70 
through the rigorous environment and through 
intraspecific competition. 
If the gene pool of the Salton Sea fish has 
been altered, then the morphological changes 
might be due to such genetic differences. This 
cannot be known without rearing experiments. 
Experiments on the metabolic physiology of the 
adults suggest slight genetic divergence has 
taken place (Barlow, 1961c). 
My opinion is that the morphological pecu- 
liarities of the fish from the Salton Sea can be 
ascribed primarily to direct modification caused 
by the chemical composition of the water in 
which they develop; the same could be said of 
the San Felipe fish from a saline pool. 
Conclusions 
The investigation of the species Gillichthys 
mirabilis has revealed a complex situation in 
which genetic divergence is evidently inter- 
twined with phenotypic plasticity. 
Over the range of mirabilis, the adaptive 
norms seem to shift gradually, but with some 
plateaus. A comparison of fin indices and means 
of the fin counts indicates the following ar- 
rangement : ( 1 ) Populations of mirabilis are 
nearly the same genetically on the California 
coast, though the Salton Sea and San Francisco 
populations may be somewhat distinct. (2) A 
genetic cline may exist on the outer coast of 
Baja California. (3) The fish from the lower 
region of the Gulf of California are similar to, 
but slightly different from, those on the outer 
coast of southern Baja California. (4) The pop- 
ulations found in the northern part of the Gulf 
have diverged from those of the southern part 
to a greater degree than can be accounted for, 
in a consistent manner, by environmentally in- 
duced changes. 
Investigations of the body form of mirabilis 
support the conclusions based on countable 
characters. The body form of fish from Califor- 
nia is about the same in each of the populations, 
although local differences exist. Fish from the 
Gulf of California, as a group, differ from those 
from California; no adult specimens were avail- 
able from the outer coast of southern Baja Cali- 
fornia. Within the Gulf populations, fish in the 
upper northern region of the Gulf are separable 
from those in the southern region. 
PACIFIC SCIENCE, Vol. XVII, January 1963 
Thus mirabilis is seen as one widely ranging 
species whose geographic subdivisions are con- 
sistent with known faunal regions. For example, 
the fish fauna of San Francisco Bay is isolated 
from its southern counterpart, and the degree 
of differentiation of the mirabilis population 
there reflects this separation. The broad transi- 
tion between faunas along the outer coast of 
Baja California also seems to be an area of tran- 
sition for mirabilis, though more samples are 
needed from this region. Likewise, the morpho- 
logical separation of the mirabilis of the Gulf ! 
from those of the outer coast is in keeping with 
the known distribution of this species. Finally, 
the division of mirabilis from the Gulf into 
northern and southern groups is in harmony 
with faunistic and oceanographic findings 
(Hubbs, I960; Walker, I960; Roden, 1938). 
Subspecific names are not proposed for the 
geographic variants. Until more collections are 
available it would only create confusion to name 
the geographic forms as they are known at pres- 
ent. Even if they were better known, I doubt 
that subspecific names would be of utility here. 
None of the evidence suggests that any group, j 
or groups, of populations is evolving into a dis- 1 
tinct species. The findings indicate only that in | 
this widely distributed species appreciable dif- 
ferences exist; these are thought to reflect adap- 
tive changes. 
SUMMARY 
1. Gillichthys mirabilis Cooper is found in | 
coastal sloughs from Central California south to ' 
near Cabo San Lucas, and then again in the cen- ; 
tral and northern parts of the Gulf of California. 
It has been introduced into the Salton Sea. 
2. The fish spawn from about January to 
June and reach maturity within 1 year. The lar- 
vae have a brief pelagic phase, during which 
time the species is distributed to new habitats. 
3. The head is shortest in specimens of mira- 
bilis from the Salton Sea and most depressed 
in those from the Gulf of California. The length 
of the upper jaw and the height of the median 
fins are greatest in samples from the Gulf and 
least in those from the Salton Sea. Specimens 
from the upper Gulf region have the dorsal fins 
separated more than in mirabilis from else- 
where. 
