310 
PACIFIC SCIENCE, VoL XVII, July 1963 
TABLE 2 
Length-Frequencies of 407 H. atra Measured 
in a Pool on the Northeastern Shore of 
Kabelle Island, Sept. 14, 1961 
LENGTH 
INCHES 
NUMBER 
LENGTH 
INCHES 
NUMBER 
2 
5 
6 
98 
3 
8 
7 
29 
4 
78 
8 
14 
5 
159 
9 
8 
10 
2 
quired for measuring. Both the high tempera- 
ture and the slight, unavoidable disturbance of 
adjacent individuals when picking up specimens 
for volume measurement resulted in a small 
amount of contraction. After handling, the speci- 
mens were deposited in another part of the pool. 
Table 2 gives the length-frequencies. These 
measurements show a single major peak at about 
5 inches ( 12.7 cm), and fail to affirm the pres- 
ence of other expected frequency groups. 
Figure 5 is a histogram showing the volume- 
frequency. As with length-frequency there is 
a single peak ( at about 60 cm 3 ) . 
From lines fitted by inspection to the scatter 
diagram of Figure 6 the relationship of volume 
(V) to length (L) in the size range 5-15 cm 
was estimated to be: 
V= 5.4 L 0 - 94 
The rather abrupt increase in slope beyond 
15 cm is emphasized by the visually fitted 
line: 
V = 0.019L 30 
A few additional measurements of specimens 
longer than 25 cm suggest that this slope of 
3.0 continues throughout the upper size-range. 
TEMPERATURE TOLERANCE 
Consideration of their habitat, exposed to the 
sun in shallow pools at low tide, would suggest 
a high temperature tolerance for H. atra. On 
partly cloudy days after at least 2 hr of con- 
tinuous sunshine, the temperatures of 69 pools 
containing H. atra ranged from 31.1 to 39.4 C, 
with mean and standard deviation of 36.19 and 
2.14 C. The temperatures, determined with clin- 
ical centigrade thermometers, of eight black sea 
cucumbers were consistently higher than simul- 
taneous temperatures in the sand-coated sea 
cucumbers alongside them (by an average of 
0.25 C, standard deviation 0.16 C). After more 
prolonged exposure to sunlight this differential 
would be expected to increase. Shaded thermom- 
eters on the bottoms of the pools gave readings 
lower than within the sand-coated sea cucum- 
bers, while thermometers in direct sunlight on 
the pool bottoms approximated those within 
the black sea cucumbers. Extreme differences 
between pool- and specimen-temperatures at any 
one place and time barely exceeded 1 C Some 
degree of reflective insulation from the radiation 
of the sun is imparted to H, atra by the coating 
of light-colored sand with which it habitually 
covers its body, enabling it to retain a slightly 
lower body temperature than it does when not 
sand-coated, or than does the naked, black H, 
leucospilota when out in the open. 
The warmest pool in which a H. leucospilota 
was found was 38.4 C, but the animal was not 
feeding. The usual habitat of H. leucospilota is 
cooler than that of H. atra because of its habit 
of seeking the shade, and protection of rocks. 
Water coming in over the reef at high tide has 
a temperature of about 29 C. When this cool 
water saturates the beach it remains cool if 
overlain by strata of slab rock, but is warmed 
by the sun if the sand is exposed. H. atra oc- 
curred in pools of higher temperature, 39.4 C. 
The upper limit was not determined. Crozier 
(1915: 281) gives the maximum temperature 
of the habitat of H. surinamensis as 31.8 C and 
states that muscle coagulates at 42 C. This is 
only about 3 C above the highest pool tempera- 
tures in which H. atra has been encountered. 
H. atra is unusually tolerant of heat, and is al- 
most the only macroscopic organism living in 
these warm pools at low tide on the warmest 
days. 
Fig. 4. Photomicrographs showing calcareous deposits in the integument. A, Tables of H. atra. B, Tables 
and buttons of H. leucospilota. 
