Morphological Development of Veliger Larval and Juvenile Stages of the Surf Clam Spisula sachalinensis

Morphological development of veliger larvae and juveniles of the hatchery-reared surf clam was examined. Larvae settled by 0.26 mm shell length (SL) with the gills and the siphons remaining scarcely differentiated. Although the inner demibranch began to develop just after settlement, the outer one did not do so until about 1.3-1.5 mm SL. After settlement, the primary exhalant siphon arose at first by 0.3 mm SL. The inhalant siphon appeared between 0.4 and 0.6 mm SL, and the exhalant one between 2.2 and 2.5 mm SL. Although distinct in early juveniles, the byssal gland degenerated by 5.0 mm SL. Digestive organs developed rapidly, once a shell length of 3-5 mm SL was reached. The juvenile stage consists of four substages that differ in their mode of life. Newly settled juveniles depend on interstitial seawater because of the lack of the inhalant siphon. Development of the inhalant siphon enables the juveniles to draw in seawater lying above the bottom surface. Elongation of the siphons enables the juveniles to burrow deeper into the seabed. Juveniles almost completely attain the adult features and mode of life by 15 mm SL. The organs do not develop harmoniously in the surf clam. Some morphological changes tend to trigger transitions in the mode of life before there has been sufficient development of organs and functions required for the subsequent life style. Such inharmonious organogenesis probably results in deficiencies in adaptation at the transitions.