Minutella cf. minuta ( Indonesia )

Simon, Eric & Hoffmann, Jana, 2013, Discovery of Recent thecideide brachiopods (Order: Thecideida, Family: Thecideidae) in Sulawesi, Indonesian Archipelago, with implications for reproduction and shell size in the genus Ospreyella, Zootaxa 3694 (5), pp. 401-433 : 405-412

Minutella cf. minuta ( Indonesia)

Pl. 1, Figs 1–5; Pl. 2, Figs 1–8

Material investigated. The material was collected from the shipwreck “ Mutiara ”, water depth: 30 m, Bay of Palu, off Donggala Harbour, Donggala near Palu, Province of Central Sulawesi, Indonesia. 131 complete shells, 19 dorsal valves and 11 ventral valves were obtained from the sieved sediment. 53 complete adult specimens, nine complete juvenile specimens and 11 ventral valves were obtained from air dried samples of fragments of the metal wall of the shipwreck. Six complete adult specimens and three complete articulate juvenile specimens were obtained from fragments of the metal wall preserved in ethanol. The material investigated in this study is deposited in the collections of the Belgian Royal Institute for Natural Sciences (RBINS) and some samples are deposited in the Museum für Naturkunde Berlin.

Description. Fully grown adult shells are rather small ( Table 2 ), rarely reaching more than 2 mm, longer than wide; strongly ventribiconvex (Pl. 1, Fig. 1d–e). The relationships between ratios length/width (L/W) and width, length of dorsal valve/width (LDV/W) and width, thickness/width (T/W) and width, and length of hinge line/width (WH/W) and width are illustrated in Text-Fig. 2. The shell is endopunctate, with large punctae. The anterior commissure is rectimarginate (Pl. 1, Fig. 1e). The lateral commissure is straight. The hinge line is straight and its length represents 50 to 75% of the maximum width of the shell ( Table 2 ).

TEXT-FIGURE 2. Scatterplots of morphometric measurements of Minutella cf. minuta (Cooper, 1981) specimens. L: length (mm); W: width (mm); LDV: length of dorsal valve (mm); T: thickness (mm); WH: length of hinge line (mm). Relationships between ratios L/W and width, LDV/W and width, T/W and width, and WH/W and width. Linear regression and correlation coefficient (r) indicated.

The ventral valve is elongate, often cemented by its posterior part but sometimes additionally by its ventral surface. The interarea is transversely striated and flanks a long, slender and dorsally convex pseudodeltidium (Pl. 1, Figs 1a, 5a). The teeth are short, robust and covered with secondary shell material. Their surface is finely corrugated (Pl. 1, Fig. 6). A very narrow smooth groove, the peripheral groove (after Zumwalt 1976, p. 29; “g” in Pl. 1, Fig. 5b), runs along the internal side of the commissure. On the internal side of this groove a row of strong tubercles is developed. The valve floor is granulose or smooth, densely punctate, appears roughly striated (Pl. 1, Fig. 5a), but is not tuberculated. The hemispondylium (Pl. 1, Figs 5b, 6) comprises two long, thin prongs which are subparallel to slightly diverging anteriorly and fused with the posterior part of the ventral valve floor. No septum was observed in the ventral valve floor.

The dorsal valve is smaller than the ventral valve, transversely ovate, often gently rounded anteriorly but sometimes emarginated. Numerous growth lines are visible on the external valve surface. The valve is slightly convex and the protegulum is placed at the tip of the valve (Pl. 1, Figs 1c–d). The surface of the protegulum is wrinkled (Pl. 1, Fig. 1b). Internally, the dorsal valve has a progressively raised profile towards its posterior part (Pl. 1, Fig. 4g; Pl. 2, Fig. 1d, 1e). The outer margin is strongly tuberculated but an external very narrow smooth flange is observed (this part of the shell is easily broken and often it is only partly visible; “fl” in Pl. 2, Fig. 1d). A straight, thin, very narrow median septum is developed. Anteriorly the septum has short divergent margins. The ventral side of this septum is ornamented with small spines and a short groove is visible anteriorly but it does not extend toward the median and posterior part (Pl. 1, Fig. 4e; Pl. 2, Fig. 1a). The peribrachial ridge is heavily tuberculated. The brachial cavities are covered with canopying spicules protecting the brood pouches. The brachial cover built by the canopying spicules is rather stout, made of subovate holes having a variable diameter and a subconical structure (Pl. 2, Fig. 1b). The marsupial holes are large with irregular spiny margins. The brachial bridge is broad and its ventral side is finely denticulated (Pl. 2, Fig. 1h). The interbrachial ridge is very spiny and irregular (Pl. 1, Fig. 4h; Pl. 2, Fig. 1f). The visceral gap is relatively large and it is divided by a very slender, lamellar calcitic pole. The pole PLATE 1. Minutella cf. minuta . Size of the specimens indicated with scale bars.

Fig. 1. RBINS- RI-BRA 9. A complete articulated adult shell still attached in living position to a shell fragment. This specimen has been collected from the sieved sediment found in the ship wreck 1a: Dorsal view. 1b: Prominent protegulum with wrinkled surface. 1c: Ventral view illustrating the attachment of specimens by the ventro-posterior part of the ventral valve. 1d: Lateral view showing the irregular growth lines. 1e: Anterior view showing the ventribiconvex aspect of the shell.

Fig. 2. RBINS-RI-BRA 10. A juvenile specimen collected alive and preserved in ethanol. Dorsal valve with preserved lophophore. At this stage of growth the lophophore is schizolophe with 40 relatively thick tentacles. The median septum is emerging anteriorly.

Fig. 3. RBINS-RI-BRA 11. Specimen at a later stage of growth, collected alive and preserved in ethanol. Dorsal valve with preserved lophophore. The lophophore is schizolophe with 60 tentacles. The median septum is more developed and canopying spicules are covering the brachial cavities. 3a: Ventral view. A small fragment of the ventral valve remained attached to the dorsal valve. 3b: Detail of the lophophore tentacles that are spaced by the top of the canopying spicules. Fig. 4. RBINS-RI-BRA 12. An adult specimen collected alive and preserved in ethanol. Dorsal valve with preserved lophophore. The lophophore with up to 75 relatively thin tentacles covers the brood pouches which are protected by the canopying spicules. The narrow straight median septum is clearly defined. 4a: Ventral view. 4b: Oblique anterior view. A narrow groove is developed on the ventral side of the median septum. 4c: Lateral view. Strongly tuberculated outer margin and cardinal process. 4d: Posterior view. The visceral foramen is divided by the slender calcitic pole which is not fused with the base of the cardinal process. Lateral adductor muscles are visible on either side of the visceral foramen and diductor muscle scars on the posterior margin of the cardinal process. 4e: The same dorsal valve in ventral view, bleached for removing all organic parts. Canopying spicules are forming subovate holes (marsupial holes). 4f: Oblique anterior view. The intrabrachial ridges are spiny and the crest of the median septum is very narrow. 4g: Lateral view. The spiny intrabrachial ridges and posterior outgrowth of the calcitic pole are visible. 4h: Oblique posterior view. The marsupial holes and the posterior outgrowth of the calcitic pole are well-defined. One median adductor muscle scar in the visceral cavity is visible.

Fig. 5. RBINS-RI-BRA 13. A ventral valve of an adult specimen. 5a: Dorsal view with interarea, pseudodeltidium, teeth and roughly striated ventral valve floor. 5b: Anterior view with the hemispondylium consisting of two long prongs. Fig. 6. RBINS-RI-BRA 14. A smaller adult ventral valve of a specimen still attached in living position to the substrate. Detailed oblique anterior view with anterior part of the pseudodeltidium and hemispondylium. The teeth are corrugated.

is attached to the brachial bridge by a short conical structure which appears as a triangle from the anterior (Pl. 1, Fig. 4e; Pl. 2, Fig. 1g). The calcitic pole is never fused with the ventral surface of the cardinal process and is protruding posteriorly as a flabelliform plate which is sometimes quite broad and irregular (Pl. 2; Fig. 1e, 8c). The cardinal process is short, strong and subquadrate with thick lateral lobes. The median lobe is anteriorly subdued. Lateral adductor muscle scars, placed on either side of the visceral gap, are subovate or fan-shaped and clearly visible (“las” in Pl. 2, Fig. 1h). The schizolophous lophophore is made of 60–70 tentacles in adult specimens (Pl. 1; Figs 3a, 4a). The imprints of the lophophore groove and lophophore muscles scars are faint and arranged along the inner side of the peribrachial ridge.

Remarks. The Minutella specimens from Indonesia are morphologically very similar to Minutella minuta s.s. from Samper Bank, Madagascar, as described by Cooper, 1981, and thus assigned to Minutella cf. minuta . However, they differ in some small morphological details. The median septum has a very similar structure in both species as it is narrow and has spiny ventral margins. In M. minuta s.s. the median septum is slightly wider and a groove is reaching to the middle of its length. In M. cf. minuta the groove is not present. The canopying spicules in M. minuta s.s. are slender and the holes formed by these spicules vary in shape. In M. cf. minut a the holes formed by the canopying spicules are more regular, subovate and relatively large. The calcitic pole of M. minuta s.s. is lamellar and develops lateral outgrowths. In M. cf. minuta the calcitic pole is more slender and develops posterior outgrowths.

PLATE 2. Minutella cf. minuta . Size of the specimens indicated with scale bars.

Fig. 1. RBINS-RI-BRA 14. A small adult dorsal valve with intact canopying spicules. 1a: Ventral view. 1b: Detail of the canopying spicules forming subovate holes with subconical margins. 1c: Oblique anterior view. 1d: Oblique lateral view. A very narrow smooth flange (fl) is visible at the commissure. 1e: Oblique lateral view showing the very thin median septum and the posterior outgrowth of the calcitic pole. 1f: Posterior view. The calcitic pole is very slender and the ventral side of the intrabrachial ridges is ornamented with strong spines. 1g: Detail of the cardinal process and the brachial bridge. The median triangular elevation of the calcitic pole is well-defined. 1h: Posterior view with fan-shaped lateral adductor muscle scars and diductor muscle scars on the posterior margin of the cardinal process.

Fig. 2. RBINS-RI-BRA 15. Earliest juvenile stage of growth observed in the material collected. The brachial bridge starts to develop. The bilobed cardinal process is small and does not extend far beyond the posterior margin of the valve. The peribrachial ridge is defined by tubercles on the lateral margins. Anteriorly the peribrachial ridge is a smooth ridge. The visceral foramen is undivided. The valve floor is smooth. 2a: Ventral view. 2b: Oblique lateral view. 2c: Oblique posterior view.

Fig. 3. RBINS-RI-BRA 16. Juvenile stage with two fused subparallel spikes developing in the middle of the valve floor. The body wall is formed between the two spikes and is expanding laterally (visible on the left part, the right part is broken). The cardinal process is bilobed. The peribrachial ridge is defined by a row of pointed tubercles. The median septum and calcitic pole are absent. 3a: Ventral view. Small fragments of the ventral valve remained attached to the posterior part of the dorsal valve. 3b: Oblique lateral view. 3c: Detailed view of the posterior part of the shell. The brachial bridge is slightly damaged on the left side.

Fig. 4. RBINS-RI-BRA 17. Growth stage with a trilobed cardinal process. In the median part of the brachial bridge the calcitic pole is formed by an outgrowth pointing downwards. The intrabrachial ridges are extending laterally forming the body wall. The peribrachial ridge becomes more prominent.

Fig. 5. RBINS-RI-BRA 18. Specimen with complete dorsal valve and posterior parts of the ventral valve including the hemispondylium. In the dorsal valve the two central fused spikes are growing backwards forming a V-shaped structure. The intrabrachial ridges are reinforced. The median septum appears in the middle of the valve floor. 5a: Ventral view. 5b: Oblique anterior view. 5c: Oblique lateral view. The slender calcitic pole is well developed and its posterior outgrowth is visible.

Fig. 6. RBINS-RI-BRA 19. Slightly more advanced stage of growth. The peribrachial ridge is now defined by two rows of strong tubercles. 6a: Ventral view. 6b: Detailed view of the calcitic pole with its posteriorly directed spur. Fig. 7. RBINS-RI-BRA 10. Growth stage with the median septum reaching the anterior part of the valve and connecting to the peribrachial ridge. Several brachial cavity tubercles are secreted in order to draw the margins of the brachial lobes. 7a: Ventral view. A fragment of the posterior part of the ventral valve remained attached to the dorsal valve. 7b: Oblique anterior view. On of the intrabrachial ridge is broken.

Fig. 8. RBINS-RI-BRA 20. In this dorsal valve the construction of the brachial lobes is progressing. The median septum is now diverging anteriorly and fused to the peribrachial ridge forming a triangular structure. 8a: Ventral view. 8b: Oblique lateral view. 8c: Detailed view of the posterior region with calcitic pole. Additional outgrowths are produced by the anterior part of the brachial bridge.

However, all these characters are slightly variable in the material investigated from Donggala. Such a variation has already been pointed out by Hoffmann and Lüter (2010, pp. 150–152) between Minutella cf. minuta , material collected from Okinawa ( Japan), Minutella cf. minuta , collected from Astrolabe Reef, Dravuni, Fiji, Pacific Ocean, and Minutella cf. minuta , collected from Lizard Island, Australia. This indicates that small morphological variations do not simply allow the erection of distinct species in the genus Minutella originating from the Indo- Pacific region. However, the investigated material from Indonesia supports the observation that all Indo-Pacific forms of the genus Minutella share one common character, the blade-like median septum, in contrast to their Caribbean relatives, which exhibit an anteriorly diverging median septum (see also Hoffmann and Lüter 2010).

Tables

TABLE 2. Minutella cf. minuta (Cooper, 1981). Morphometric measurements were taken from 48 specimens. Ratios were calculated for each individual and are not based on the maximum and minimum values of the entire sample. All measured shells are dead adult specimens extracted from the sieved sediments collected from the holds of the shipwreck. L: length (mm); W: width (mm); T: thickness (mm); LDV: length of dorsal valve (mm); WH: width of the hinge line (mm). L / W = ratio length to width, T / W = ratio thickness to width, LDV / W = ratio length of dorsal valve to width, WH / W = ratio width of hinge line to width.

n = 48	L [mm]	W [mm]	T [mm]	LDV [mm] WH	L/W	T/W	LDV/W	WH/W
MIN	1.40	1.21	0.47	0.87 0.60	1.07	0.35	0.67	0.47
MAX	2.14	1.67	1.27	1.44 1.07	1.37	0.82	1.06	0.74
MEAN	1.74	1.43	0.91	1.20 0.87	1.22	0.64	0.84	0.61
Standard error	± 0.02	± 0.02	± 0.03	± 0.02 ± 0.02	± 0.01	± 0.02	± 0.01	± 0.01