Biology of Pachygrapsus crassipes —HIATT 
141 
of Lyngbya (two species), and a sparse supply 
of Cladophora sp. It is the foregoing group of 
plants which form the greater part of the food 
consumed by P. crassipes in central California. 
In addition to the minute algae the following 
macroscopic forms are significant in the life of 
this crab: 
IJlva lactuca L.: This species is sparsely represented 
•in the first biotope on the tops and sides of the 
rocks just below high-tide level; it is of less im¬ 
portance here than in the second biotope to be 
described. 
Vucus furcatus Ag.: An abundant species which sup¬ 
plies this animal both cover and food. 
Enteromorpha sp.: A common species which pro¬ 
vides considerable food. Longer fronds furnish 
effective cover for small crabs (many of the 
small, recently molted individuals may be col¬ 
lected under this alga). 
Grateloupia Setchelii Kylin and Rbodoglossum af¬ 
fine (Harv.) Kylin: Both species are common in 
high tide pools and are consistently used by the 
animals for food. 
Endocladia muricata (Harv.) J. Ag.: This species 
grows on the rocks in and immediately below 
crevices near high-tide level. It is a relatively 
important source of food. 
The algal forms listed below are, as a group, 
less important than the above types. Algal fronds 
broken from plants lower in the strand or from 
pelagic types may often be found floating in 
the high tide-pools; these floating pieces are 
often seized by the crabs and partially devoured; 
thus, they are important integers in the life 
equation of this animal. 
Cryptopleura lobulifera (J. Ag.) Kylin: Found 
floating in tide pools at high-tide level. 
Laminaria Sinclarii (Harv.) Farlow: Found floating 
in tide pools at high-tide level. 
Macrocystis pyrifera (L.) C. A. Ag.: Found float¬ 
ing in tide pools at high-tide level. 
Pelvetiopsis limitata (Setchell) Gardner: On rocks 
and in tide pools just below high-tide level. 
Gelidium sp.: Growing in rock crevices at high- 
tide level. 
Gigartina leptorhynchos J. Ag.: Growing in tide 
pools at high-tide level. 
Iridophycus sp.: Growing on rocks just below high- 
tide level. 
Egregia Menziesii (Turn.) Aresch.: Growing on 
rocks just below high-tide level. 
Cladophora gramine a Collins: Growing in crevices 
containing sand just below high-tide level. 
Gastroclonium Coulteri (Harv.) Kylin: Growing 
on rocks just below high-tide level. 
Agardhiella Coulteri (Harv.) Setchell: Growing on 
rocks just below high-tide level. 
Corallina officinalis L.: Growing in tide pools just 
below high-tide level. 
Gigartina Harvey ana (Kuetz.) Setchell and Gard¬ 
ner: Growing on rocks in tide channels just above 
low-tide level. 
Gigartina corymbifera (Kuetz.) J. Ag.: Growing on 
rocks in tide channels just below low-tide level. 
Cryptopleura violacea (J. Ag.) Kylin: Growing on 
rocks in tide channels just above low-tide level. 
The second and less common biotope is found 
along the outer coast, as well as along man¬ 
made shore lines which are met most commonly 
in estuaries and bays. This type is characterized 
by the presence of large, relatively stable bould¬ 
ers, which occupy an area of the strand coincid¬ 
ing with the intertidal range of this species. 
On the outer coast these boulders generally rest 
upon a solid substrate of smooth rock or upon 
other boulders. Along estuaries, e.g., the Oak¬ 
land Estuary in the San Francisco Bay, granite 
boulders have been arranged two or three deep 
along the shore, extending from low- to high- 
tide level; very few project sufficiently high 
to be in a position comparable to the splash 
zone of the outer coast. At the upper limit of 
these boulders along the estuarine strand the 
terrain becomes flat, and the boulders gradually 
decrease in number and size. The crevices 
requisite for the existence of this species are 
formed by the contiguous surfaces of the 
boulders. The crab population in this habitat 
shows a progressive decrease from the large 
boulders placed low on the strand to the smaller 
ones farther shoreward. This gradient in popu¬ 
lation may be attributed to the fact that the 
boulders extending shoreward are smaller, 
stacked in a more shallow layer, and tend to be 
more deeply imbedded in mud or sand; such 
conditions manifestly reduce the number of 
available refuges. In these areas where environ¬ 
mental conditions for P. crassipes are met along 
a rocky embankment, shoreward of which occurs 
a narrow, relatively muddy tidal flat, the usual 
linear order of grapsoid habitation on the strand 
becomes inverted. In this terrain one frequently 
encounters both Hemigrapsus nudus Dana and 
H. oregonensis Dana farther shoreward than P. 
crassipes; whereas in the first biotope both these 
species frequent comparatively lower fasciations 
in the littoral zone than the subject of this study. 
