Oikopleura is a genus of small, free-swimming tunicate found in marine environments across the globe. These animals are a type of plankton, drifting with ocean currents, and measure only a few millimeters in length. They belong to the phylum Chordata, making them distant relatives of vertebrates.
Unique Anatomy and Body Plan
The body of Oikopleura is divided into two sections: a compact trunk and a long, muscular tail. The trunk houses all the essential organs, including a pharynx for feeding, a stomach, a heart, and the gonads. The separate tail is specialized for movement and generating feeding currents.
Unlike many other tunicates that lose their chordate features during development, Oikopleura retains several of these characteristics throughout its life. Its tail contains a flexible rod called a notochord, which provides structural support, and a dorsal nerve cord. This lifelong retention of a tadpole-like body plan is a defining feature of the organism.
The Gelatinous House
Oikopleura constructs an intricate, external gelatinous structure known as a house. This structure is not part of the animal’s body but is secreted from a specialized sheet of cells on its trunk. The house is made of mucus and cellulose-like compounds and serves as a filter-feeding apparatus.
The animal resides within this transparent bubble, using the rhythmic beating of its tail to pump large volumes of water through it. The house has a multi-stage filtration system; a coarse outer mesh blocks large particles, while a fine inner filter concentrates microscopic food like bacteria and phytoplankton. This system allows the animal to feed on some of the smallest organisms in the ocean.
The delicate filters of the house can quickly become clogged with debris. To solve this, Oikopleura rapidly abandons its house when it is no longer functional. The animal can shed a clogged house and inflate a new, pre-formed one in seconds, a process it may repeat multiple times a day.
Rapid Life Cycle and Reproduction
The life history of Oikopleura is characterized by rapid growth and a short lifespan. Depending on water temperature and food availability, many species live for only a few days, progressing from a fertilized egg to a sexually mature adult in under a week. For instance, Oikopleura dioica can complete its entire life cycle in just five days under optimal conditions.
This accelerated development is an example of neoteny, where an organism reaches sexual maturity while retaining juvenile or larval physical characteristics. This strategy allows it to begin reproducing very quickly, taking advantage of favorable environmental conditions like plankton blooms.
Reproduction is a terminal event for the animal. Most species are hermaphrodites, developing male and then female reproductive organs. At the end of its life, the organism releases its eggs and sperm into the water, a process during which its body wall ruptures, leading to its death.
Ecological Significance
Oikopleura plays a role in marine food webs as a primary consumer. By grazing on the smallest forms of plankton, they convert this biomass into a food source for larger animals. This makes them a link between microscopic life and higher trophic levels, including fish larvae and other invertebrates that prey upon them.
The discarded houses of Oikopleura are important to the ocean ecosystem. These abandoned gelatinous structures, laden with filtered organic matter, sink from the surface waters toward the deep sea. This sinking material becomes a component of “marine snow,” a shower of organic debris that provides food for deep-sea organisms.
This process is part of the biological carbon pump, the mechanism by which the ocean sequesters carbon from the atmosphere. As the carbon-rich houses and fecal pellets sink, they transport carbon from the surface to the deep ocean, where it can remain for long periods. Because these discarded houses contribute significantly to the total carbon flux, Oikopleura is an important organism in the global carbon cycle.