“Dacrydium angulare n. sp.
Figs 46-48, 51.
Material: S. Atlantic, 34°35'S 17°31'E, 1849 m (993 fath.), 1958, 'Vema', st. 54, 14 animals (4 with broken shells), 1 valve, alkohol-preserved. Type-locality. Holotype and most paratypes in ZMC, except 3 paratypes in USNM no. 822398.
Diagnosis: A Dacrydium of rounded-oblong modioliform shape and with antero-ventral shell region acuminate and pointed. Anterior margin with two more or less distinct angulations, one off the lower end and one off the upper end of internal series of hinge teeth. The latter continuous, not interrupted by pit to primary ligament, and provincular series and teeth formed later not separated from each other. Primary ligament remains functional, secondary ligament absent. Distinct internal ridge antero-ventrally extending obliquely towards posterior limit of anterior adductor scar. Fine, radial striae on umbonal region. Prodissoconch rather evenly convex, 159-165 µm in diameter. Outer demibranchs present (in larger specimens). Labial palps minute. Kidneys large, sac-like.
Description: Shell (Figs 46, 47) thin, fragile, transparent-milky (possibly clear-glassy in small and fresh specimens), somewhat convex. In lateral view oblong-rounded and modioliform. Umbones small, rapidly widening, beaks very slightly prosogyrate. Antero-dorsal margin rather short, almost straight, with a more or less distinct angulation at transition to widely and evenly curving dorsal margin Vertex indistinct, above centre of longitudinal dimension or slightly behind. Posterior margin semicircular, evenly connecting with ventral margin. The latter straight or, in some larger specimens, weakly arcuate, connected by bend with antero-ventral margin. The latter with a distinct angulation below umbo (off the lower end of ventral series of hinge teeth). Antero-ventral shell region acuminate and of pointed appearance, bounded posteriad by an oblique, broad, weak and shallow furrow or sulcus (corresponding with internal ridge). Dorsal shell flexure rather weak (best seen in transparency), almost straight from dorsal flank of umbo to postero-dorsal margin, not expressed at shell edge. Shell surface with exceedingly fine commarginal lines, with somewhat stronger ones at intervals, and slightly stronger ventrally and antero-ventrally. Sharp microscopic radial striae to outside of prodissoconch boundary (resembling those of D. vitreum, Fig. 49). Shell texture generally with a microscopic radial pattern. Periostracum thin, filmy, smooth, somewhat iridescent, slightly yellowish ventrally. Shell interior somewhat dull. Anterior adductor scar distinct (Fig. 47), other scars and pallial line indiscernible. Hinge-line rather short, almost straight (Fig. 47). The series of teeth of provincular derivation and that formed later continuous, not interrupted by the fairly shallow pit to the primary ligament. Ventral series rather long, provincular teeth between prodissoconch and primary ligament persisting even in larger specimens. Dorsal series with rather gradual transition from provincular teeth to those formed later, rather broad sub-distally. Primary ligament moderately strong, secondary ligament absent. A distinct brown band of presumably fused periostracum along the series of hinge teeth, except distally, and slightly invading the latter (Fig. 47). Subligamental ridge fairly strong, but short, somewhat keeled, separated from dorsal hinge-plate by a narrow furrow. (Even though a secondary ligament is absent, the term ‘subligamental ridge' is used, as it is homologous with similar formations in other mytilids which possess it.) The antero-ventral ridge extending from ligament pit rather short and low. Separated from the former a distinct, oblique antero-ventral ridge towards posterior limit of anterior adductor scar. It becomes indistinct before reaching the ventral shell margin. – Prodissoconch (Fig. 48) almost equilateral, evenly rounded, moderately convex, somewhat swollen below provinculum, with distinct inframarginal zone (corresponding to prod. II) 7-8 µm wide ventrally, and with finely granular-punctate surface. Origin of primary ligament nearly central. Dimensions of 4 prodissoconchs (length × height, µm): 159 × 132, 161 × 133, 164 × 134, 165 × 136.
Dimensions (length × height × width, mm): 3.85 × 2.45 × 1.9 (holotype), 3.85 × 2.4 × 1.95, 3.8 × 2.45 × 1.9, 3.6 × 2.3 × 1.7, 3.5 × 2.35 × 1.7, 3.35 × 2.05 × 1.6, 3.3 × 2.15 × 1.7, 2.85 × 1.9 × 1.4, 2.75 × 1.9 × 1.3, and 2.2 × 1.5 × 1.05. Largest specimen (shell broken) about 4.1 mm long. — Angle between central region of hinge-line and ventral margin varies between 65° and 72º.
Anatomy: In general the soft parts of the species resemble those of D. ockelmanni. Anterior adductor in cross-section as large as posterior adductor, the two portions of which are distinct and of same size. Anterior retractors fine, attaching to dorsal side of umbonal cavities. They give off a ventral pair of muscles which, above the oral field, support inner and outer labial palps and attach to the sides of the entrance to the anterior extension of the infrabranchial cavity (Fig. 51). Mattson & Warén (1977) call these muscles in D. ockelmanni ‘pedo-byssal retractors'. However, the term 'labial palp suspensors' seems more appropriate. The posterior byssus retractors comprise two distinct pairs, the most posterior one strongest and near dorsal attachment more or less distinctly subdivided into 2 pairs of bundles. Foot fairly large, of normal shape. Byssus cavity distinct. None of the specimens examined have byssus threads and in one the heel of the foot and the tissues forming the byssus cavity are torn off. Outer and inner demibranchs present. According to conditions in a 2.2 mm long specimen, the former begin to appear at a shell length of 1.8-2.0 mm. Anteriorly, inner demibranchs are grown on to the sides of the oral field (Fig. 51). The first filaments of outer and inner lamellae are incomplete and attached to the latero-ventral epithelia along their whole length. The descending region of the first complete filaments (those of outer lamellae) are strong and about twice as thick as the following filaments. (Conditions in D. ockelmanni are similar.) Inner and outer labial palps minute, each with 2 blunt folds and attached over most their length (Fig. 51). As the inner palps are mere lateral extensions of the lower lip, only the outer borders of the oral groove are present. Laterally to outer palps they curve backwards and thin out. To either side of entrance to extension of infrabranchial cavity there are forward extensions of the visceral mass penetrated by the labial palp suspensors and the anterior retractors. The mouth is rather wide (flattened in specimens examined due to dorso-ventral contraction) and with low lips. Towards the stomach the oesophagal walls become rather thick and glandular. Stomach similar to that of D. ockelmanni with region of dorsal hood pigmented a dark brown. Conjoined style-sac and mid-gut only half as long as stomach, pointing slightly to the right of mid-line. Recurrent loop of intestine latero-dorsally to the left of stomach and ahead of this with a sharp anterior turn. From above hind end of conjoined style-sac and mid‑gut the rectum in the midline. Digestive glands with the number of tubules varying with size of the animal, mainly close to stomach, but all specimens examined have a pair extending far anteriad above the oral field (Fig. 51). Separation between anterior extension of infrabranchial and posterior extension of suprabranchial cavities above anterior limit of stomach. Kidneys large, sac-like, narrowing anteriad, filling lateral spaces between posterior adductor, posterior byssus retractors, gill axes and extending forward to off posterior region of stomach. The paired gonads are long sac-like organs of superficial disposition latero-dorsally. Posterior fusion of inner mantle lobes large and muscular. The gills attach slightly below its upper border. Walls forming the exhalant opening rather muscular and somewhat tubular. Dorsal longitudinal pallial muscle well developed as are inner mantle lobes surrounding inhalant and pedal opening.
Of the 6 specimens examined, 1 has empty gonads (aparently newly spent), one is a male, and four are females. The gonadal state varies greatly. One female, 3.5 mm long, has about 40 ripe and about 20 half-ripe eggs in gonads. Apparently ripe eggs (preserved) are about 95 µm in diameter, yellowish and very yolky.
Variation: This is small as to shell proportions.
Systematic remarks: The present species seems distinct from all other known dacrydiines. Its shape is rather different from that of D. modioliforme Thiele. In this respect it seems closer to D. albidun Pelseneer from the Antarctic. However, compared with the latter, it is markedly more oblong and more pointed antero-ventrally and also the anterior margin and hinge appear to differ. According to most of its shell characters and anatomy the present species is a Dacrydium s. s.
Biology: The shells of all specimens show many fine, randomly orientated scratches indicative of rough handling during collecting and sorting. This may explain why none were found with a byssus. The anatomical findings certainly indicate that the species is byssiferous. Probably, it also spins a nest. Its development is lecithotrophic and size and appearance of the prodissoconch as well as egg-size suggest a pelagic larval phase.”
(Ockelmann, 1983: 114-118)
