Post-hatching development of circular mantle muscles in the squid Loligo opalescens

Post-hatching development of the circular muscles in the mantle of squid was studied morphometrically 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, CMP fibers increase at a much higher rate than do SMR fibers. As a result, 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 apparently remains constant to adulthood. These structural changes are consistent with developmental changes in muscular activity. During slow, jet-propelled swimming, 1-week-old animals show mantle contractions that have twice the relative amplitude and frequency of those in adults. The presence of Na-channel protein in mantle muscle was detected bio-chemically by using site-directed antibodies; the protein was found to be preferentially expressed in CMP fibers. These results suggest that SMR fibers are an important source of locomotory power at hatching, but become progressively less important during the first 8 weeks of development as CMP fibers assume the dominant role in jet locomotion.