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Trends in the Change with Growth of the Proportional Dimensions in the Squaloidea
Contrasted with bony fishes, sharks have few of those characters which enable the systematist to recognise species or genera by simply counting structures. The number of gill-openings serves to distinguish two sub-orders from the remainder, while dental formulae have rather wider use within several sub-orders and particularly the Galeoidea. In the Squaloidea the number of teeth per row is generally less constant than in the Galeoidea, and is increased throughout the life of individuals of many species. Because of the lack of discernible fin rays and countable scales, shark systematists must rely mainly on morphological differences, and particularly proportional dimensions to distinguish species. Compared with differences in shape, outline or appearance, differences in proportional dimensions are readily stated or described, and therefore are used extensively. In view of this reliance on proportional dimensions it might be expected that information on the nature and extent of any change with growth of the proportional dimensions of sharks would be readily available and assembled to show the growth trends. Generally speaking this is not the case, especially in the Squaloidea, although many authors, and particularly those writing in the last few decades, do give some indication of the range of proportions in their specific descriptions. Such ranges of proportions are valuable, but unless they are qualified by information on the sizes of the specimens they refer to (and this is seldom made plain) their full value is not realised.
There would be little importance in having information on change with growth of dimensions if such change is of small extent. Judging by the paucity of references to the subject, it appears a tacit assumption by most shark systematists that change with growth is of small account. This is not supported by the present study where particular attention has been paid to growth change and where it has been found to be of considerable importance.
The following contribution to our information on growth change in sharks is based on seven species of six genera of Squaloidea, but is probably applicable in general terms to all sharks. Because of the small amount of material available for most of the species studied, the account is not regarded as definitive. Rather the aim has been to present a framework showing the main trends, which can be filled out and modified as further data become available. Methods adopted for the study have therefore been kept as simple as possible. For each species, measurements of various regions of the body were made from specimens of all sizes available, from juveniles and even embryos to the largest adults. These measurements, converted to percentages of total length, were then plotted against total length. On the assumption, following Olsen (1954, p. 391) that growth is isometric, straight lines were fitted, by eye, to the plots. Proportional dimensions, as in Table II were read from these straight lines for two size-groups only, juveniles and large adults. The size-groups are not specified in further detail because of the few data available, nor is any indication given of the scatter on the plots which was moderate but within recognisable trends.
The results, in Table II, show that the head, trunk and tail do not grow at a uniform rate, but that the trunk is a region of accelerated growth. Thus in large
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[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
| Species | Snout tip to 5th gill | 5th gill to Upper Caudal Origin | Upper Caudal Origin to Tip of Tail | 5th Gill to 1st Dorsal Spine | 1st Dorsal Spine to Pelvic Origin | Pelvic Origin to 2nd Dorsal Spine | 2nd Dorsal Spine to Upper Caudal Origin |
|---|---|---|---|---|---|---|---|
| J = Juveniles. | A = Large Adults. | C = Change with Growth. | |||||
| J A C | J A C | J A C | J A C | J A C | J A C | J A C | |
| Squalus acanthias | 22 19 -3 | 57 63 +6 | 21 18 -3 | 15 16 +1 | 13 20 +7 | 14 12 -2 | 15 15 0 |
| Squalus blainvillii | 24 21 -3 | 55 60 +5 | 21 19 -2 | 11 12 +1 | 14 19 +5 | 16 16 0 | 14 13 -1 |
| Etmopterus baxteri | 24 20 -4 | 52 61 +9 | 24 19 -5 | 13 15 +2 | 15 22 +7 | 11 11 0 | 13 13 0 |
| Centrophorus squamosus | 24 20 -4 | 56 64 +8 | 20 16 -4 | 14 18 +4 | 21 24 +3 | 9 11 +2 | 12 11 -1 |
| Centroscymnus crepidater | 26 23 -3 | 50 59 +9 | 24 18 -6 | 13 15 +2 | 19 26 +7 | 8 7 -1 | 10 11 +1 |
| Scymnodon plunketi | 20 17 -3 | 55 65 +10 | 25 18 -7 | 15 20 +5 | 17 24 +7 | 9 8 -1 | 14 13 -1 |
| 5th Gill to 1st Dorsal Origin | 1st Dorsal Origin to Pelvic Origin | Pelvic Origin to 2nd Dorsal Origin | 2nd Dorsal Origin to Upper Caudal Origin | ||||
| Dalatias licha | 27 19 -8 | 49 61 +12 | 24 20 -4 | 11 16 +5 | 17 25 +8 | 7 4 -3 | 14 16 +2 |
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adults the trunk, measured from 5th gill-opening to upper caudal origin, comes to occupy from 5% to 6% (Squalus spp.) to 10% to 12% (Scymnodon plunketi-Dalatias licha) more of the total length than it does in juveniles.
The head and tail show a corresponding proportional decrease in length, which may be uniformly distributed between them as in Squalus acanthias and Centrophorus squamosus, or the tail may show a proportionately greater decrease than the head (more than twice as great in Scymnodon plunketi), or less frequently the head may show the greater proportional decrease (Dalatias licha)
Within the trunk itself, growth is not uniform, but is fastest anterior to the pelvic fins. In most squaloids (the exceptions being some of the Dalatiidae, and the Echinorhinidae) the 1st dorsal fin is well in advance of the pelvics, and hence the anterior portion of the interspace between the 1st and 2nd dorsal fins lies in this region of accelerated growth. As a consequence the distance from pelvic fin to 1st dorsal compared with pelvic to 2nd dorsal is an increasing ratio with increase of total length. Accelerated growth also occurs anterior to the 1st dorsal fin, to at least as far forward as the pectoral axis. This is evidenced in part by the data in Table II showing the increase in length from the 5th gill to the 1st dorsal spine. However, a better indication is given by comparison of the relative positions of the pectoral fin tip when adpressed to the side and the origin of the 1st dorsal spine or fin; in large adults the level of the 1st dorsal spine or fin is from 1% to 5% of the total length rearward of its position in relation to the pectoral fin tip in juveniles. This is not due to a marked decrease in the proportional length of the pectoral fins, for compared with the total length their length may remain reasonably constant, or more often will show a slight increase.
The relationship of height to length of base of the dorsal fins between juveniles and adults differs so markedly that it is evident that a very different growth rate operates on the vertical dimension to that on the longitudinal. Compared with total length, the bases generally show a slight relative increase in length, as would be expected in view of the accelerated growth rate of the trunk region. This is not invariable as in my material of Etmopterus baxteri the 1st dorsal base decreases relative to the total length, though the 2nd dorsal base increases. In contrast the heights of both fins either decrease or remain constant compared with the total length. The consequences are that in adults the heights of the dorsal fins relative to the lengths of the bases are less than they are in juveniles. This applies even to the 1st dorsal of E. baxteri where, although the base decreases proportional to the total length, the decrease in height is even more. Diminishing dorsal fin heights therefore appear to be a growth feature of most if not all squaloids, though the same cannot be said of the Galeoidea where at least the Mako and some carcharhinids show the reverse trend in their 1st dorsal fins.