Cidaris blakei (A. Agassiz 1878)

Cidaris blakei (A. Agassiz)

Cidaris blakei.—For a complete synonymy see Mortensen, 1928, p. 307; for additional descriptions see A. Agassiz, 1883, p. 10, and Mortensen, 1928, p. 307.

The test of Cidaris blakei is moderately to slightly flattened above and below (Plate 7: figure 1), and its height is approximately two-thirds the horizontal diameter. The test is round at the ambitus, the interambulacral midzones are not sunken. There is a slight incurving of the basicoronal plates at the edge of the peristome.

The ambulacra are distinctly sinuate above the ambitus, less sinuate below. This is apparently due to the very large areoles on the interambulacral plates above the ambitus. The pore pairs are slightly oblique and the ambulacral marginal tubercles are high on each plate, tending to crowd above the inner edge of the inner pore that is low on the plate. Small granules are common on the lower edge of the plates below the marginal tubercles which are in uniform series. Adjacent to the marginal tubercle is an inner tubercle low on the plate. This inner tubercle and the marginal tubercle lie in an oblique position very nearly parallel to the oblique position of the pore pair of the same plate. If an additional inner tubercle is present on the plate, its position is commonly at random but crowding the midline edge of the plate. Although I have not observed it, Mortensen (1928, p. 307) reported the additional inner tubercles as being in a uniform series. The inner tubercles adjacent to the marginal tubercles are in a uniform series.

The areoles are large and occupy a major portion of the interambulacral plates. Above the ambitus the areoles are very large and only very newly introduced plates have rudimentary tubercles. The areoles of the two or three lowermost plates are commonly confluent. Crenulation is not diagnostic but is weakly present in the uppermost tubercles of some specimens. The scrobicular ring of tubercles is very distinct. These tubercles are considerably larger than the almost granular secondary tubercles (Plate 6: figure 3).

The uppermost primary spines of some specimens resemble the fan-shaped leaves of the ginkgo tree (Plate 8: figures 1, 2; Plate 9: figure 2). On such specimens the great widening of the spine tip is progressively decreased with descending position of the spines toward the adoral side. Spines below the ambitus are quite slender and lack the widened tip. The variation among specimens is remarkable and ranges from those as shown in Plate 8 to specimens possessing only slender-pointed tipped spines. Indeed, specimens with little or no widening of the spine tip are common. Some slender spines bear a very small flared tip, like a funnel cone. The spinules are easily visible but not thorny. The oral primary spines are rather distinctly serrate. The secondary spines are small, slender, and almost pointed, and contrast with the broader more bluntly tipped scrobicular spines.

The slender, only slightly flattened, almost pointed marginal spines of the ambulacra, slender extrascrobicular spines, and the almost granular size of the extrascrobicular tubercles are very diagnostic and are the most significant features of specimens lacking the fan-shaped spines. The very large areoles are important features also, but C. rugosa has large areoles and resembles some specimens of C. blakei.

The color was reported by A. Agassiz (1883, p. 12) as being brilliant vermilion. Most of the color has been lost from specimens stored in alcohol or dried. The spines may have been white, for they show no trace of color in the shaft.

COMPARISON WITH OTHER SPECIES.—Cidaris blakei and Cidaris rugosa have been collected together at several localities and can be easily identified when they bear their highly characteristic spines (Plate 8: figures 1, 2, 7), but some specimens of these species have only slender primary spines similar to those of Stylocidaris affinis (Plate 20: figure 4). Specimens that lack the distinctive spines may be identified by examining the extrascrobicular area of the interambulacral plates, the granular tubercles of the apical system, and the tips of the marginal spines. The extrascrobicular area of C. blakei occupies a smaller percentage of the plate and is covered with more delicate secondary tubercles than on C. rugosa (Plate 6: figures 3, 4). The granular tubercles of the apical system of C. blakei are also smaller and less prominent than those of C. rugosa (Plate 6: figures 5, 6).

The inner tubercles of the ambulacra are commonly in more uniform series in C. blakei. On specimens of C. rugosa with a single inner tubercle on each ambulacral plate a partial vertical overlap of tubercles occurs and a single zigzag series is formed along the midline (Plate 7: figure 4). Large specimens of C. rugosa commonly have inner tubercles of the ambulacra one above the other adjacent to the marginal tubercles of a single plate (Plate 7: figure 9). This condition may exist on a majority of the plates with additional inner tubercles toward the midline. These specimens tend to show a uniform series of inner tubercles. The inner tubercle arrangement is variable and therefore of little help in identification.

The marginal spines of C. rugosa are noticeably broader and more bluntly tipped than those of C. blakei (Plate 8: figures 8, 9).

DISTRIBUTION.—Cuba to Barbados in the West Indies at depths of 315 to 420 meters.

Cidaris blakei is a species of sea urchins of the family Cidaridae. Its armour is covered with spines of three types, one unique type being extended and fan-like, making it easily recognized. Alexander Agassiz first described it scientifically in 1878. It is present on the seabed in deep waters in the Gulf of Mexico and the Bahamas.