morphometric character

morphometric character
a measurement of a body part, e.g. head length; a character based on measurement. In fish, measurements are taken on a straight line basis, not around the curve of the body with the exception of such measurements specifically intended to measure roundness, e.g. girth. Any measurement should be defined or referenced to a standard work as some may include soft parts or only be from bony margin to bony margin. Note that North American students of fishes mostly use straight-line measures between fixed points on the body, e.g. predorsal distance is from the anterior base of the dorsal fin to the tip of the snout while older European literature and some modern works use measures between verticals, e.g. predorsal distance is from a point on the flank directly below the dorsal fin origin in the mid-line of the fish. The latter methodology means that the sum of snout length, eye diameter and postorbital length equals head length, while the former methodology would not (e.g. postorbital length would be the longest distance from the orbit to the margin of the operculum and this is often at an angle, not on a mid-line). Some measurements include:-
1) Total length - from the anteriormost part of the head to the tip of either lobe of the caudal fin when that fin is normally splayed
2) Standard length - from the anteriormost part of the snout (even when the lower jaw projects) to the end of the hypural plate (the end of the plate is found by flexing the caudal fin; in small fish it may be seen by shining a strong light through the caudal region). Standard length can be an inaccurate measurement. The end of the hypural plate is obscured by scales, flesh and caudal rays. Its position is determined by flexing the caudal fin; this flexure is taken to be the end of the hypural plate. Small fish have thin, delicate bones and the flexure may be at the anterior base of the hypural plate, at the origin of the caudal fin rays which articulate with and overlap the end of the hypural plate, or even between the last whole vertebra and the hypural plate. Large fish have a broad flexure which can give a variety of measurements by independent observers. If fish are comparatively small then strong illumination helps to discern the end of the hypural plate, in large fish this is somewhat inaccurate
3) Head length - from the anteriormost part of the snout to the bony margin of the opercle (excluding the opercular membrane)
4) Body depth - maximum straight line depth excluding fins or fleshy and scaly structures at fin bases
5) Body width - maximum distance from one side of the body to the other
6) Head depth - from the occiput vertically to the breast or lower head surface
7) Head width - the distance between the opercles when in their normal, closed position. The opercles are gently pressed into a closed position if greatly dilated
8) Snout length - from the anteriormost part of the snout or upper lip at the mid-line to the bony front margin of the orbit
9) Orbit diameter - greatest diameter between the bony rims of the orbit. This distance is not always horizontal
10) Postorbital length - greatest distance between the posterior bony orbit margin and the bony opercular margin
11) Interorbital width - least bony width between the orbits over the top of the head in a straight line
12) Predorsal length - from the base of the anteriormost dorsal fin ray to the tip of the snout or upper lip
13) Prepelvic length - from the base of the anteriormost pelvic fin ray to the anteriormost point on the head (snout or upper lip)
14) Preanal length - from the base of the anteriormost anal fin ray to the anteriormost point on the head (snout or upper lip)
15) Length of caudal peduncle - the oblique distance from the insertion of the anal fin to the mid-point of the end of the hypural plate
16) Depth of caudal peduncle - the least depth of this structure from the mid-line of the ventral surface
17) Length of the longest dorsal and anal fin rays - from the structural base of the ray to its tip
18) Length of the dorsal and anal fin bases - from the anteriormost ray base (the origin of the fin) to the point where the fin membrane contacts the body behind the last ray (the insertion of the fin)
19) Length of the pectoral and pelvic fins - from the extreme base of the uppermost, outermost or anteriormost ray to the tip of the fin
20) Distance between pectoral and pelvic fin bases - used principally in Cyprinidae and Cobitidae, this and the following measurement are from the extreme base of the anteriormost, uppermost or outermost ray of the appropriate fin to the anterior base of the next fin
21) Distance between the pelvic and anal fin bases - as above
22) Length of fin spine - from the base of the spine to its tip. In pungent spines, as in catfishes, this excludes soft rays or membranes distal to the sharp tip, but in more flexible spines, which may taper gradually as in Cyprinidae, this measurement includes the soft tip

Dictionary of ichthyology. 2009.

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