Doliolida
Encyclopedia
Doliolida are small marine animals of the Tunicata phylum, related to salps and pyrosoma
s. The Doliolid body is small, typically 1–2 cm long, and barrel-shaped; it features two wide siphon
s, one at the front and the other at the back end, and eight or nine circular muscle
strands reminiscent of barrel bands. Like all tunicate
s, they are filter feeder
s. They are free-floating; the same forced flow of water through their body with which they gather plankton
is used for propulsion - not unlike a tiny ramjet
engine. Doliolids are capable of quick movement. They have a complicated life cycle
consisting of sexual and asexual generations. They are nearly exclusively tropical animals, although a few species can be found as far to the north as North California
.
ual and asexual
generations. The sexual generation consists of individuals featuring eight muscle bands, each having male
or female
gonads. These individuals are called gonozooids. Fertilized eggs
produce slightly different individuals, featuring nine muscle bands, no gonads, and two stalks growing from each individual's body: the shorter one at the ventral side, and the longer one growing from the dorsal
edge of the posterior siphon. Each of these asexual individuals, nicknamed "nurses", produces an astonishing number of asexually grown progeny, both sexual and asexual zooids in three sequential "generations".
The "nurse" produces buds (which grow into new zooids) in its ventral stalk, but the buds grow and mature on its dorsal stalk. Each bud is an aggregate of a few dozen cells
, and the way it gets to its final place is the first peculiarity of doliolid reproduction. Buds are immobile, but are actively carried by special mobile cells, called phorocytes - "carrier cells", shaped like amoeba
s. Each bud is transported by several phorocytes, which follow a clearly defined path across the "nurse's" body: up the ventral stalk, in a spiral along the left side of the "barrel", and finally onto and along the dorsal stalk.
The first buds are planted in pairs on both sides of the dorsal stalk. They grow into zooids not unlike the "nurse", attached to its dorsal stalk with their own dorsal stalks. They are not barrel-shaped, though; their intake siphons are so wide that the individual is more like a spoon than a barrel. These zooids feed the whole colony, which shares common blood circulation (two blood-filled sinuses extend from the "nurse" into complete length of the dorsal stalk). As this first generation grows, the "nurse's" feeding role is gradually diminished, and at the point where the colony's nutrition is supplied by the stalk zooids the "nurse" loses most of its organs, becoming a purely generative and propulsive agent, dragging its huge "tail" of grape-like stalk through the water.
As the dorsal stalk grows and more zooids are planted on its sides, the phorocytes begin to plant a second batch of buds in two more rows between the first two, on the dorsal side of the stalk. These grow into asexual zooids that are smaller, barrel-shaped like the "nurse", and attached to the "nurse's" stalk with their ventral stalks. They do not have a dorsal stalk themselves. This "generation" is called phorozooids, "carrier zooids" because of their later function.
Finally, when the two phorozooid rows on the "nurse's" stalk are filled up and the first phorozooids grow big enough, the phorocytes begin to plant subsequent buds on the stalks of phorozooids, which are still attached to the main colony at this point. It is only this third batch of buds that eventually grows into gonozooids - the sexual generation.
As phorozooids mature, their stalks detach from the "nurse's" stalk, and they swim away on their own, carrying budding gonozooids on their own stalks. The "nurse" and its battery of feeding zooids goes on until all carriers leave, and then the whole colony dies off. The carriers go on as long as it is required for the gonozooids on their stalks to grow and detach, and then they die off too.
Gonozooids detached from the phorozooid swim free, mate, and produce fertilized eggs - from which springs the next generation of asexual zooid "factories", and the cycle repeats. The total number of zooids produced by a single "nurse" colony can reach tens of thousands - explosive growth unprecedented in the animal kingdom.
Pyrosoma
Pyrosomes, genus Pyrosoma, are free-floating colonial tunicates that live usually in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or conical shaped colonies made up of hundreds to thousands of individuals, known as zooids...
s. The Doliolid body is small, typically 1–2 cm long, and barrel-shaped; it features two wide siphon
Siphon
The word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir,...
s, one at the front and the other at the back end, and eight or nine circular muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
strands reminiscent of barrel bands. Like all tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...
s, they are filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
s. They are free-floating; the same forced flow of water through their body with which they gather plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
is used for propulsion - not unlike a tiny ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
engine. Doliolids are capable of quick movement. They have a complicated life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
consisting of sexual and asexual generations. They are nearly exclusively tropical animals, although a few species can be found as far to the north as North California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
.
Doliolid life cycle
Doliolids alternate through sexSex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...
ual and asexual
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
generations. The sexual generation consists of individuals featuring eight muscle bands, each having male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
or female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
gonads. These individuals are called gonozooids. Fertilized eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
produce slightly different individuals, featuring nine muscle bands, no gonads, and two stalks growing from each individual's body: the shorter one at the ventral side, and the longer one growing from the dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
edge of the posterior siphon. Each of these asexual individuals, nicknamed "nurses", produces an astonishing number of asexually grown progeny, both sexual and asexual zooids in three sequential "generations".
The "nurse" produces buds (which grow into new zooids) in its ventral stalk, but the buds grow and mature on its dorsal stalk. Each bud is an aggregate of a few dozen cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, and the way it gets to its final place is the first peculiarity of doliolid reproduction. Buds are immobile, but are actively carried by special mobile cells, called phorocytes - "carrier cells", shaped like amoeba
Amoeba
Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...
s. Each bud is transported by several phorocytes, which follow a clearly defined path across the "nurse's" body: up the ventral stalk, in a spiral along the left side of the "barrel", and finally onto and along the dorsal stalk.
The first buds are planted in pairs on both sides of the dorsal stalk. They grow into zooids not unlike the "nurse", attached to its dorsal stalk with their own dorsal stalks. They are not barrel-shaped, though; their intake siphons are so wide that the individual is more like a spoon than a barrel. These zooids feed the whole colony, which shares common blood circulation (two blood-filled sinuses extend from the "nurse" into complete length of the dorsal stalk). As this first generation grows, the "nurse's" feeding role is gradually diminished, and at the point where the colony's nutrition is supplied by the stalk zooids the "nurse" loses most of its organs, becoming a purely generative and propulsive agent, dragging its huge "tail" of grape-like stalk through the water.
As the dorsal stalk grows and more zooids are planted on its sides, the phorocytes begin to plant a second batch of buds in two more rows between the first two, on the dorsal side of the stalk. These grow into asexual zooids that are smaller, barrel-shaped like the "nurse", and attached to the "nurse's" stalk with their ventral stalks. They do not have a dorsal stalk themselves. This "generation" is called phorozooids, "carrier zooids" because of their later function.
Finally, when the two phorozooid rows on the "nurse's" stalk are filled up and the first phorozooids grow big enough, the phorocytes begin to plant subsequent buds on the stalks of phorozooids, which are still attached to the main colony at this point. It is only this third batch of buds that eventually grows into gonozooids - the sexual generation.
As phorozooids mature, their stalks detach from the "nurse's" stalk, and they swim away on their own, carrying budding gonozooids on their own stalks. The "nurse" and its battery of feeding zooids goes on until all carriers leave, and then the whole colony dies off. The carriers go on as long as it is required for the gonozooids on their stalks to grow and detach, and then they die off too.
Gonozooids detached from the phorozooid swim free, mate, and produce fertilized eggs - from which springs the next generation of asexual zooid "factories", and the cycle repeats. The total number of zooids produced by a single "nurse" colony can reach tens of thousands - explosive growth unprecedented in the animal kingdom.