Species Structure of the Gobiid Fish Gillichthys mirabilis 
from Coastal Sloughs of the Eastern Pacific 1 
George W. Barlow 2 
A RECENT ISSUE of Systematic Zoology ( I960, 
vol. 9, nos. 3, 4) was devoted entirely to a 
symposium entitled "The Biogeography of Baja 
California and Adjacent Seas.” One recurring 
theme was the affinity between forms occurring 
on the Pacific Coast in the Californian province 
(Hubbs, I960: 134), and those in the northern 
part of the Gulf of California. Between these 
areas, in the main part of the Gulf, the fauna 
was said to differ, being primarily Panamic in 
origin (Garth, I960; Hubbs, I960; Walker, 
I960). 
The outer-coast species found in the upper 
Gulf of California are Pleistocene relicts, now 
trapped in a cul-de-sac of warm, tropical water 
(Hubbs, 1948: 463; I960: 137). According to 
Walker (I960: 130), most of the northern 
species found there differ from their cognates 
on the Pacific Coast. As the symposium re- 
vealed, however, there has been no extensive 
study comparing populations of one species on 
the Pacific Coast with those of the same species 
in the Gulf of California. 
The immediate objective of this investiga- 
tion, although it was initiated before the sym- 
posium was conceived, was to establish the 
nature of the variation between populations of 
Gillichthys mirabilis Cooper. The goal was to 
ascertain the degree of differentiation of the 
form from the upper Gulf in relation to the 
over-all structure of the species. A brief study 
(Barlow, 1961^) of G. seta (Ginsburg) has 
served as comparative material to gauge the 
differences observed in G. mirabilis. G. seta is 
a close but distinct species restricted to an ex- 
1 A revised portion of the dissertation submitted in 
partial fulfillment of the requirements for the degree 
of Doctor of Philosophy at the University of Cali- 
fornia, Los Angeles, California. Manuscript received 
September 21, 1961. 
2 Department of Zoology, University of Illinois, 
Urbana, Illinois. 
treme rocky habitat found only in the upper 
Gulf. 
G. mirabilis has been cited as one example of 
a species from the Pacific Coast which is also 
found in the upper reaches of the Gulf of Cali- 
fornia (Fig. 1). It occurs in the central region 
of the Gulf as well. There is a break in its 
distribution around Cabo San Lucas, however, 
separating the species into a Gulf group and a 
Pacific Coast group (details in Barlow, 1961^). 
At the inception of the present investigation 
G. mirabilis from the northern part of the Gulf 
of California was considered by some ichthyolo- 
gists to be a distinct species, G. detrusus Gilbert 
and Scofield. Differences of opinion existed 
(Barlow, 19 6D), and the issue was clouded by 
comparisons with a third species, the then un- 
described Gillichthys seta. As will be seen, the 
evidence supports the conclusion of Starks and 
Morris (1907: 227) that detrusus should be 
regarded as a synonym of mirabilis. 
In 1930 G. mirabilis from the San Diego 
area in southern California was introduced into 
the Salton Sea. The fish has become well estab- 
lished in that odd saline lake in the Colorado 
Desert of California. This has proved to be a 
fortunate experiment. 
ACKNOWLEDGMENTS 
With pleasure I acknowledge the timely as- 
sistance of the following people in gathering 
material for the study: H. P. Arai, W. J. Bald- 
win, Raymond Cannon, L. H. Carpelan, G. L. 
Durall, W. I. Follett, R. H. Linsey, J. B. Makern- 
son, F. W. Munz, K. S. Norris, P. E, Pickens, 
J. H. Prescott, J. C. Quast, E. S. Reese, R. H. 
Rosenblatt, Octavio Salazar, George Sims, R. R. 
Whitney, and John Wintersteen. Charles Baxter 
kindly prepared the chart of the study region, 
and Gerta M. Barlow assisted in the preparation 
of the manuscript and many of the illustrations. 
47 
