Reference: Bin/. Bull 193: 30-46. (August, 1947) Larval Shell Muscles in the Abalone Ha Hot is kamtschatkana L. R. PAGE Department of Biology, University o/'l "icloria. I'icloria. British Columbia. Canada I <S'H'JA'5 Abstract. I used light and electron microscopy to in-vestigate shell-attached muscles in larvae of Haliolis kamtschatkana Jonas, 1845, because an early descrip-tion of these muscles in //. tuberculata by Crofts ( 1937, 1955) has featured prominently in theories about gastro-pod evolution. Larval shell muscles in //. kamtschat-kana can be grouped into two categories. The first cate-gory consists of the larval retractor muscle (LRM) and the accessory larval retractor muscle (ACC); these are striated muscles in which myofilaments begin differen-tiating before the head and foot rotate relative to the pro-toconch (this rotation is known as onlogenetic torsion). Collectively, these muscles ultimately insert on tissues within the larval head and mantle, but the ACC and mantle fibers of the LRM degenerate as metamorphic competence is achieved. The second category consists of two nonstriated pedal muscles that differentiate after cephalopodial rotation. The left pedal muscle is an-chored on the back of the protoconch, to the left of the shell-attachment plaque for the LRM. It projects into the foot primarily, but also gives rise to muscle slips extend-ing into the mantle fold. The right pedal muscle is an-chored on a calcareous septum secreted along the vis-ceral rim of the protoconch. The new data force a recon-sideration of the ancestral homologues of larval shell muscles in abalone, because Crofts may have misidenti-fied the accessory larval retractor muscle as a precursor of one of the later pedal muscles. Introduction The form and development of shell-attached muscles in vetigastropods and patellogastropods, which I will call Received 2 December 1996; accepted 28 April 1997. Abbreviations: LRM = larval retractor muscle, ACC = accessory lar-val retractor muscle. 'archaeogastropods' (see Hickman, 1988; Haszprunar, 1993), have had considerable impact on theories about early gastropod evolution. Studies by Crofts ( 1937, 1955) on muscle morphogenesis in Ha/iotis tuberculata and several other archaeogastropods have been particularly influential. Crofts identified two shell muscles in larvae of//, nibercitluta: the larval (or velar) retractor muscle and the future columellar muscle. She suggested that the precocious differentiation of the larval retractor muscle relative to the columellar muscle initiates the mechanical twisting of the developing larval body known as ontoge-netic torsion. Crofts ( 1937, 1955) also indicated that the pair of larval muscles become the two adult shell muscles in H. tuberculata, a condition viewed as primitive within a molluscan class in which most members have only one adult shell muscle. The diagrams in Figure 1A, B depict Crofts' (1937, 1955) interpretation of the larval shell muscles in //. tuberculata and several other archaeogas-tropods during an early and late stage of premetamor-phic development. Crofts' (1937. 1955) descriptions endorsed and ex-tended the hypothesis that the two shell muscles in ar-chaeogastropod larvae, and their postmetamorphic de-rivatives, are bilateral homologues that have lost their ancestral symmetry (Garstang, 1929). The two are pre-sumed to be descendant remnants of serially duplicated, symmetrical pairs of dorso-ventral shell muscles as re-tained by extant monoplacophorans and chitons (Knight, 1952; Stasek, 1972; Wingstrand, 1985). This hypothesis has influenced interpretations of paleontolog-ical data (see Yochelson, 1967; Harper and Rollins, 1982; Runnegar and Pojeta. 1985) and speculations about evolutionary diversification of shell muscles among extant larval and adult gastropods (Fretter and Graham, 1962; Fretter, 1969; Haszprunar, 1985). Despite the widespread influence of Crofts' (1937, 30
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