Question

List the four distinguishing chordate traits. Which of these traits are retained by an adult tunicate?

Short answer

Adult tunicates retain pharyngeal slits; they lose the notochord, dorsal nerve cord, and post-anal tail.

Step-by-step solution

Understanding Chordate Traits

Chordates are animals belonging to the phylum Chordata. The four distinguishing traits of chordates are: 1) the notochord, 2) a dorsal (back) hollow nerve cord, 3) pharyngeal slits, and 4) a post-anal tail. These traits may not be present in all stages of a chordate's life, but they are typically found during some part of their development.

Identifying Adult Tunicate Traits

Tunicates, also known as sea squirts, are marine chordates. In their larval stage, they exhibit all four chordate features, but undergo significant transformation as adults, retaining only the pharyngeal slits as evident distinguishing traits. The notochord, dorsal nerve cord, and post-anal tail are lost as the larva metamorphose into an adult.

Key concepts

Notochord

The notochord is a flexible, rod-like structure found in all chordate embryos. It functions as a primary skeletal support. The presence of the notochord is one of the key traits that define chordates.
During embryonic development in vertebrates, the notochord is eventually replaced by the vertebral column, or spine, once the organism matures. This original structure can still be observed in the early stages of development and plays a crucial role in directing the formation of the surrounding neural structures and tissues.
In simpler chordates or in their larval stages, the notochord may remain present throughout their life, offering structure and providing sites for muscle attachment which facilitate movement.

Dorsal Hollow Nerve Cord

The dorsal hollow nerve cord is another vital characteristic of chordates. Unlike other groups of animals, chordates possess a nerve cord that runs along the dorsal side (back) of the body and has a cavity (hollow).
This structure serves as the central nervous system, eventually developing into the spinal cord and brain in vertebrates. It allows for complex neurological processes and sensory information processing.
The dorsal hollow nerve cord's structure is initially formed during the early embryonic development stage, a result of neurulation, where ectodermal cells fold and close to create the neural tube.

Pharyngeal Slits

Pharyngeal slits are a series of openings that connect the inside of the throat to the outside of the neck. They play a critical role in filter-feeding in aquatic species and are associated with respiration.
While invertebrate chordates use these structures primarily for feeding, in vertebrates, the pharyngeal slits contribute to developing parts of the ear, tonsils, and other neck and throat structures.
Tunicates, a group of marine chordates, retain their pharyngeal slits into adulthood, serving as a distinguishing feature despite undergoing significant morphological changes from their larval forms.

Post-anal Tail

The post-anal tail is an extension of the body past the anal opening. This trait is distinct in chordates, and it serves multiple functions depending on the species. It is often involved in movement and balance.
In aquatic chordates, the post-anal tail is crucial for locomotion, helping organisms swim efficiently. While in terrestrial animals, it serves various functions ranging from balance to communication.
In the larval stage of tunicates, the post-anal tail is present, but it disappears as they mature into adults, showcasing the diversity and adaptability of chordate traits.

Tunicates

Tunicates, commonly known as sea squirts, belong to the subphylum Tunicata and are fascinating creatures within the chordate family. These marine organisms exhibit all chordate traits during their larval stage.
As larvae, they are free-swimming and exhibit the full suite of chordate characteristics, including the notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail.
Upon reaching adulthood, tunicates undergo a dramatic metamorphosis. The adult form becomes sessile, adhering to surfaces, and retains only the pharyngeal slits. This transformation highlights the wonderful adaptability and complexity of chordate development.