Tunicates are marine invertebrates found in oceans worldwide, commonly known as “sea squirts” in shallow waters. A group of these animals has colonized the deep sea, evolving varied forms and strategies to survive its crushing pressure, constant cold, and profound darkness. Their adaptation to these extreme conditions allows them to occupy niches far removed from the sunlit surface.
Surviving the Crushing Depths
The immense hydrostatic pressure of the deep sea, which increases by one atmosphere every ten meters, puts stress on biological structures. Deep-sea tunicates have gelatinous bodies that lack gas-filled spaces, which would be crushed under such pressure. Their high water content, often exceeding 95% of their wet weight, allows them to achieve near-neutral buoyancy and equalizes the pressure inside and outside their bodies.
Temperatures in the abyss hover just above freezing, slowing metabolic processes. Deep-sea tunicates have adapted with reduced metabolic rates to conserve energy in the cold, food-scarce environment. Their bodies are composed of acidic mucopolysaccharides, and their tunic contains cellulose, providing support without the energetic cost of dense tissues.
Without sunlight, vision is of little use, and these animals rely on mechanoreception and chemoreception to perceive their surroundings. They use specialized siphons to interact with their environment, detecting water movements and chemical cues that might signal the presence of food or predators.
An Array of Undersea Siphons
The deep sea hosts a variety of tunicate forms, each reflecting a different survival strategy. They exist as solitary individuals or extensive colonies. They can be either sessile, attached to the seafloor, or pelagic, drifting through the water column.
One example is the predatory tunicate, Megalodicopia hians. Unlike its filter-feeding relatives, this species has a large, jaw-like oral siphon it can snap shut on prey like small crustaceans and fish. It sits anchored to the seafloor, an ambush strategy well-suited to an environment where food is sparse.
Other deep-sea tunicates, known as larvaceans, are pelagic and construct intricate mucus “houses” that act as feeding filters. These structures trap tiny particles of organic matter, known as marine snow, that drift down from above. Some houses are several meters across, and when the filters clog, the larvacean abandons its house and quickly builds a new one.
Daily Life in Perpetual Darkness
For the majority that are filter feeders, life is a patient process of continuously pumping water through their bodies. They use a mucous net from an organ called the endostyle to trap microscopic food particles. This efficient filtering is necessary in the nutrient-poor waters of the deep sea.
Reproductive strategies are adapted for finding a mate in the vast abyss. Many deep-sea species are hermaphroditic, possessing both male and female organs, which increases the chance of reproduction. Some colonial species also reproduce asexually by budding. Species with a larval stage have a “tadpole” larva that must find a suitable place to settle and metamorphose.
Unseen Architects of the Abyss
Deep-sea tunicates play an important role in the abyssal ecosystem. As filter feeders, they are important for processing organic matter. They consume vast quantities of marine snow, helping to cycle nutrients within the deep-sea food web. They repackage this particulate matter into their bodies and waste, making it available to other organisms.
The discarded mucus houses of giant larvaceans contribute significantly to the deep-sea carbon cycle. These large, carbon-rich structures sink rapidly to the seafloor, providing a concentrated food source for bottom-dwelling organisms and sequestering carbon. The stalks and bodies of sessile tunicates also provide physical structure on the soft sediment of abyssal plains, creating attachment points and habitat for other deep-sea fauna.