ONTOGENY OF FOREGUT MORPHOLOGY IN THE EUPHAUSIID EUPHAUSIA-PACIFICA

The ontogeny of the foregut in Euphausia pacifica was investigated by direct observations, using scanning electron microscopy with the dry fracturing method. The results show how the adult structures of the gastric mill are formed by the progressive transformation of larval structures. The gastric mill is lacking in all calyptopis stages, but the lateral process appears in calyptopis II and III. By furcilia I, the lateral process is replaced by the basic adult form of the gastric mill. The development of the gastric mill is gradual. In early furcilia stages it consists of a pair of poorly chitinized lateral teeth and a pair of undeveloped cluster spines. The number of spines on the lateral teeth increases from 2 in furcilia II to 6 in furcilia VI. The filter press does not appear in any larval stages. Significant changes in foregut morphology occur between calyptopis I and II, and between calyptopis III and furcilia I, although they do not closely follow the change of diet. It is likely, therefore, that ontogenetic changes in foregut morphology are more related to the phylogenetic history of euphausiids than to diet. Comparison of the morphology of the mandibles and the foregut for the larval stages of E pacifica indicates that there is no inverse correlation between the mandibles and the gastric mill in masticatory ability during ontogeny.