Population Dynamics in a Sublittoral Epifauna 
Willis E. Pequegnat 1 
At PRESENT we have little evidence that suc- 
cessional changes following predictable patterns 
occur among epifaunal communities living on 
natural rock-reefs in the shallow sublittoral of 
the open ocean. If this phenomenon does occur 
here, it can be detected by sustained observa- 
tions at a single study site. In the period from 
1957 to I960 I conducted an intensive study of 
the epifaunas of two submarine hogbacks lo- 
cated at different depths off the coast of Corona 
del Mar, California. Some observations made 
during this period indicate that both gradual 
and disruptive changes do occur in these epi- 
faunal communities. Attention is called in this 
paper to an abrupt change in population density 
of a predominant species that occurred on part 
of the shallower reef, and to the widespread 
biotal adjustments that ensued. 
The epifauna of the deeper reef (9.5-18.5 m) 
remained quite stable throughout the study pe- 
riod (Fig. 1, Reef 500). Nevertheless, this is 
a dynamic stability with an appreciable flow of 
individuals through the community structure. 
For example, in the rock oyster, Chama pellucida, 
which is dominant on the upper part of this 
reef, natality and growth counterbalance mor- 
tality from all causes and such erosive factors 
as Lithophaga burrowings, resulting in a stable 
community as determined by consecutive sam- 
plings over a prolonged period. Some seasonal 
population fluctuations were observed, especially 
among sponges and ectoprocts, but I have not 
detected there any changes that can be construed 
as serial stages moving toward a climax differing 
from any extant community. 
But this stability appears not to be charac- 
teristic of a significant part of the epifauna that 
1 Formerly at National Science Foundation, Wash- 
ington 25, D. C. Present address: Department of 
Oceanography and Meteorology, Texas Agricultural 
and Mechanical University, College Station, Texas. 
Manuscript received May 2, 1962. 
was present on the shallower reef (2-11.2 m) 
when the first observations were made (Fig. 1, 
Reef 200 ) . Although the temporal changes in 
communities observed here cannot be presented 
as a typical ecological succession, they have had 
profound influences upon the composition of 
the epifauna of this rock-reef. My purpose in 
calling attention to these changes is to stimulate 
other investigators to follow the course of simi- 
lar population shifts that may occur in regions 
under their purview. Additional data may make 
it possible to delineate the role of successional 
heterochrony in creating the complex epifauna 
present on submarine reefs at any given point 
in time. 
The central species in the present study is the 
common mussel, Mytilus edulis . Beginning in 
late January, 1959, the Mytilus population on 
the inshore reef began a decline in density, 
which was not recognized as of singular im- 
portance at the time. However, before this de- 
cline leveled off it resulted in an almost complete 
disappearance of this species from the reef. The 
basic cause is unknown, although the early sharp- 
ness of decline suggests that disease may have 
been a contributing factor. Still, the decline did 
not occur on the deeper reef, some 300 m distant, 
although it must be pointed out that the maxi- 
mum population densities here were only a 
hundredth of those on the shallower reef. In 
addition to the possibility of disease, it is known 
that an unusually large population of Pisaster 
giganteus was preying on the dense mussel popu- 
lation during preceding weeks. Although the 
decline extended through a year, the bulk of 
the Mytilus population disappeared in half this 
time. Unfortunately observations were not made 
during part of this period, nor has it been pos- 
sible to follow developments after Mytilus was 
nearly wiped out. Nevertheless, sufficient data 
are available to provide a picture of the biotal 
shifts that accompanied the decline. The dis- 
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