERENCE+BETWEEN+LANCELETS+AND+TUNICATES+&ots=tE-kjdV1M5&sig=Frf9E6fsJJidu8tAWPEH7PsTtfw