Biology Reference
In-Depth Information
3.3.5 The saccopharyngiformes
The order Saccopharyngiformes is composed of four families: the
Cyematidae (bobtail snipe eels), the Saccopharyngidae (swallowers), the
Eurypharyngidae (gulpers or pelican eels) and the Monognathidae (one
jaw gulpers) (Nelson, 2006). It is evident from their common names that
all the fi shes in this order are highly peculiar in their adult body shape.
They are deep-sea inhabitants and until the classifi cation of Greenwood et
al. (1966), they had been placed in widely separated taxonomic positions.
Although some researchers have doubted that they are true bony fi shes
(Tchernavin, 1946), their larvae do appear to be leptocephali (Fig. 3g). The
shape of the fi ns of saccopharyngiform leptocephali is similar to that of
the anguilliforms. The body is deep. The snout is long and straight. The
myomeres are somewhat V-shaped.
3.4 Morphological Changes During Metamorphosis
3.4.1 Elopiform metamorphosis
The elopiforms consist of only two families and two genera,
Elops
(tenpounder) and
Megalops
(tarpon). The developmental patterns during
metamorphosis of both genera have been described in detail.
Larvae of
E. saurus
at various stages, including leptocephali,
metamorphosing larvae, juveniles and adults, were captured on the
South Atlantic coast of the United States. Sequential changes during
metamorphosis were compiled from the specimens as shown in Fig. 4
(Gehringer, 1959). The metamorphosis of
E. saurus
was divided into three
phases: early, mid, and late.
At the end of the leptocephalus stage, the larvae reach 40-45 mm in
total length (Fig. 4b). During the early metamorphic period (Fig. 4b-d), the
body shortens to about 25 mm but the head size remains constant. During
mid metamorphosis (Fig. 4e and f), a marked change in body form from a
leaf-like to a typical fi sh-like shape occurs, and the body further shortens
to about 20 mm. Subsequently, it increases in length to 25 mm. During late
metamorphosis, as the fi sh becomes a miniature adult (Fig. 4g), the length
increases from 25 to 60 mm.
At the beginning of metamorphosis, the anal and dorsal fi ns are
rudimentary or small (Fig. 4a-c) and, subsequently during metamorphosis,
these fi ns develop a distinctive shape and shift forward (Fig. 4d-g). The
pelvic fi n buds appear at the early metamorphic stage (Fig. 4d) and by
the late metamorphic stage the full complement of pelvic fi ns has formed
(Fig. 4g). The body of the leptocephalus is transparent. However, larval
melanophores are sparsely scattered along the digestive tract, between
the myomeres along the mid-lateral line of the body, on the caudal fi n,
