STRUCTURE OF TROCHOPHORE LARVA
Life cycle of many Polychaetes (Annelids) includes a free swimming trochophore larva. This larva is characteristic of many protostomous groups (e.g., molluscs). Trochophore is a minute, ciliated, unsegmented, and almost pear-shaped pelagic creature, with oral and aboral surfaces recognizable. Thus, the early, presegmental trochophore is characterized by a locomotor ciliary band just anterior to the region of the mouth. This ciliary band, the prototroch, areises from special cells, called trochoblasts. Most trochophores also bear an apical ciliary tuft associated with an apical sense organ (eye and cerebral ganglia). In addition there is often a perianal ciliary band called the telotroch. Many trochophores bear larval sense organs such as ocelli and statocyst, as well as a pair or larval protonehpridia.
The cilia of ciliary girdles, rings or bands, when beating give the appearance of a rotating wheel, hence the name trochophore (Gr. Trochos= wheel; phoros= bear).
The fully formed trochophore may be divided into three regions: 1. pretrochal region comprising area about and above the mouth; 2. pygidium consisting of telotroch and anal area; and 3. growth zone, lying between the mouth and telotroch. In Polychaetes, the growth zone forms all the trunk segments.
When the trochophore is fully formed, there is a sensory apical organ or plate bearing a tuft of cilia. Brain, as a ganglion is present just beneath the apical organ. A preoral ciliary band, which is called prototroch is present just above the equator of the body. A postoral ciliated band, called as metatroch is present just behind the mouth. Another ciliary band, the telotroch may be present infront of the anus. All these ciliary bands help in locomotion and feeding of the trochophore.
Digestive tract of trochophore larva is complete, and comprises stomodaeum, stomach, intesting and proctodaeum. A spacious fluid-filled blastocoel surrounds the larval gut. It contains mesenchyme cells, larval muscle fibers and a pair of larval kidneys or protonephridia, one on each side of the gut. The nephridia open by a common passage. Trochophore lacks segmentation and coelom.
Trochophores may be planktotrophs (i.e., feeding on plankton) and have long planktonic lives, or they may be lecithotrophs (i.e., a non feeding larva which utilizes yolk as a source of nutrition) and have a short planktonic existence.
The prototroch functions as the swimming organ, and in a feeding trochophore, it also collets the suspended food particles. The feeding trochophore also has a food groove between the preoral prototroch and the postoral metatroch.
GROWTH AND DEVELOPMENT
The trochophore larva grows and elongates by proliferation of tissue in the growth zone. Due to metamorphosis, planktonic life of the trochophores ends and loss of various larval structures occur, i.e., protonephridia, muscle bands and ciliary girdles, apical plate and brain. Size of the larva is increased gradually. Segments of the larva are produced from mesoderm. Body segments are marked externally by setae. Such segmental larvae are sometimes called polytroch larvae. The region anterior to the protrochal ring becomes the prostomium, while the protrochal region forms the peristomium. The body continues to elongate as more segments form, and the juvenile worm finally drops from plankton and assumes the lifestyle of a young polychaete.
SIGNIFICANCE OF TROCHOPHORE LARVA
The trochophore larva is of great phylogenetic significance. Many animals such as polychaetes, sipunculans, bryozoans, molluscs, develop a trochophore larva, of course, with certain modifications. This has led some embryologists (e.g., Hatscheck, 1878) to suggest a theory (the so-called trochophore theory) that these animal groups have descended from a common hypothetical ancestor called trochozoon having trochophore-like features. The animal groups thought to have common ancestry may also include other phyla (Platyhelminthes and Arthropoda) which may have lost the trochophore somewhere in the evolution.
There is a striking resemblance between certain rotifers and the trochophore larvae of some annelids, molluscs and others. This indicates that the rotifers and the animals closely related to the ancestors of the annelids, mollusks and the other groups.
Trochophore larva (theory) accounts satisfactorily in many ways for a common ancestor of the coelomate prostomia but not for the acoelomate groups. Many authorities, however, believe that the resemblance of trochophore larvae may be coincidental, the result of adaptive radiation and not of evolutionary significance or actual relationship.