Tunicates are not vertebrates, but frequent confusion arises from their close evolutionary relationship. This connection is not obvious in their adult forms, as the answer to their classification lies hidden within their life cycle. A shared ancestry links them to all backboned animals, including humans. This relationship is revealed by features that emerge only during the tunicate’s earliest stages of life.
Defining Tunicates and Vertebrates
Tunicates, which include the commonly known sea squirts, are marine invertebrates that in their adult form appear quite simple. Most adult tunicates are sessile, meaning they are permanently attached to a surface like a rock or the seafloor. They have sac-like bodies with two openings, called siphons, that are used to draw in and expel water for filter-feeding and respiration. Their body is covered by a protective outer layer called a tunic, which is unique because it contains a type of cellulose, a material more commonly associated with plants.
This stationary, bag-like organism contrasts sharply with the body plan of a vertebrate. Vertebrates are defined by the presence of a vertebral column, or backbone, which encases the spinal cord and provides primary structural support. This diverse group encompasses all fish, amphibians, reptiles, birds, and mammals. The visible differences between an adult sea squirt and an active vertebrate make their close relationship surprising.
The Chordate Connection
Despite their stark physical differences as adults, tunicates and vertebrates are grouped together in the same biological phylum, Chordata. This classification is based on a set of five anatomical features that all chordates possess at some point in their development, revealing a shared heritage.
The five defining traits of chordates are:
- A notochord, a flexible, rod-shaped structure that provides skeletal support. In vertebrates, it is present in the embryo and is later replaced by the vertebral column.
- A dorsal hollow nerve cord, which in vertebrates develops into the brain and spinal cord.
- Pharyngeal slits, which are openings in the pharynx. In aquatic chordates like tunicates, they are used for filter-feeding, while in fish, they develop into gills.
- A post-anal tail, an extension of the body past the anus.
- An endostyle, a groove involved in filter-feeding.
The Tunicate Life Cycle Transformation
The key to the tunicate’s place within Chordata lies in its life cycle. Tunicates begin life not as a sessile sac, but as a free-swimming, tadpole-like larva. This larval stage clearly displays its chordate characteristics. The larva has a tail containing the supportive notochord and a dorsal nerve cord, allowing it to swim freely after hatching.
This mobile phase is brief, sometimes lasting only a few hours while the larva finds a suitable surface to settle on. Once it attaches itself, it undergoes a dramatic metamorphosis into its adult form. During this transformation, the tail, notochord, and most of the nerve cord are absorbed by the body. The animal then reorganizes its internal structure into the simple, sac-like adult, losing the features that so clearly mark it as a chordate.
Evolutionary Placement of Tunicates
Because tunicates possess chordate features in their larval stage, they are classified as invertebrate chordates within the subphylum Tunicata (also called Urochordata). While they lack a backbone and are therefore not vertebrates, their position on the evolutionary tree is significant.
For many years, scientists believed another group of invertebrate chordates, the lancelets, were the closest relatives to vertebrates. However, extensive genetic and molecular analyses have overturned this view. Recent phylogenetic studies, which compare large sets of genes across different species, have provided strong evidence that tunicates are the closest living relatives of vertebrates. This finding means that the ancestors of vertebrates may have shared many characteristics with the tadpole-like tunicate larva. The simple adult form of the tunicate is a highly specialized adaptation, not a reflection of its primitive origins.