The neurocranium of Cetorhinus maximus (GUNNERUS, 1765)
by
Kunihiko Izawa and Terukazu Shibata
(published in: IZAWA, K. & SHIBATA, T. (1993): A young basking shark, Cetorhinus maximus, from Japan. Japanese Journal of Ichthyology, 40 (2): 237-245, figs 1-4)
Snout and neurocranium:
There are no previously published references to the ventral groove under the snout. A photograph of a young basking shark caught in Portobello, English Channel, in June 1950, was included in Matthews and Parker (1950, pl. 8). Apparent traces of the ventral groove can be seen in the photograph, although the authors made no mention of it. The fish length was estimated from the photograph as 2.1 -2.4 m, but is in fact believed to have been about 3 m. judging from the snout shape. The ventral groove had disappeared in individuals a little larger than the present specimen, according to the following figures; Pavesi (1874, pl. 1, fig. 3 [2.95 m]). Pavesi (1878, pl. 3, fig. 3 [3.25 m]), Carazzi (1904, fig. 1 [3.37 m]), Gervais and Gervais (1876, pl. 13, figs. 2, 3 [3,65 m]), Gudger (1935, fig. 3 [4.36 m]) and Matthews (1962, photograph of a basking shark of uncertain length). Furthermore, the ventral groove had disappeared completely from the median rostral cartilage in an 8.53 m specimen studied by Barnard (1937, fig. 1), in which the median rostral cartilage was very stout, being almost as wide as long, and had the ventral side almost flattened, although slightly depressed.
Considering the functional implications of the juvenile snout, it seems clear that newly-born basking sharks have a long snout with a ventral groove similar to the present 2.6 m specimen. Yano (1978) has speculated that its function is connected with oophagy (following observation of the same specimen dealt with here). In addition, a second function is possible. The flap-like juvenile snout with a ventral groove would assist feeding in the early, free-living young in respect to increasing the water current into the mouth and/or feeding efficiency, compensating for a presumably lesser ability to swim and feed. Photographs of a feeding basking shark in Hallacher (1977, fig. 1) show that when the fish swims with the mouth fully open, the snout receives water directly on to the ventral side, owing to the axis of the snout and head being somewhat upwardly directed. The ventral groove on the juvenile snout would clearly enhance such a feeding attitude.






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