Fig. 5. Pattern on dorsal surface of snout of some American species and subspecies of the genus Trionyx. Note the gradual transition in pattern from that of hartwegi (b) and asper (c) to that of emoryi (h).

  1. T. ferox (UMMZ 102276, × 1/3)
  2. T. spinifer hartwegi (KU 46742, × 3/4)
  3. T. spinifer asper (KU 50842, × 1)
  4. T. spinifer pallidus (KU 2958, × 1/2)
  5. T. spinifer pallidus (KU 2934, × 1/2)
  6. T. spinifer pallidus (KU 2947, × 1/2)
  7. T. spinifer guadalupensis (TU 10165, × 2/3)
  8. T. spinifer emoryi (KU 48218, × 2/3)
  9. T. muticus muticus (KU 48236, × 2/3)

Geographic Variation

Geographic variation occurs in Trionyx spinifer and T. muticus. The variant populations of spinifer are segregated into six subspecies, those of muticus into two. In the subspecies of spinifer there is both group variation and clinal variation.

Group Variation

The six subspecies of spinifer can be separated into two groups on the basis of the juvenal pattern. One group (subspecies spinifer, hartwegi and asper) has a pattern of dark spots or ocelli of various sizes on the carapace, whereas the other group (subspecies pallidus, guadalupensis and emoryi) has a pattern of small white dots or tubercles on the carapace. The two groups differ also in the manner in which the mottled and blotched pattern first appears on the carapace of females. Usually, contrasting lichenlike figures initially surround the dark spots or ocelli on the carapace in females of the spinifer group (less evident in pallidus), whereas females of the emoryi group usually lack a contrasting pattern early in ontogeny. In general, the two groups differ in the degree of pigmentation. The spinifer group has larger marks and more contrasting patterns on the head and limbs, and more extensive pigmentation on the ventral surface than members of the emoryi group. T. ater is more closely related to those subspecies of the emoryi group but differs in having the ventral surface heavily speckled with black and an over-all blackish, dorsal coloration; the underlying pattern of ater resembles that of emoryi.

Clinal Variation

Several characters are arranged in a geographical gradient or cline. Some characters are relatively uniform and represent a terminus in the spinifer group. Some characters change gradually and successively through the subspecies pallidus and guadalupensis, and terminate in emoryi and T. ater. Some characters of ater, in turn, show affinity with T. muticus and T. ferox.

Pattern on Snout