Variability in Sonic Muscle Size and Innervation among Three Sciaenids: Spot, Atlantic Croaker, and Weakfish
Comparisons of sonic muscle width, body cavity length, and sonic muscle innervation patterns were made for three species of sciaenids: weakfish, Cynoscion regalis; Atlantic croaker, Micropogonias undulatus; and spot, Leiostomus xanthurus. Body cavity length and sonic muscle width (rostrocaudal extent), both normalized for TL, were significantly different among the three species, increasing from spot to croaker to weakfish. However, the percentage of the body cavity length including sonic muscle was greater in the croaker and weakfish than in the spot. The sonic muscles of sciaenids are innervated by a number of true spinal nerves rather than by paired occipital spinal nerves. The number of sonic nerves increased from spot (4–5 nerves from vertebrae 4–9) to croaker (6–8 from vertebrae 3–10) to weakfish (8–9 nerves from vertebrae 3–12), paralleling the increase in muscle width. Although the number of nerves and their vertebra of origin varied within each species, they were bilaterally symmetrical within an individual. The spinal nerves emerged from two (dorsal and ventral) foramina in croaker and spot and from the sole foramen observed in weakfish vertebrae. Therefore, a longer body cavity supports a wider sonic muscle that is innervated by a greater number of spinal nerves.Abstract
Mean body cavity length and sonic muscle width varied among the three species, increasing from spot to croaker to weakfish. (A) Mean ± SD body cavity length expressed as a percentage of TL. (B) Mean ± SD sonic muscle width expressed as percentage of TL. (C) Mean ± SD sonic muscle width expressed as a percentage of body cavity length. Spot, n = 8; croaker, n = 8 (4 male and 4 female); weakfish, n = 9. Means denoted with different symbols differed significantly
Drawings of sonic muscle innervation in the (A) weakfish, (B) croaker, and (C) spot. Spinal nerves with branches innervating the sonic muscle before continuing on to the hypaxial musculature are noted as dashed lines. The pathways of spinal nerves just rostral and caudal to those innervating the sonic muscle are also noted. Vertebrae are numbered from the rostral end of the fish to beyond the body cavity, but not all (25) vertebrae are shown or numbered
Photomicrographs of the vertebral foramen of (A) croaker, (B) spot, and (C) weakfish. The positions of the single or double foramina (F) are noted by the arrows. Dorsal is at the top of the image, and rostral is to the left. Scale bar = 5 mm
Line drawings showing the pathway of a sonic nerve. (A) Croaker vertebral column illustrating the general pathway of the spinal nerve innervation of the sonic muscle (not visible). Dorsal is at the top of the figure and rostral is to the left. The switch from solid to dashed line indicates where the nerve passes through and medial to the fascia separating the hypaxial and sonic muscles (drawing by T. L. Vance). BC, dorsal extent of body cavity beneath fascia; C, vertebral centrum; F, fascia lying on the sagital midline between left and right epaxial musculature; NS, neural spine containing the neural arch; PR, pleural rib; VF, vertebral foramen. (B) Cross-section of a croaker illustrating the position of the bilateral sonic muscles (SM) and the pathway of the sonic nerve (SN) from vertebrae to the lateral surface of the sonic muscle (modified from Ono and Poss, 1982). The arrow indicates the point at which the nerve crosses through (medial to) the fascia separating the hypaxial and sonic muscles. E, epaxial musculature; H, hypaxial musculature; SB, swim bladder