V I I . DESERT FISHES
463
Vertebral numbers, and some other features of fishes, plotted as a function of temperature or other environmental factor, seem often to produce
a V-shaped curve. The number of rays, vertebrae, or body lengths, as noted
by Sweet and Kinne (1964), are minimal at some intermediate temperature or salinity. This, and other related phenomena have been discussed
by Barlow (1961c) and more recently by Blaxter (1969). Developmental
rates and sequences, along with differing sensitivities to environmental
influences by diverse structures and processes, combine to create an
exceedingly complex situation when fishes are considered as a group.
Generalizations exist, but the many exceptions in these instances often
weaken the rules.
Although salinities and temperatures have been emphasized as influencing body form, other environmental factors can also come into play. Low
oxygen tensions may induce morphological variations (Kinne and Kinne,
1962a). Increased light intensity (McHugh, 1954) and increasing duration
of light (Lindsey, 1958) have been shown to effect changes in vertebral
numbers. Lindsey suggested that some clines in meristic features may
be induced more by day length than by temperature, mostly because of
the marked regularity of change in day length with latitude.
4. Delayed Hatching
The annual fishes, cyprinodontids of Africa and South America that produce delayed-hatching eggs capable of withstanding desiccation (Myers,
1952), mostly occur in tropical zones with pronounced seasonality of rainfall. But some are distributed in semiarid regions, especially in eastern
Africa (Turner, 1964). They are a diversified group, still poorly known
taxonomically (Weitzman and Wourms, 1967). Much of the interest generated in them comes from aquarists (Myers, 1952; Sterba, 1967), and
more recently, from their possibilities for use as larvivores in mosquito control (Haas, 1965; Bay, 1966).
These annual fishes are characteristic of more or less ephemeral habitats, although some species appear to live equally well in more permanent
waters (Hildemann and Walford, 1963). Fertilized eggs may persist in
moist sediments below crusted muds of dried pools for many months
(Foersch, 1961), and even when in permanent waters they appear to go
through a period of quiescence prior to hatching (Haas, 1965). The embryos pass into a latent phase, and hatching is triggered in response to
a decline in dissolved oxygen that typically occurs soon after such a pond
is filled with water (Peters, 1963). Each species appears to have a particular threshold of oxygen (or something else) that stimulates its final development, and within a given species not all eggs develop and hatch at the
same time.
463
Vertebral numbers, and some other features of fishes, plotted as a function of temperature or other environmental factor, seem often to produce
a V-shaped curve. The number of rays, vertebrae, or body lengths, as noted
by Sweet and Kinne (1964), are minimal at some intermediate temperature or salinity. This, and other related phenomena have been discussed
by Barlow (1961c) and more recently by Blaxter (1969). Developmental
rates and sequences, along with differing sensitivities to environmental
influences by diverse structures and processes, combine to create an
exceedingly complex situation when fishes are considered as a group.
Generalizations exist, but the many exceptions in these instances often
weaken the rules.
Although salinities and temperatures have been emphasized as influencing body form, other environmental factors can also come into play. Low
oxygen tensions may induce morphological variations (Kinne and Kinne,
1962a). Increased light intensity (McHugh, 1954) and increasing duration
of light (Lindsey, 1958) have been shown to effect changes in vertebral
numbers. Lindsey suggested that some clines in meristic features may
be induced more by day length than by temperature, mostly because of
the marked regularity of change in day length with latitude.
4. Delayed Hatching
The annual fishes, cyprinodontids of Africa and South America that produce delayed-hatching eggs capable of withstanding desiccation (Myers,
1952), mostly occur in tropical zones with pronounced seasonality of rainfall. But some are distributed in semiarid regions, especially in eastern
Africa (Turner, 1964). They are a diversified group, still poorly known
taxonomically (Weitzman and Wourms, 1967). Much of the interest generated in them comes from aquarists (Myers, 1952; Sterba, 1967), and
more recently, from their possibilities for use as larvivores in mosquito control (Haas, 1965; Bay, 1966).
These annual fishes are characteristic of more or less ephemeral habitats, although some species appear to live equally well in more permanent
waters (Hildemann and Walford, 1963). Fertilized eggs may persist in
moist sediments below crusted muds of dried pools for many months
(Foersch, 1961), and even when in permanent waters they appear to go
through a period of quiescence prior to hatching (Haas, 1965). The embryos pass into a latent phase, and hatching is triggered in response to
a decline in dissolved oxygen that typically occurs soon after such a pond
is filled with water (Peters, 1963). Each species appears to have a particular threshold of oxygen (or something else) that stimulates its final development, and within a given species not all eggs develop and hatch at the
same time.