Morphology
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2
3 
4
Evactinopora radiata is a single, free-living bryozoan colony which forms a star-shaped skeleton. The star shape is defined by the rays which radiate from a central core, with or without a central pit or depression. The colony has a many-sided star-shaped form with the number of rays varying from four to nine. The rounded base of the star rests on the sea floor and the rays of the star radiate upwards.
The individual bryozoan animals, the zooids, are found on the faces of the rays or blades. Although at first glance there does not appear to be a regular arrangement of the zooids, figures 3 and 4 show that the zooids appear to be arranged with relatively equal spacing along the blades. Figure 2 shows that the area along the edge of the blades has no zooids -- in fact, it appears that there is a line of demarcation between the border of the blade and the area in which the zooids are found. Figure 1, on the other hand, shows zooids all the way to the edges of the blades. We have not seen zooids on the base of the star.
There is a wide variation in the number of rays on
the colony. A comparison of our findings with those of Weller (1909) is shown
in the following table. The most common number of rays on a colony in our
collection is five rays, whereas the most common number of rays in Weller's
collection is six. There were no specimens found with three rays.
| Number
of Rays |
Our Count |
Our Percent |
Weller Count |
Weller Percent |
| 4 | 10 | 5.7% | 3 | 2.1% |
| 5 | 56 | 32.2% | 39 | 26.7% |
| 6 | 48 | 27.6% | 44 | 30.1% |
| 7 | 44 | 25.3% | 37 | 25.3% |
| 8 | 16 | 9.2% | 18 | 12.3% |
| 9 | 0 | 0.0% | 5 | 3.4% |
| Total | 174 | 100.0% | 146 | 100.0% |
Sizes of the specimens in our collection are shown in the following table.
| Number
of Rays |
Count | Avg size
Diameter (mm) |
Size Range (mm) |
| 4 | 10 | 5.6 | 4.0 - 7.2 |
| 5 | 56 | 5.5 | 3.2 -11.5 |
| 6 | 48 | 7.7 | 3.0 -12.5 |
| 7 | 44 | 7.7 | 4.1 -12.0 |
| 8 | 16 | 6.9 | 3.2 - 9.1 |
Note that the size of the colony does not increase by adding to the number of rays. This indicates that the skeleton as a whole grows as the colony ages. The thickness of the blades diminishes with increasing length, such that it is almost impossible to prepare an entire specimen from the matrix in which it is embedded.
The drawing below is a composite of several specimens of the species Evactinopora radiata, showing the morphology of the star-shaped specimen and the location of the individual zooids on the blades.
In many of the specimens, there appeared to be a pit or circular depression in the center of the bottom side of the base. Pictures of the base with and without pits is shown below. In addition, two pictures of the upper and lower sides of the same colony shows that the pit does not go entirely through the colony skeleton.
The distribution of the pits was found to be as shown
in the following table.
| Number
of Rays |
Count | Number with pit |
Percent with pit |
Percent in
Small sizes |
Percent
in Large sizes |
| 4 | 10 | 1 | 10.0% | ||
| 5 | 56 | 18 | 32.1% | 66.7% | 33.3% |
| 6 | 48 | 10 | 20.8% | 70.0% | 30.0% |
| 7 | 44 | 13 | 29.5% | 53.8% | 46.2% |
| 8 | 16 | 10 | 62.5% | 40.0% | 60.0% |
| Total | 174 | 52 | 29.5% | 63.5% | 45.5% |
The function of the pit is not known. It is possible that it is a holdfast attachment site. The larger percentage of pits were found in the smaller sizes than in the larger sizes, although this varied by the number of rays present. We have found no evidence of an attachment structure for these colonies. The center of gravity of the colony is near the thickened base of the skeleton, so the base would rest on the seafloor and the rays would be held upright. The number of rays themselves could be related to the environmental conditions, with the higher number of rays offering better stability to the colony.
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