Zostera marina in Alaska — McRoy 
511 
origin, populating both sides of the Pacific 
Ocean in one direction and through the Indian 
Ocean and the Mediterranean Sea to both sides 
of the Atlantic Ocean in the other. Setchell 
(1935:572) suggested that a route of this sort 
could have occurred at a time, probably in the 
early Tertiary, when the Tethys Sea covered 
much of the Northern Hemisphere. The other 
possibility for dispersion is a one-way move- 
ment. This is really an argument for a one-way 
dispersion route through the Pacific Ocean 
rather than through the Indian Ocean. This 
path would result logically in the present dis- 
tribution of the species. The theory demands 
that the Atlantic and its adjacent seas were 
populated by migration through the Arctic dur- 
ing a prehistoric milder climate, which should 
be entirely possible, for, as the distribution in 
Alaska illustrates, Zostera marina is a cold-toler- 
ant species. If continuity through the Arctic 
once existed, relict populations would be ex- 
pected along the Arctic coast. These do exist in 
the White Sea, the Barents Sea, the Kara Sea, 
and Hudson Bay (Zenkevitch, 1963:195-198; 
Blinova, 1962:150; Setchell, 1920:567; Por- 
sild, 1932:91). Ekman (1953:160-164) de- 
scribes similar patterns for many species of 
marine invertebrates and vertebrates that have 
discontinuous distributions in the Pacific and 
Atlantic oceans. Additionally, Durham and 
MacNeil (1967:343) report that a large num- 
ber of species, more than 125, of marine in- 
vertebrates have dispersed from one ocean to 
the other mostly in one direction — into the 
Arctic- Atlantic ; they consider these migrations 
to have occurred during the late Cenozoic. 
The evidence available indicates that Z. 
marina originated in the western Pacific and 
dispersed to the north along the coast of Asia 
and then around to North America. Its toler- 
ance to low temperatures permitted it not only 
to cross the subarctic Pacific to North America, 
but also to populate the Arctic and move even- 
tually to both shores of the Atlantic. Perhaps 
in times of cooler climates migration continued 
into the north coast of the Mediterranean Sea 
and its adjacent seas. No other species of Zos- 
tera has this widespread distribution, nor is 
there another one tolerant to low temperatures. 
The single closely related species that has a 
similar, but more restrictive, temperature toler- 
ance is Phyllospadix scouleri Hook which is 
endemic to the temperate Pacific Ocean. This 
species provides an example of the limited dis- 
tribution of a less cold-tolerant species origi- 
nating in the same area; it never reached the 
Atlantic. The opposite situation is illustrated by 
the distribution of Zostera nana Roth, a species 
less tolerant to cold water but more so to warm 
water; it is limited to the western Pacific, but 
has also been able to move through the Indian 
Ocean to populate parts of Africa, the Mediter- 
ranean (both north and south) and parts of the 
southern Atlantic coast of Europe. 
SUMMARY AND CONCLUSIONS 
Zostera marina , eelgrass, is a common inhabi- 
tant of the Alaska coast, occurring from the la- 
goons on the north coast of the Seward Penin- 
sula to the southern limit of Alaska and beyond. 
New records of Zostera in Alaska are from 
Adak and Atka in the Aleutian Islands, Chag- 
van and Nanvak bays and Nunivak Island, and 
Lopp and Ikpek lagoons on the Seward Penin- 
sula. In Prince William Sound the distribution 
of Zostera was markedly altered by uplift asso- 
ciated with the earthquake of March 1964. 
Zostera grows in the soft sediments of shal- 
low, protected marine bays, inlets, and lagoons. 
It is excluded from large river deltas, glacial 
fjords, and arctic environments. The distribu- 
tion in Alaska is disjunct, a result of environ- 
mental restrictions rather than a lack of disper- 
sion mechanisms. Global dispersion is a result 
of oceanic circulation and waterfowl migrations. 
Vegetative growth is the most important means 
of extending coverage of a restricted area. In 
view of the present distribution of the species 
and the geological structure of the Alaska coast, 
no extensions of the range of Zostera are ex- 
pected. Populations may exist, of course, in 
remote places along the coast. 
Alterations in the present distribution are 
possible only with major geological or climatic 
changes. Seismic uplift or depression of the 
south coast of Alaska could eliminate a large 
portion of the present populations. On the 
other hand, amelioration of conditions in arctic 
waters by a change in ice conditions could per- 
mit further extension of the present range. 
The circumboreal distribution of Z. marina 
