On the feeding and behaviour of the larva of Branchiostoma lanceolatum

Collections of Branchiostoma lanceolatum (Pallas) made in mid-May and mid-July at Helgoland before and after spawning have established that the larvae leave the amphioxus ground about June and therefore presumably become planktonic. Metamorphosing larvae and young adults can be collected on the ground in late August and early September and are either the same larvae returning, or others from a neighbouring ground within the same circulating current system. An examination of the gut contents of 67 larvae collected from the plankton at Helgoland in August showed that 30% of the animals had ingested calanoid copepods or other organic material of a size similar to that of the larval mouth. A few larvae had also taken small particles evidently by a ciliary mechanism. In 50% of the larvae the gut was empty. It has been found that, in addition to a muscular mouth and gill bars richly supplied with nerves, both the gut wall and the body wall are muscular and capable of passing, by peristalsis, large food masses that distend the body. The visceral muscles of the larva resemble the coelomyarian fibres of the Nematoda. The larva appears, therefore, to be both microphagous and macrophagous. Evidence from the swimming behaviour and from reports of the vertical distribution of larvae in the sea is discussed. It is suggested that the larvae normally swim upward with the mouth and gills closed and then sink passively in the horizontal position with the pharynx expanded and the open mouth directed downward. In the event of large organism such as a copepod or a mass of organic material coming into contact with the adhesive lower left surface of the larva, it could be captured by the mobile lower lip and engulfed. The straightening of the larval tail, the great increase in the number of eyecups and the growth of the metapleura at metamorphosis are suggested as factors leading to the settlement of the young adult. Attention is drawn to the possible significance of the structure of the larva in interpreting the relationships of the cephalochordates.