Schizaster compactus from Western Australia 
319 
Figure 3 Variation in gonopore number in four adult 
specimens of Schizaster (Schizaster) compactus 
showing individuals with either two, three of 
four gonopores. In those specimens with 
three gonopores, the anterior gonopore may 
be either gonopore 2 or 3. Bar represents 1 
mm. 
posterior petals and the number of pore pairs in 
the ambulacra. As the test increases in size 
ambulacrum III becomes relatively narrower 
(Figure 4A), the width being 24.8%TL in the 
smallest specimen, decreasing to 18-19%TL in the 
largest specimens. A similar relative decrease in 
the width of ambulacrum III adorally was recorded 
in Schizaster (Ova) myorensis by McNamara and 
Philip (1980a, figure 6D). Ambulacrum III also 
shows a progressive increase in number of pore 
pairs throughout ontogeny, from 18 in each row in 
the smallest specimen to a maximum of 38 in the 
largest specimens (Figure 5C). However, this is not 
achieved by a lengthening of ambulacrum III and a 
posterior migration of the apical system, which is 
the situation in Schizaster (Ova) myorensis, rather 
the rate of pore pair production within the 
ambulacrum would have been greater during 
ontogeny. 
Of the other ambulacral characters, the only other 
to show significant shape changes are the posterior 
petals, which show a slight relative increase in 
length (from about 14%TL to almost 20%TL) 
(Figure 4B). This length increase coincides with the 
increase in number of pore pairs from eight to an 
average of 17 in the largest specimens (Figure 5A). 
The anterior petals similarly show an increase in 
the number of pore pairs in each row, from 17 to 
30 (Figure 5B). This persistent production of pore 
pairs throughout ontogeny also occurs in S. (Ova) 
myorensis (McNamara and Philip 1980a). Although 
there is a slight increase in length of the anterior 
petals during ontogeny, it is highly variable (see 
below). There appears to be no correlation between 
those individuals with longer anterior petals and a 
greater number of pore pairs. This implies that the 
pore pairs are relatively more closely packed in 
larger individuals. Petal width, position of the 
apical system, and test height and length all show 
little or no ontogenetic variation. This is in contrast 
to the situation in S. (Ova) myorensis, in which the 
test shape changes markedly during growth, 
becoming both relatively flatter and narrower 
(McNamara and Philip 1980a), and the apical 
system migrates posteriorly. 
Mortensen (1951, p.222) observed that the three 
specimens of S. (Schizaster) compactus at his 
disposal, which were twice the size of Koehler's 
specimens, showed a more prominent 
development of the keel in interambulacrum 5 
aborally. The Western Australian material similarly 
shows the adult specimens to have a more 
pronounced keel than the small juvenile (Figures 
1C,F). 
Test Length (in mm) 
A 
Test Length (in mm) 
B 
Figure 4 Plots of. A, width of ambulacrum III and, B, 
length of posterior petals, expressed as 
percentages of test length against test length 
for Schizaster (Schizaster) compactus. 
