LARVAL DEVELOPMENT OF A SPIONID POLYCHAETE PARAPRIONOSPIO PINNATA (EHLERS)

Abstract

2-segment larva measures 281 mk. It has 3 reddish-brown eyes. The first segment is prominent due to its large setal sac. A neurotroch runs posteriorly from the mouth and ends in a small ciliated pit which is located in segment 1. Posterior to the ciliated pit there is a region with relatively long cilia. These cilia move slowly and are directed towards the ciliated pit. There are two pairs of ciliated cells on the ventral side in segment 1. The anterior pair is directed longitudinally, while the posterior is directed diagonally. A gastrotroch is observed in segment 2. It consists of four ciliated cells which are situated two by two laterally in the segment. The pygidium has two anal cirri. Stiff cilia, which are seen to hold setae, are present dorsally on the base of anal cirri. The digestive apparatus consists of three parts and is densely ciliated inside. 7-segment larva measures 653 mk. Two pairs of eyes become dark and appear almost black. The parapodia are well developed except in segment 2, where the long larval setae are not found. Two pairs of bundles of stiff cilia , that serve to hold the larval setae along the dorsal side of the body on swimming, occur dorsally on the pygidium. Behind the pharynx a muscular oesophagus is noticeable in segments 1 and 2. It has strongly developed walls and has no cilium inside. Anlagen of palpi appear first in the 17-segment larva, lateral to the nuchal organs. Branchiae develop first in the 11-segment larva as small protuberances on segment 2. As the larva grows, they become larger and bipinnate, similar to the characteristic form of the adult. A second pair of branchiae become visible first in the 22-segment larva on segment 3. The anal cirri increase generally, but the number of them varies with individuals from none to ten. The stiff cilia on the anal cirri are shed, but another pair on the pygidium remain. As the segments are added, the muscular oesophagus shifts posteriorly, in the 24-segment larva it is located in segment 5, and in 35-segment larva it is located in segments 7 to 8. A specimen with 37 segments and a length of 5380 mk is the largest among the larvae examined. The palps aquire basal sheath. The notopodial and neuropodial lamellae are differentiated in the anterior segments except in segment 1. The neuropodial hooded hooks appear first in the 27-segment larva in segment 10 and the succeeding segments. The hooks have six small teeth in two rows above the main fang. In some of the oldest larvae, a sabre seta occurs in segment 10 and the succeeding segments. A single spinous seta, which is not of the same type as the sabre seta, occurs in some of the neuropodia except in segment 1. This kind of seta is observed only in this stage. Most of the larval anal cirri have fallen off and an anlage of an adult anal cirrus appears mid-dorsally on the pygidium. In a 36-segment settled juvenile, the peristomium grows dorsally, the neurotroch, the gastrotrochs and the telotroch fall off shortly after settlement. The ciliated pit also disappears. There is a dorsal ridge connecting the bases of the first pair of branchiae. The first segment has neither parapodial lamellae nor setae and fuses with the peristomium resulting in parapodial loss. Notopodial slended hooded hooks are present in the segments posterior to 31-35. The dorsal crests, which are present in segments 22-36 in the adults, are hardly visible in this stage. On either side of the mid-dorsal anal cirrus two small protuberances of lateral cirri become visible.