Introduction
Orbinia johnsoni (Moore, 1909) inhabits, according to Smith and
Carlton (1975) protected sandy beaches. This species is known from
Marin County to La Jolla, California, in both low intertidal and
subtidal locations. Hartman (1957) describes the worm as being
80-90 mm long, but the worms used in this study, after being relaxed
in magnesium chloride and preserved in formaldehyde, were found to
range from 161 to 225 mm, with a mean length of 190 mm.
Though the descriptions of Moore (1909) and Hartman (1957)
are full descriptions, they do not give any information on the
biology and feeding behavior of the orbinids. This study proposes
to contribute information on these two factors.
Two questions are raised. First, what is the population density
of the worms? Second, what is their feeding behavior? The latter
concern is further divided into four categories. First, when do they
eat? Second, how long does it take for food to travel through their
gut? Third, what do they eat? Fourth, where do they find their food?
Materials, Methods and Results
The population density of Orbinia johnsoni from the Boatworks
Beach (fig. 1) east of Mussel Pt., Pacific Grove, California, was
measured by taking a transect between 3.5 ft and -1.0 ft. tide level.
The worms were found between 3.1 ft. and -0.5 ft., seaward of a pop¬
ulation of Nerinides acuta (Spionidae). Only occasional Nephtys
californiensis (Nephtyidae) occurred with Orbinia. The sand grain
size on the beach is fairly homogeneous (+1-+20). The worms are
uncovered 3 to 4 days out of a monthly tidal cycle.
To collect the worms, a hollow metal cube with a volume of
15,625 cubic cm, enclosed on four sides and open at the top and bottom,
was pounded into the sand at intervals of 2 m throughout the length
of the transect. The cube was pushed into the sand until the upper
limits were flush with the surface of the sand. All the sand inside
the cube was then dug out and placed on a wire screen with holes
larger than the sand but significantly smaller than any organisms
living in the sand. Seawater was poured upon the screen to filter
the sand through it. After all the san was drained away, all the
orbiniids living in the sand remained scattered upon the screen. They
were then counted and recorded (fig. 2). Nine samples were taken.
The range was 7-94 worms per sample. The mean density was 27 orbiniids
per 15,625 cc.
When the worms eat was investigated by collecting worms at various
times during a 36-hr period, coinciding with a series of low tides.
Comparing volumes of fecal material from different times of day with
respect to tidal levels could disclose if there was periodicity in
their eating habits that conformed to the rise and fall of the tide.
Worms were collected at 6-hr intervals and at every low tide for a
36 hr period. Ten worms were obtained during each collection. Each
worm was brought to the laboratory within one-half hour after collection,
cleaned thoroughly of sand and mucous, and placed in a petri dish
partially filled with sea water. The worms seemed to defecate all of
their gut contents in 5-6 hours. The petri dishes were kept on a sea
table with running water for as much as 36 hours. The 36-hr collection
yielded ninety worms. The last thirty worms, however, were placed in
syracuse dishes rather than petri dishes. All of these worms died
before they had defecated completely; death was attributed to the
small size of the dishes. Only the first 60 fecal pellets (obtained
from the first 24 hrs of collection), consequently, were used for
determining fullness of the guts. After 36 hours the fecal pellets were
removed from the petri dishes using a pipette. The volume of each
fecal pellet was measured by placing it in a graduated centrifuge
tube filled with sea water. The feces, mostly sand, would always
quickly settle to the bottom. The volume of each worm was measured
by the amount of water it displaced in the tube. A ratio was attained
for each worm's defecation by dividing the volume of the fecal pellet
by the volume of the worm after defecation. The mean for each of the
six sets of ten ratios was attained to arrive at six separate data
points. The fecal pellets were then mounted on slides and preserved
for later examination.
A comparison of attained volume feces/volume of worm ratios to
tidal cycles within the 24-hr period shows a parallel relation between
high ratios and high tides, in addition to the close relation between
low ratios and low tides (fig. 3). All of the data points, however,
have extremely wide ranges. Some of the ranges vary from 0.0 to 1.0.
The standard deviation for each of the data points is extremely large,
making the means statistically insignificant.
A follow-up study was conducted in search of further evidence
that might support the finding that orbiniids have full guts during
high tides and empty guts during low tides. Worms were collected
every three hours for a 24-hr period. Eight sets of ten worms, the
product of the collection, were placed individually in separate petri
dishes partially filled with sea water which were, in turn, kept on
a sea table. Again, this procedure was completed for each set of ten
worms within one-half hour after collecting them from the field.
They were left to defecate for 36 hours. Ratios of volume of feces/
volume of worm were acquired by the same methods applied in the preceding
experiment. The means for each of the 8 sets of ten ratios was obtained
to arrive at 8 separate data points. The 80 fecal pellets were then
mounted on slides and preserved for later use.
The mean defecation over time does not appear to conform to tide
levels (fig. 4). Once again, ranges and standard deviations are very
large.
Comparing all 140 of the defecation ratios to tide levels in a
scatter diagram, where each fecal pellet's relative volume is represented
individually sheds more light on the extent to which feeding behavior
could be a function of tide (fig. 5). Here one notes low ratios
clustered about low tide levels and high ratios clustered about high
tide levels. Furthermore, primarily low ratios occur during outgoing
tides and mainly high ratios are found in incoming tides.
Determining how long it took for food to go through the worm's
gut was done by placing worms for one hour in petri dishes that were
partially filled with methylene blue stained sand. Then the worms were
cleaned off and moved to clean petri dishes filled partially with sea
water. The dishes wereplaced on a sea table and spot checked every
fifteen minutes for defecation of blue sand. Fig. 6 summarizes the
results of the four worms that did defecate the blue sand.
Discerning what the orbiniids ate was done by analyzing the
preserved fecal pellets retained from the earlier experiments. 170
slides were analyzed. These were compared to the composition of sand
samples from the orbiniids' habitat (fig. 7). The worms were found
to eat about 932 sand, 42 organic detritus, 12 diatoms and less than
1 percent of both blue green algae and bacteria. Not all the animals,
however, had extremely high percentages of sand in their fecal pellets.
Some had as low as 5 percent sand and as high as 95 percent organic
matter in the feces. This large variation justifies an analysis of the
percentages of organic matter in the fecal pellets at varying tide
levels. Such an investigation might offer an explanation for the
large percentages of organic detritus in some feces. Fig. 8 shows that
15 of the 16 fecal pellets with organic matter constituting more than
20 percent of the feces were collected during tide levels of 2 ft or
lower. Furthermore, the majority of these worms were collected during
an outgoing tide.
An examination of the size of the fecal pellets that have high
percentages of organic matter reveals that they are relatively large
in size (fig. 9). A breakdown of the feces by percentage of organic
composition shows that unidentifiable organic detritus accounts for most
of the increase in amount of organic matter (fig. 10).
To find out where in a column of sand the worms get their food
3 narrow aquaria, each 40 cm long, 40 cm deep, and 1.5 mm wide, were
filled with sand. One orbiniid was put in each aquarium with constantly
circulating sea water. The aquaria were placed in a darkroom lit
by a dim red light. Observations were made on the movements of the
thorax in two overlapping twelve hour shifts (fig. 11), in order to
determine vertical movement and hence food-getting behavior.
Discussion
The population density of the orbiniids is most highly concentrated
at 0.0 ft tidal level. Here they are found 27 per 15,625 cubic cm.
The worms are not in uniform concentrations but are clustered.
Food apparently becomes available on incoming high tides since
the guts are full at this time, even though there is a lack of statistical
significance to prove this (fig. 7). It is conceivable that the
incoming tide triggers their feeding response since high tide finds
them with their guts the fullets. It takes 3-3.5 hrs for food to pass
through the gut which is ample time for feeding during an incoming to
high tide, defecate during outgoing to low tide and be ready to feed
again for the next high tide.
Though the average worm appears to ingest sand either preferentially
or concurrently with other food (fig. 10), some worms showed upward
of 512 of their fecal contents (fig. 10) as being derived from
organic material. A possible explanation of this might be in being
near a pocket of decaying seaweed which commonly occur on this beach.
During outgoing tides the receding wave action has a tendency to leave
rows of wrack on the beach. This wrack could seep into the sand when
the tide is out and eventually provide a food source for the worms.
The worms are probably bottom feeders. In the field they were
never observed to be closer than 10 cm below the surface of the sand.
In aquaria their tendency was to go to the bottom and stay there.
Summary
Orbinia johnsoni were studied from a small sandy beach near Mussel
Pt., Pacific Grove, California, where they are most abundant at low
tide levels in fine sand. They were found to have a mean length of
190 mm.
The orbiniids were found with their guts the fullest during
incoming to high tides. It is plausible that this is when they are
feeding. It takes about 3 to 3.5 hrs for food to travel through the
length of the gut. The orbiniids eat 93 percent sand and seven percent
organic detritus. Special note should be taken that some food selectivity
appears to be involved and that high percentages of organic matter in
the feces are found in worms collected during low, outgoing tides.
Evidence suggests that the worms are bottom feeders, not coming to the
0
surface to feed.
ACKNOWLEDGEMEN
I wish to acknowledge Dr. Isabella and Dr. Donald P. Abbott for
their time, patience and endless energy.
References
Hartman, 0. 1957. Orbiniidae, Apistobranchidae, Paraonidae. Allan
Hancock Pacific Expeditions 15: 257.
Moore, J.P. 1909. Polychaetous annelids from Monterey Bay and San
Diego, California. Proc. Acad. Nat. Sci. Phila. 161: 260-262.
Smith, R. and J. Carlton (eds.). 1975. Light's Manual, Intertidal
Invertebrates of the Central California Coast. 3rd ed. Univ. of
Calif. Press. Berkeley. 716 pp.
FIGURE CAPTIONS
Figure 1: Map of a portion of Boatworks Beach, showing location of
Orbinia johnsoni.
Figure 2: Population density per 15,625 cm’. Mean density: 27; range:
7 to 94; sample size: 9. The population is most highly concentrated
at around O foot tidal level.
Figure 3: Mean defecation over time compared to tide levels.: Solid
line: tide level; dotted line: mean defecation.
Figure 4: Mean defecation over time compared to tide levels. Solid
line: tide level; dotted line: mean defecation.
Figure 5: Defecation ratios compared to tide levels. V: the worm was
collected during an outgoing tide;A: the worm was collected during an
incoming tide.
Figure 6: Time for food to go through gut.
Figure 7: Detritus: feces compared to sand samples. A: sand; B: organic
detritus; C: diatoms; D: blue-green algae; E: bacteria.
Figure 8: Organic detritus (2) in feces at varying tide levels.
V: the worm was collected during an outgoing tide; A: the worm
was collected during an incoming tide.
Figure 9: Percentage of organic matter in feces compared to fecal
volume. Each dot represents one fecal pellet.
Figure 10: Breakdown of feces by percentage of organic composition.
A: sand; B: organic detritus; C: diatoms; D: blue-green algae; E:
bacteria.
Figure 11: Positions of thorax in aquaria. Solid line: mean depth of
thorax along vertical axis, representing depth of sand; dotted line:
mean distance moved each hour. 0 - sand level.
FI GURE 1
LOWER
LIMIT
Vot
— —0.5 1t



UPPER
LIMIT
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O

ROCK
TRANSECT
SCALE
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tif
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RATIO: VOL FECES/VOL WORM
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13431 3011
m
8
RATIO: VOL FECES/VOL WORM
1A31 3011
58
5
S






I
RATIO: VOL FECES, VOL WORN
55
WORMS
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m

DETRITUS (%) IN FECES
EACH SAMPLES
SAND (%)
03
N

s.
J
5
S5
S

I
SS
ORGANIC DETRITUS (%)
55

5
8
8 8
VOLUME (ML)


. . .


. .
0.2
o

5

8
5

8
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-
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A-
DEPTH IN SAND
(CM)