POSTEMBRYONIC GROWTH CHANGES IN THE ISOPOD PENTIDOTEA RESECATA (STIMPSON) WITH REMARKS ON THEIR TAXONOMIC SIGNIFICANCE

¹ Pacific Marine Station, College of The Pacific, Dillon Beach, California

In an attempt to find constant characteristics which may be relied upon as specifically diagnostic in the marine isopod Pentidotea resecata, the following features proved to be especially significant constant features regardless of the size of the specimen: (1) structural interrelationships of the frontal laminae, (2) epimeral plate length in relation to the length of the lateral border of the corresponding perion segment, (3) the character of the setation and certain features of peraeopod morphology, (4) general body shape.

Features showing numerical increase which was found to be directly proportional to the size of the animal and thus believed to be of very limited taxonomic utility include: (1) number of segments to the flagellum of the second antennae, (2) number of segments to the palp of the maxilliped, (3) number of setae of the maxilliped and of the peraeopods.

It would seem to be necessary, in view of the findings, to reexamine the status of species and genera which owe their existence exclusively or in part to characteristics here shown, in this species at least, to be variable in different age groups.

One species Idothea rufescens Fee, apparently based on immature specimens, is considered a synonym of Pentidotea resecata (Stimpson).