What structures common to all chordates do adult tunicates possess? Why are tunicates considered chordates?
The correct answer and explanation is :
Answer:
Adult tunicates possess pharyngeal slits as the only chordate structure.
Explanation:
Tunicates, also known as urochordates, are marine invertebrates that belong to the phylum Chordata. Although they undergo significant morphological changes during their life cycle, they are considered chordates because of the key structures they exhibit during their larval stage.
In their larval stage, tunicates possess all four hallmark characteristics of chordates:
- Notochord – a flexible rod that provides structural support.
- Dorsal hollow nerve cord – a nerve cord running along the back.
- Pharyngeal slits – openings in the pharynx used for filter feeding and respiration.
- Post-anal tail – an extension of the body beyond the anus.
However, as tunicates transition into their sessile adult form, they undergo metamorphosis, during which they lose the notochord, dorsal hollow nerve cord, and post-anal tail. The only chordate feature retained in adult tunicates is the pharyngeal slits, which are used for filter feeding.
Despite losing most chordate traits in adulthood, tunicates are classified as chordates because their larval stage clearly exhibits all chordate characteristics. This is a crucial aspect of evolutionary classification, as scientists group organisms based on their developmental features rather than just adult morphology.
Tunicates are of great interest in evolutionary biology because they are believed to be closely related to vertebrates. Their simplified adult form suggests that they have undergone a secondary loss of chordate features due to their sedentary lifestyle, rather than never having them at all.
Thus, while adult tunicates only retain pharyngeal slits, their classification as chordates is justified by their larval anatomy and genetic relationships with other chordates.