Comprehensive Description

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Description

Dacrydium panamensis n.sp.
Text-figs. 53, 54

 

 

1961 Dacrydium, WOLFF, p.150, fig. 19.

 

 

Material:

 

St. 716, E. Pacific, Acapulco-Panama (9°23'N, 89° 32'W), 3570 m, 6 May 1952. Gear: HOT. Bot­tom: dark muddy clay. Bottom temp.: (1.8°C). –23 specimens.

 

St.726, E. Pacific, Gulf of Panama (5°49'N, 78° 52'W), 3670-3270 m, 13 May 1952. Gear: HOT. Bottom: clay. Bottom temp.: (2.0°C). – 6 speci­mens.

 

 

Diagnosis:

 

A Dacrydium having a small umbo, an elongate resilium and a very small and indistinctly marked anterior crenulated area on the hinge. The anterior adductor scar is pear-shaped, the posterior scar small and circular.

 

 

Description:

 

The shell is white, somewhat semitransparent and has a small umbo. The antero-ventral edge is pro­jecting and the ventral edge is straight. The post­erior part of the shell is broadly rounded. The sculpture consists of a concentric striation which is extremely delicate at the umbo, becoming more distinct towards the periphery. The resilium is well developed and elongate. The anterior crenulated area is very small and not distinctly marked off. The posterior crenulated area is distinct, increasing in breadth posteriorly, and forms about 27 % of the total length of the shell. The anterior adductor scar is distinct, pear-shaped. A triangular depres­sion runs from the scar towards the hinge. The posterior adductor scar is rounded and is about half the size of the anterior one.

 

The dorsal edge has a prominent ridge running from the hinge to about half the distance to the posterior end of the shell. The soft parts: the exhalant opening is a simple slit and the mantle edge is completely smooth, without any papillae. The foot is stout and cylindrical, with a byssal slit, but none of the specimens examined appeared to have a byssus.

 

 

Measurements and proportions:

 

Nos. 1, 3, 4 are from St.726, no.2 from St. 716.

 

 

No. L H B H/L B/L

 

1 3.9 3.0 0.7 0.77 0.18

 

2 4.4 3.0 1.2 0.68 0.27

 

31 4.7 3.4 1.0 0.72 0.21

 

4 5.0 3.6 1.0 0.72 0.20

 

1. Type.

 

 

Variation:

 

From the survey it appears that there is some variation in the H/L ratio. It was also observed that the shape of the anterior end varies somewhat.

 

 

Remarks:

 

D. panamensis differs from D. vitreum in having a smaller umbo, in lacking the antero-ventral cal­losity on which the adductor scar is located. It dif­fers also in having a much smaller anterior crenu­lated area and a relatively much shorter posterior crenulated area. There are also differences in the shape and size of the adductor scars, and in the location of the posterior scar.

 

THIELE (1912, p. 226, p1. 17, fig. 9) described D. modioliforme from the Antarctic, at the "Gauss" station, at about 400 m depth. The specimen figured has a rather large umbo and a distinctly concave ventral edge, and would appear to be a species dif­ferent from the present one. THIELE (in: THIELE & JAECKEL 1931, p. 12) recorded D. modioliforme from "Valdivia" St.152, the Antarctic, 4636 m. I have examined the sample, which consists of a single specimen with dry soft parts. It seems to differ in shape from THIELE'S previous figure (1912) of D. modioliforme, for instance by having a straight ven­tral edge. On the whole it resembles D. panamensis in its general shape, but differs in its somewhat larger umbo.

 

 

Reproduction: Specimens nos. 1 and 4 from St. 726 were females, the gonads containing apparently mature eggs having diameters ranging from 67 to 74 µ. Two mature males were observed in the sample from St.726.

 

 

Distribution: Known only from the central part of the E.Pacific, 3270-3670 m, 1.8°-2.0°C.

 

 

Type: ZMUC.

 

Type locality: "Galathea" St.726.”

 

 

(Knudsen: 1970: 91-92)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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