POSTHATCHING DEVELOPMENT OF CIRCULAR MANTLE MUSCLES IN THE SQUID LOLIGO-OPALESCENS

Post-hatching development of the circular muscles in the mantle of squ id was studied morphemetrically to identify structural changes and to quantify hyperplasia and hypertrophy of the muscle fibers. Superficial , mitochondria-rich (SMR) fibers and central, mitochondria-poor (CMP) fibers are present at hatching. Although both fiber types increase in size and, even more so, in number during post-hatching development, CM P fibers increase at a much higher rate than do SMR fibers. As a resul t, the relative proportion of SMR to CMP fibers shifts from about 1:1 in a hatchling to about 1:6 in an 8-week-old animal; it then apparentl y remains constant to adulthood. These structural changes are consiste nt with developmental changes in muscular activity. During slow, jet-p ropelled swimming, 1-week-old animals show mantle contractions that ha ve twice the relative amplitude and frequency of those in adults. The presence of Na-channel protein in mantle muscle was detected biochemic ally by using site-directed antibodies; the protein was found to be pr eferentially expressed in CMP fibers. These results suggest that SMR f ibers are an important source of locomotory power at hatching, but bec ome progressively less important during the first 8 weeks of developme nt as CMP fibers assume the dominant role in jet locomotion.