Ascidians, commonly known as sea squirts, are a diverse group of marine invertebrates that often go unnoticed despite their widespread presence in ocean environments. These unique creatures, belonging to the class Ascidiacea, are biologically significant and inhabit various marine substrates worldwide, from shallow coastal waters to the deep sea. They present a wide array of forms and play distinct roles within their ecosystems. With over 2,500 described species, ascidians are a fascinating subject for understanding marine life and evolutionary biology.
Understanding Ascidians
Ascidians are classified under the subphylum Tunicata, part of the phylum Chordata, which also includes vertebrates. Despite their sac-like, stationary adult form, their chordate classification is supported by their larval stage characteristics. Adult ascidians typically appear as rounded or cylindrical organisms, ranging from 0.5 to 10 centimeters in size, and are usually found attached to hard surfaces such as rocks, coral, jetties, and ship hulls.
Their external appearance includes a tough outer layer called a “tunic,” which provides structural support and protection. The tunic is often quite rigid, and in some species, it has root-like extensions that help the animal firmly grip the surface. Ascidians possess two distinct siphons: an incurrent (oral) siphon through which water enters, and an excurrent (atrial) siphon through which filtered water, wastes, and gametes are expelled. When disturbed, many species will forcibly eject water from these siphons, earning them the common name “sea squirts.”
Life Cycle and Body Structure
The life cycle of ascidians involves two distinct stages. The initial stage is a free-swimming, tadpole-like larva, a few millimeters long. This larval form possesses the defining characteristics of chordates, including a notochord (a flexible rod supporting the body), a dorsal hollow nerve cord, rudimentary pharyngeal gill slits, and a post-anal tail. The larva’s tail, containing muscle cells, enables locomotion and dispersal.
After a brief planktonic period, often lasting minutes to a few hours, the larva settles on a suitable substrate. It then undergoes a complex and rapid metamorphosis, which can take several days to complete. During this process, the larval tail, notochord, and nerve cord are reabsorbed, and the sessile adult form begins to develop. Its internal anatomy becomes specialized for filter feeding. Water is drawn in through the oral siphon into a large pharyngeal basket, perforated by numerous ciliated gill slits (stigmata). These cilia create a water current that draws food particles, such as phytoplankton, onto a mucus net secreted by the endostyle. The food-laden mucus is then transported to the digestive system, while filtered water exits through the atrial siphon.
Role in Marine Ecosystems
Ascidians play a significant part in marine ecosystems, primarily through their filter-feeding. They actively pump water, filtering out plankton and particulate organic matter. This process contributes to water clarity by removing suspended particles, acting as natural water purifiers. Food consumption is influenced by the concentration of food particles in the surrounding seawater and their retention efficiency.
Some ascidian species form dense aggregations, particularly in low intertidal and subtidal habitats, which can alter the structure of benthic communities. Their ability to attach to various natural and artificial substrates, including rocks, docks, and ship hulls, makes them common fouling organisms. In some regions, certain ascidian species are considered invasive, as they can spread rapidly, often introduced via shipping. These invasive species can outcompete native organisms for resources and space, sometimes forming extensive colonies that smother other filter-feeding organisms like mussels, clams, and oysters.
Remarkable Biological Traits
Ascidians possess several unique biological features. Their tunic, the protective outer layer, is composed of cellulose, a polysaccharide typically found in plants, making ascidians one of the few animals known to produce it. This cellulose, combined with proteins and calcium salts, forms a tough yet flexible external skeleton. The tunic can vary in composition and structure between species, influencing its properties, such as hardness and flexibility.
Colonial ascidians reproduce asexually through budding, where new individuals (zooids) grow from existing ones, often interconnected by a shared external vascular system. This budding allows colonies to expand, sometimes covering several square meters of substrate. The ability to reproduce asexually is considered an extension of their regenerative capabilities. Ascidians display robust regenerative potential, regenerating lost or damaged body parts, or even an entire body from small fragments. This capacity is particularly pronounced in colonial species but has also been observed in solitary species, where a dissected individual can regenerate all its missing organs from separated fragments. This ability in solitary, sexually reproducing ascidians, such as Polycarpa mytiligera, is a recent discovery. The evolutionary proximity of ascidians to vertebrates, with tunicates being the closest invertebrate relatives, makes them valuable models for studying biological processes like development, regeneration, and the evolution of chordates.