Ascidian reproduction and larvae

Not surprisingly, these fixed organisms almost always have free-swimming larvae that find places to settle and ensure recruitment and gene flow between populations. The larval tail, with a complex array of muscles and nerves, stiffened by the rod of notochord cells and guided by pigmented sense organs, propels the larva in specified directions (rather than passively drifting around as do the ciliated larvae of most other invertebrates). Responding to gravity and light, the larvae first move up to the light and away from the sea floor for dispersal, and then down toward the sea floor and into shaded places for settlement and metamorphosis (when the tail is withdrawn into the larval haemocoele and is resorbed).

Fertilisation (either internal or external) is probably synchronised by pheromones. Life histories patterns contribute to the success of sexual reproduction, gene flow and population maintenance. In some respects, these patterns are different in solitary and colonial species. Solitary forms (with very few exceptions) have large gonads, produce large numbers of gametes and are fertilised externally. Their larvae are free-swimming for relatively long periods and are subject to dispersal away from the parents. In colonies, large numbers of replicated zooids, each with gonads, compensates for their small size (compared to the gonads in solitary species) and ensures adequate concentrations of gametes and opportunities for fertilisation. In an adaptation possibly associated directly with internal fertilisation and incubation of embryos, colonial zooids have relatively large testes and small ovaries. Retention of tailed larvae within the parent so their free swimming life is short (often less than 10 minutes) ensures settlement close to

1. bilaterally symmetrical ancestor

2. embryo of imaginary ancestor; dorsal body wall shortened to form a gut loop

5. bending up of gut loop in pallial body wall beside pharynx (phlebobranch and stolidobranch)

Figure 27.2 Diagrammatic representation of the development of the ascidian body from an imaginary bilaterally symmetrical ancestor. (Figure: P. Kott.)

1. bilaterally symmetrical ancestor

2. embryo of imaginary ancestor; dorsal body wall shortened to form a gut loop

5. bending up of gut loop in pallial body wall beside pharynx (phlebobranch and stolidobranch)

Figure 27.2 Diagrammatic representation of the development of the ascidian body from an imaginary bilaterally symmetrical ancestor. (Figure: P. Kott.)

parent colonies. This strategy maintains relatively large populations and hence opportunities for fertilisation (as does the profusion of suitable habitats in tropical locations).

Despite pressures to maintain populations by reducing the length of larval life, tailed larvae persist in most ascidian species and maintain gene flow by chains of recruitment through the vast geographic ranges that appear to characterise the tropical ascidian fauna. Direct development, without the intervention of tailed larvae occurs only in some species in open sea floor habitats.

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