Contributions to Zoology, 71 (1/3) (2002)Peter Damen; Wim J.A.G. Dictus: Newly-discovered muscle in the larva of Patella coerulea (Mollusca, Gastropoda) suggests the presence of a larval extensor
Discussion

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Hypothetical model for larval extension

The discovery of an undescribed muscle, the transverse muscle, allows us to present a hypothetical model to explain the extension of the larval body out of the larval shell (Fig. 5). Fig. 5A shows a fully retracted larva with a closed operculum. The larva retracts into its shell and closes the operculum when it is disturbed, e.g. by “teasing” the larva. Fig. 5B shows a larva that is partly retracted. This is an intermediate state between the fully extended and the fully retracted state that is also obtained when larvae are treated with MgCl2 to relax the muscles. When larvae are left undisturbed for some time, they can become fully extended (Fig. 5C). Since treatment with MgCl2 immobilizes the musculature, the situation of Fig. 5B represents the resting situation in which no muscular activity is present.

FIG2

Fig. 5. Hypothetical model explaining the extension of the larval body out of the larval shell. In this model, the main larval retractor (mlr) pulls the larval body back into the shell. The accessory larval retractor (alr), together with the transverse muscle (new), are the antagonists of the main larval retractor and are responsible for the extension of the larval body out of the shell. Note that in the schematic drawings only the muscles involved in retraction and extension are depicted. The gray area represents the larval body that functions as a ‘hydrostatic skeleton’. (A) Fully retracted larva. The main larval retractor (mlr) is contracted. The accessory larval retractor (alr) and the transverse muscle (new) are relaxed (extended). (B) Partly retracted larva. The main larval retractor (mlr), accessory larval retractor (alr), and the transverse muscle (new) are all partly contracted. In this intermediate state, the main larval retractor (mlr) on the one hand, and the accessory larval retractor (alr) and the transverse muscle (new) on the other hand, are in equilibrium. (C) Fully extended larva. The main larval retractor (mlr) is relaxed (extended). The accessory larval retractor (alr) and the transverse muscle (new) are contracted. For legend see figure 1. Scale bar: 50 µm.

In the 48 to 52 h old post-torsional larva of Patella coerulea, contraction of the transverse muscle, which inserts on the two lateral shell walls, will decrease the volume that the larval body can occupy within the shell. Since the body of the larva functions as a ‘hydrostatic skeleton’, this will result in extrusion of the larval body out of the shell (see Fig. 5). The function of the transverse muscle may be compared to squeezing meat out of a sausage that is opened on one side. In addition to the transverse muscle, we think that the muscle previously known as the accessory larval retractor (alr) may also be involved in the extension process. We observed that the so-called accessory larval retractor consists of four fibers (see Fig. 1A and 4C). All four fibers reached far into the mantle and did not reach the pedal region or the velum. Since the fibers of the accessory larval retractor all end in the mantle, the accessory larval retractor cannot be involved in the retraction process of the larva. Because of its location and insertion area, we propose the opposite: the accessory larval retractor is involved in the extension process of the larva. Observation of numerous partly and fully extended larvae demonstrated that in these larvae the greater part of the shell is empty, i.e., not filled with the larva body. This can also be seen in various papers, e.g., Patten (1886), Crofts (1955), and Wanninger et al. (1999). In Fig. 5 [compare to e.g., Fig. 3A in the paper of Wanninger et al. (1999), the insertion area of the accessory larval retractor at the larval shell is visible. Since the fibers of the accessory larval retractor insert ventrally on the shell and project into the mantle, and the mantle is connected to the rim of the shell, contraction of the accessory larval retractor will result in extrusion of the larval body out of the shell. Since we expect that the accessory larval retractor is responsible for the major displacement of the larval body out of the shell, this muscle fulfills the function of a main larval extensor. The transverse muscle, which is expected to play a moderate role in the displacement of the larval body, fulfills the function of an accessory larval extensor. As a result, these two muscles form the antagonists of the main larval retractor. According to this model, the accessory larval retractor should be renamed the main larval extensor, whereas the transverse muscle may also be called the accessory larval extensor. Since no accessory larval retractor is present in this model, the main larval retractor may be renamed larval retractor.

Until now, no satisfactorily explanation for the extension of the larval body out of the shell has been given in the literature. The model presented above, which incorporates a completely different function of the accessory larval retractor, together with a function of the transverse muscle, solves this problem.