Biology Reference
In-Depth Information
to more traditional measurement schemes in ichthyology, some landmarks were chosen to
allow duplication of those measures. Traditional measurements between these landmarks
were used in a systematic study of Pygocentrus (
Fink, 1993
), and in several geometric
morphometric studies of the evolution of piranha ontogeny and the diversification of their
body forms (e.g.
Zelditch et al., 2000, 2003a
).
Selecting landmarks on the lateral body of piranhas is relatively straightforward
because specimens are essentially two-dimensional. Most of the shape variation can be
seen in that view, and little distortion is caused by viewing the animal in a plane.
Specimen bending can occur at fixation or during preservation, and such bent specimens
were not included in analyses unless they could be manually straightened with no result-
ing distortion in the lateral body shape. Data acquisition consisted of placing the specimen
in a standard view, using a specially designed container that kept the animal's midline in
the plane defined by the top edges of the container. A piece of metric graph paper was
placed on the container's edge in the same plane for calculating size. In some cases, insect
pins of various sizes were used to make landmarks more visible. The camera was placed
so that each specimen occupied approximately the same area in the viewing field, in order
to minimize distortion.
There are few landmarks on the post-cranial lateral body of piranhas, and almost all
landmarks chosen are from around the perimeter of the body. Had the data been taken
from radiographs, some internal osteological landmarks could have been used. However,
it was decided that data would be taken from entire specimens, partly to facilitate applica-
tion to identification keys. Most of the landmarks chosen are at boundaries or extremes of
structures, or are skeletal features accessible without x-rays.
Landmark 1 represents the anterior point of the head, and is taken where the two
premaxillary bones articulate at the midline. Because this point is directly on a vertical
from the plane of the specimen, no special marking is required. The landmark involves
soft tissues, and thus could be affected by desiccation of the specimen.
Landmarks 2, 3, 7 and 12
16 all represent skeletal features, representing extremal
points, intersections of structures, or borders of bones. Landmark 2 is the anterior border
of the epiphyseal bar
a small extension of bone that spans a large fossa in the dorsal
neurocranium
and was chosen to provide information on the shape of the head. The
landmark is found by inserting a pin through the skin of the midline dorsal to the orbital
region, where the pin just penetrates past the bar into the brain cavity. Although this land-
mark is constantly available in piranhas, some related fishes show ontogenetic change in
the width of the bar, such that the bone grows anteriorly as the fish grows, independent of
head shape changes. Landmark 3 lies at the posterior tip of the supra-occipital bone of the
neurocranium. It lies just under the skin at the dorsal midline, and is found by moving a
fingernail along the midline until the junction between bone and muscle is found. A pin
is inserted at that point for purposes of digitizing. Landmark 7 represents the posterior
termination of the hypural bones of the caudal skeleton, traditionally a point used in the
calculation of standard length (tip of snout to base of caudal fin). In piranhas, there is a
concavity in the hypural bones at the lateral midline such that the bone lies anterior to the
rest of the posterior border of the caudal skeleton, so the actual point measured is where
the bone would be in other teleosts. This is less problematic than it might seem, since the
actual measurement is done at the area where the caudal fin base can be bent laterally.
