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Dictycoryne truncatum (Ehrenberg, 1861)

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Benson, 1966, p. 235-237; pl. 15, fig. 1:

Dictycoryne cf. truncatum (Ehrenberg)

?Rhopalodictyum truncatum Ehrenberg, 1861, Akad. Berlin Monatsb. (1861), p. 301; Haeckel, 1887, Challenger Rept., Zool., vol. 18, pp. 589-590.

Test with three, broad, spongy arms, equally to bilaterally disposed, generally with, but in several specimens without, a spongy patagium between. Central region not spongy, consisting of 5-10 or more concentric latticed discoidal shells; arms narrow proximally for a short distance, becoming broad and thick, elliptical in section, distally. In specimens without a patagium the arms have a definite margin; in those with a patagium margin of arms indefinite, appears to merge with the spongy patagium; in several specimens arms observed only as triangular dark areas.

Measurements; based on 30 specimens from stations 46, 56, 60, 71, and 81: angle A 112°-139°, mean 126°; angle B 92°-116°, mean 108°; angle C 114°-144°, mean 127°; ratio of angle A to B 1.03-1.43, mean 1.17, of angle C to B 1.07-1.43, mean 1.18; diameter of outermost concentric shell of central region (11 specimens with number of shells ranging from 5-10) 68-117 µm; maximum breadth of arms 86-153 µm; length of base of triangular test 172-343 µm, of altitude 177-348 µm.

Remarks. Specimens lacking a patagium and having arms with a definite margin closely resemble Ehrenberg's illustration of Dictyastrum angulatum Ehrenberg (1872b, P1. 8, fig. 18) in symmetry, size, and shape of the arms. Haeckel (1887, p. 524) in his discussion of this species also states that this figure seems to represent a Spongodiscid (spongy arms), but in the same locality (Philippine Sea) he observed a true Dictyastrum of quite the same form, with concentric rings in the arms and three to four concentric rings (shells?) of the central disc. The internal structure of the arms of the Gulf specimens appears to be spongy and does not consist of distinct, internal, concentric rings. Specimens lacking the patagium correspond to the genus Rhopalodictyum Ehrenberg. Haeckel (1887, pp. 589-590) states that Rhopalodictyum truncatum Ehrenberg is probably identical with Ehrenberg's figure of Dictyastrum angulatum and differs from it apparently in the presence of spongy arms. Haeckel placed the latter species in tentative synonymy with the former. The majority of specimens of this species from the Gulf have a patagium; therefore, this species was placed in the genus Dictyocoryne Ehrenberg. Because according to Haeckel, Rhopalodictyum truncatum Ehrenberg is similar to Dictyastrum angulatum Ehrenberg but has spongy arms, and because specimens from the Gulf with spongy arms but lacking the patagium are similar in appearance to the illustration of the latter, the Gulf species is tentatively identified with the former, which, however, was not illustrated by Ehrenberg.
Popofsky (1912, p. 137, text fig. 52) illustrated a form which he tentatively identified as Euchitonia mülleri Haeckel. This form, however, is identical with D. cf. truncatum . It appears to be spongy and has an incompletely developed patagium.

Distribution. This species is cosmopolitan in the Gulf but is rare at all stations where it is present. It is absent at stations 90, 203, and 214. Its highest frequency (l.7%) was observed at station 194 in the northern Gulf, indicating some tolerance for waters of higher than average salinity and temperature. Its frequency in the southern half of the Gulf, however, is slightly greater than in the northern half. It does not appear to be controlled by upwelling.
Rhopalodictyum truncatum Ehrenberg was reported from the Philippine Sea by Ehrenberg and Haeckel and from the tropical Pacific by Haeckel. The form illustrated by Popofsky (1912, text fig. 52) which is similar to the Gulf species was reported from near the Cape of Good Hope. Due to the uncertain taxonomic status of this species little can be stated about its world-wide distribution. It appears to be confined to tropical or subtropical seas.
Benson 1966


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