Colonial tunicates are marine invertebrates often found attached to various underwater surfaces. Often called “sea squirts,” they are known for forming colonies. They are filter feeders, drawing water through their bodies to capture small particles for sustenance. They are a widespread presence in marine environments around the globe.
Anatomy and Colonial Structure
Colonial tunicates are intricate organisms composed of numerous individual, genetically identical units known as zooids. These zooids are not separate entities; instead, they are interconnected and embedded within a shared outer layer called a tunic. This tunic can vary in texture, appearing gelatinous or leathery, and often contains blood vessels connecting the zooids.
Each zooid possesses two distinct openings: an incurrent (or inhalant) siphon and an excurrent (or exhalant) siphon. Water enters through the incurrent siphon, passes through a sieve-like branchial basket that traps food particles, and is then expelled through the excurrent siphon.
Colonial tunicates display diverse colors such as tan, cream, yellow, orange, red, purple, or greenish-grey. Their shapes can range from sheet-like mats to lobed or bulbous forms. They are commonly found on hard substrates like rocks, dock pilings, boat hulls, and other marine organisms.
Life Cycle and Propagation
Colonial tunicates reproduce both asexually and sexually, allowing for growth and dispersal. Asexual reproduction occurs through budding, where new zooids form from existing ones, expanding the colony. Colonies also propagate through fragmentation, where pieces break off and reattach elsewhere to establish new growth.
Sexual reproduction involves gamete release; most colonial tunicates are hermaphroditic, possessing both male and female reproductive organs. Sperm may be released into the water and drawn into the siphons of other individuals for fertilization. Fertilized eggs typically develop within the parent colony’s tunic before hatching.
The larval stage is a free-swimming, tadpole-like form. These larvae are short-lived, often swimming for less than a day, and possess sensory organs and attachment structures to locate a suitable substrate. Once a suitable surface is found, the larva settles and undergoes rapid metamorphosis, transforming into the sessile adult zooid that begins to form a new colony.
Ecological Role and Impact
Colonial tunicates are filter feeders, processing large volumes of seawater by consuming plankton and detritus. This feeding activity contributes to water clarity by removing suspended particles from the water column. However, their widespread presence can also lead to significant ecological and economic impacts, particularly when certain species become invasive.
Invasive colonial tunicate species, such as Didemnum vexillum (pancake batter tunicate), Botryllus schlosseri (golden star tunicate), and Botrylloides violaceus (violet tunicate), pose threats to marine ecosystems. These species rapidly spread, forming dense, encrusting mats or hanging lobes that overgrow and smother native organisms. This overgrowth displaces native sessile species and prevents the settlement of their larvae, altering community structures.
Competition for food resources is another impact, as invasive tunicates can compete with native filter feeders like scallops, oysters, and mussels. This competition can reduce the food available for commercially raised shellfish, affecting their growth and condition. These tunicates also contribute to biofouling, accumulating on marine infrastructure like docks, boat hulls, and aquaculture gear. Such fouling increases drag on vessels, raising fuel costs, and adds weight to fishing and aquaculture equipment, making operations more demanding and costly. Management efforts focus on preventing their spread through hull cleaning and avoiding the transport of infested shellfish or equipment.