The Growth of Arachnoides placenta (L.) (Echinoidea) 
Judith Hines and Ron Kenny 1 
ABSTRACT: Arachnoides placenta (L.) increases in diameter 7 mm during the 
first year, 4 mm during the second year, and 4 mm in the third year after meta- 
morphosis. 
The relationship between diameter and weight of the test is approximately of 
cubic form. 
The spawning period in north Queensland probably is June to July, and the 
time of metamorphosis probably September. 
The growth characteristics are compared with other echinoid species. 
The "sand dollar,” Arachnoides placenta 
(L.), has been recorded from a wide geo- 
graphical area ranging from the Andaman 
Islands through the Philippines and along the 
northern and northeastern coasts of Australia 
(Clark, 1946). The distribution of the species 
on the Queensland coast extends from Thurs- 
day Island (Clark, 1921) to Mackay (Endean, 
1953, 1956). Clark (1938) recorded some 
observations on the habits of this "sand dollar” 
from Darwin, but no data on the growth of 
the species have been traced. 
METHODS AND RESULTS 
The animals used in this study were taken 
at Lucinda (18°31'S, 1 46° 19'E) in the mid- 
dle of the range of A. placenta along the 
Queensland coast. Lucinda beach is a gently 
sloping sandy strand with many offshore shoals. 
The spring tidal range is 8 ft. At low tide 
Arachnoides is distributed from approximately 
mean sea level to below low water of spring 
tides, lying on, or a little under, the sand 
surface. 
Collections were made at daytime spring low 
tides on 10 occasions from March 1961 to 
February 1962. In collecting, all specimens 
within a narrow strip extending from the last 
high water mark to below low tide were taken. 
In general, animals were collected by hand, but 
random digging and sieving along the length 
1 Zoology Department, University College of 
Townsville, Townsville, Queensland, Australia. Manu- 
script received May 8, 1966. 
of the traverse ensured sampling of the popula- 
tion down to a diameter of approximately 
6 mm. 
Measurements of diameter were read to the 
nearest millimeter and weights of oven-dried 
whole animals to the nearest milligram. 
The temperature data were recorded at 
Townsville Harbour breakwater (19° 15'S, 
146° 50'E) and varied from a June mean of 
20.5°C to a January mean of 31.5°C (Fig. 3). 
The records of diameters were arranged in 
1-ram class intervals (Table 1), and the data 
were smoothed prior to plotting as a series of 
monthly histograms (Fig. 2). From these 
histograms the modes of frequency distribution 
of diameter were extracted and drawn as a 
growth curve (Fig. 3). 
These results show Arachnoides increasing in 
diameter from 11 to 18 mm during the Novem- 
ber to February period of the first year after 
metamorphosis. From March to October there 
is little increase in size, but a second season of 
active growth commences in October and in 
the ensuing three to four months the diameter 
enlarges from 18 to 22 mm. In the second year 
there is a further seven- to eight-month period 
with no obvious growth, followed by another 
growing season when the animal increases from 
22 to 26 mm in diameter during approximately 
four months. 
The rate of growth during the active growing 
period decreases with age, being 2.3 mm per 
month in the first growing season, 2 mm per 
month in the second, and 1.3 rnm per month 
in the third. 
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