What is Trochophore?
Trochophore which is also known as Trochosphere, is a small and translucent larvae of marine annelids. The trochophore larva is a free swimming larva in marine animals and is found mostly in the group of molluscs. Commonly the trochophore larva belongs to the phylum of annelida, and molluscs. The trochophore larva is a type of planktonic marine larva i.e. a larva that can swim against the current and hence has several bands of cilia besides their spherical and pear-shaped body which provide them with the ability to swim.
The Trochophore Larva
The trochophore larvae exist in larval form and belong to the trochozoa clade. They include different clades which are entoprocta, annelids, molluscs, echiurans, sipunculans, and nemerteans. All together these phyla make up the Lophotrochozoa clade. The reason that the larvae belong to so many phyla is because the larvae were present in the life cycle of a common ancestor of the group. These larvae are often planktotrophic and are known so because they feed on planktons.
Typically, the trochophores are known as they are because of the wheel-band shaped cilia that they possess. The term trochophore is derived from the ancient Greek language. Splitting the name, the word trokhos means “wheel” and phero or phoreo which means “to bear, to carry” in ancient Greek. Hence, the combined name means wheel bearing. This is because of the presence of a wheel-shaped band of cilia.
The Life Cycle and Anatomy of Trochophore Larva
A typical anatomy of the larvae is given below in the trochophore larva diagram:
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Understanding the structure of trochophore larva, it can be seen that above the prototroch there is a sensory plate and an apical tuft of cilia. There is also an ocellus or a simple eye present above the prototroch. Below the prototroch there is the mouth, stomach, anus, and also other structures that also include solenocyte. The function of the solenocyte seems to be the maintenance of proper internal salt-water balance. In some species, there are one or two additional ciliary rings. In some of the molluscs such as the gastropods and the bivalves, the trochophore is known to develop into a second stage before metamorphosis into an adult form. The rotifers and the larvae of invertebrates such as phoronids and bryozoans, are also sometimes considered as trochophores because of their trochophore-like appearance.
The various life-cycle trochophore stages of the annelid Pomatoceros lamarckii which belong to the family of Serpulidae are shown in the following bright-field microscopic images:
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The life cycle of the trochophore larvae are specifically shown by the early, complete, late and meta phases which further develop into an annelid in this case. Devepeding on certain conditions as explained below the larvae have the probability to develop into an adult belonging to another phylum such as molluscs. Even in those cases though the adults may be different the life-cycle stages of the trochophore larvae remain the same as given above.
In case of an indirect development there is a larval stage and an adult stage. The larvae metamorphosizes into the adult. Whenever there is a limit on the food habit scenario in the habitat where both the larvae and the adult forms are co-existing members together, more and more taxa show a significant larva. Hence, the evolutionary significance of trochophore larvae is that since they are formed in various phyla, it not only shows a common ancestral connection between the phyla which also depicts the different connections between the different phyla but also helps in the studies relating to the metamorphosis that leads to adults in different phyla from the larval stage owing to these diverse conditions.
FAQs on Trochophore
1. What Animals Have Trochophore Larvae?
Ans: Trochophore which is also known as trochophore is a small, translucent larvae that freely swims in marine water bodies. It is a special characteristic of the marine annelids and mollusks although may also be found in other phyla such as entoprocta, echiurans, sipunculans, and nemerteans. Such a correlation depicts that these larva life-cycle stages must be present in a common ancestor to the group. These larvae are usually spherical in shape and are pear shaped which are girdled by the ring of cilia, the proto-torch which enables them to swim even against the current as well.
2. Do All Mollusks Have Trochophore Larvae?
Ans: Yes, all the mollusks have trochophore larvae. Another group of organisms that have the trochophore larvae are the annelids. The life-cycle stages of the trochophore larvae are common in-between the mollusks and the annelids. This suggests that there is an evolutionary relationship between the molluscs and annelids. It is thought both the groups had to have a common ancestor that had the life-cycle of the trochophore larvae because of which the mollusc and annelid descendents also have the same life stage of the trochophore larvae. The only specificity for the molluscs is that the trochophore larva of a mollusc develops into a veliger with large ciliated lobes especially in the case of marine snails and bivalves and few of the freshwater bivalves.
3. Do Cnidarians Have Trochophore Larvae?
Ans: The trochophore larvae are specifically characterized by the presence of two bands of cilia present around the body. The diploblastic cnidarians possess an embryonic mesoderm sandwiched in-between the ectoderm and the endoderm which is developed as the trochophore larvae metamorphosizes into a cnidarian adult. Hence, it is evident that some of the cnidarians, as they come under the clade of lophotrochozoans, have trochophore larvae.
