Thecidellina mawaliana, Simon & Lüter & Logan & Mottequin, 2018

Simon, Eric, Lüter, Carsten, Logan, Alan & Mottequin, Bernard, 2018, Recent thecideide brachiopods (Thecideida, Thecideoidea) from northern Sulawesi (Indonesia) with discovery of a new Thecidellina species (Thecidellinidae), Zootaxa 4526 (4), pp. 481-515 : 485-492

Thecidellina mawaliana sp. nov.

Table 1; Text-Figs. 2–4; Pls. 1–6

Holotype: RBINS–BT.9 (Pl. 1, Fig. 2a–g, Pl. 5, Fig. 4): articulated adult specimen.

Paratypes: RBINS–BT.10 (Pl. 1, Fig. 1a–f): articulated adult specimen . RBINS–BT.11 (Pl. 3, Fig. 1a–n): adult dorsal valve with complete development of the canopies on brachial cavities. RBINS–BT.12 (Pl. 2, Fig. 1a–g): small articulated specimen with brachial cavities still without canopying spicules and with a planodeltidium curving to right side. RBINS–BT.13 (Pl. 2, Fig. 2a–g, Pl. 4, Fig. 2a–c): juvenile articulated specimen. RBINS– BT.14 (Pl. 2, Fig. 3): small articulated specimen with its planodeltidium curving to the left side. RBINS–BT.15 (Pl. 3, Fig. 2a–f): adult ventral valve with teeth and intact hemispondylium. RBINS–BT.16 (Pl. 4, Fig. 1a–d): a very early juvenile dorsal valve. RBINS–BT.17 (Pl. 4, Fig. 3a–f): juvenile dorsal valve with a peribrachial ridge beginning its development and with appearing posterior outgrowths from the brachial bridge. RBINS–BT.18 (Pl. 4, Fig. 4a–c): juvenile dorsal valve with tubercles beginning the building of the brachial cavities. RBINS–BT.19 (Pl. 5, Fig. 1a–g): dorsal valve with developed median septum, first appearance of calcitic pole and continuing the development of brachial cavities. RBINS–BT.20 (Pl. 5, Fig. 2a–e): dorsal valve with brachial bridge and peribrachial ridge connected. RBINS–BT.21 (Pl. 5, Fig. 3): adult dorsal valve with brachial cavities with incomplete canopies. RBINS–BT.22 (Pl. 5, Fig. 6), RBINS–BT.23 (Pl. 5, Fig. 7), RBINS–BT.24 (Pl. 5, Fig. 5a–b), and RBINS–BT.25 (Pl. 5, Fig. 8a–b): several adult dorsal valves illustrating the variation of the canopies covering the brachial cavities. RBINS–BT.26 (Pl. 6, Fig. 1), RBINS–BT.27 (Pl. 6, Fig. 2), RBINS–BT.28 (Pl. 6, Fig. 3), RBINS–BT.29 (Pl. 6, Fig. 4), RBINS–BT.30 (Pl. 6, Fig. 5), and RBINS–BT.31 (Pl. 6, Fig. 7): several dorsal valves illustrating the structure and development of the lophophore. Aspects of internal ventral parts of the animal.

ZMB–Bra 2472 (Pl. 6, Fig. 6): ventral valve of a female specimen, preserved in ethanol, observed under binocular and showing the gonad with spherical female gametes.

Etymology. The species name is derived from “Mawali” the name of the shipwreck where it has been found for the first time.

Type locality. The “Mawali” shipwreck (1°26’46.74”N / 125°13’32.16”E), Lembeh Strait, North Sulawesi Province, Indonesia.

Additional material. The material studied is composed of two types of samples. The first type is dried material collected either from sieved sediment or from specimens scraped off the walls of the hold or living specimens attached on oyster shells. The second type consists of oyster shells with living specimens directly placed in ethanol by the collector. The dried material represents 285 specimens whereas the material preserved in ethanol consists of 206 specimens (both juveniles and adults). Note that many specimens in this latter sample were already dead but still attached to their support and that truly living specimens when collected were less abundant.

Diagnosis. Small-sized Thecidellina species. Elongated drop-like ventral valve with striated, triangular, flat planodeltidium. Hemispondylium arising from posterior part of the valve, connected to the ventral valve floor for only a short part of its length, with long and slender prongs acutely pointed reaching the level of the cyrtomatodont teeth. Ventral valve floor without median ridge, very coarsely tuberculate except in the deep ovate gonad pits, which have a smooth surface. Dorsal valve lid-like. Brachial bridge and intrabrachial ridge firmly connected by wide posterior outgrowths. Calcitic pole, thick, attached to dorsal valve floor and connected with anterior expansions to the posterior parts of the brachial bridge in fully adult specimens. Median septum high, thin, with acute tip, ventrally convex, tuberculate-spinous on ventral surface. Brachial cavities or brood pouches covered with thin, fragile irregular canopying spicules that become fused together through ontogeny but leaving always a lot of randomly disposed small cavities at maturity.

Mendiagnosia. Thecidellina spesies berukuran kecil. Memanjang katup ventral tetes berbentuk dengan lurik, segitiga planodeltidium. Hemispondylium timbul dari bagian posterior katup, tak lama terhubung ke lantai katup ventral, berbentuk garpu dengan dua pisau panjang dan ramping mencapai tingkat gigi cyrtomatodont. Lantai katup ventral, tanpa ridge, sangat kasar tuberculate kecuali gonads lubang-lubang dengan permukaan halus. Katup dorsal seperti penutup kaleng. Jembatan brakialis dan punggung peribrachial tegas tersambung dengan posterior persimpangan piring-piring. Calcitic pole, tebal, bergabung ke lantai katup ventral. Dalam spesimen sepenuhnya dewasa, bagian depan dari calcitic pole juga dihubungkan oleh ekspansinya ke bagian posterior jembatan brakialis. Median septum tinggi, dengan ujung akut, ventally cembung, dengan tuberkel kecil di permukaan ventral. Brakialis parit-parit (atau merenung kantong) ditutupi dengan tipis dan rapuh spikulae sekering bersama-sama melalui ontogeni untuk berbentuk satu cakupan tetapi selalu meninggalkan rongga-rongga kecil dengan bentukbentuk sangat variabel.

Description. External shell characters. Relatively small-sized, whitish thecideide ( Table 1), with endopunctate shell, slightly longer than wide in adult stage but often as long as wide in juvenile stages of growth (Pl. 1, Figs. 1a, 2a; Pl. 2, Figs. 1a, 2a). The external surface of the shell is marked by many growth lines with variable thickness. The shell is strongly ventri-biconvex, with the maximum thickness situated at mid-length or slightly backward of this for the ventral valve. The maximum thickness for the dorsal valve corresponds to the position of the protegulum (Pl. 1, Figs. 1c, 2d). The lateral commissure is variable being dorsally concave (Pl. 2, Fig. 2c), rectimarginate (Pl. 2, Fig. 1d) or slightly sinusoidal (Pl. 1, Figs. 1c, 2d). The anterior commissure is rectimarginate (Pl. 1, Fig. 2f).

The deep ventral valve has an ellipsoid to hemispherical outline in lateral profile (Pl. 1, Figs. 1e, 2d) and a very faint sulcus is sometimes developed in the middle of the anterior part of the valve (Pl. 1, Fig. 2e–f). The ventral valve is cemented to the substrate by its posterior part and often by a large portion of its ventral side (Pl. 1, Fig. 2e). The adult shell is often lifted from the substrate anteriorly (Pl. 1, Fig. 1e), its anterior part tending to be always more elevated than its posterior part. Such a development of the ventral valve is common in thecideide brachiopods as already illustrated by Pajaud (1970, p. 219, fig. 130A).

The interarea is a planodeltidium ( Logan & Baker 2013) with an isosceles triangular outline, and a flat (Pl. 1, Fig. 2d) or slightly concave (Pl. 1, Fig. 1d–e) surface. The interarea represents 22–31% of the length of the shell ( Table 1). Due to variable habitat conditions the planodeltidium can be curved to the left or the right sides (Pl. 2, Figs. 1a, 3). The surface of the interarea is striated with parallel growth lines. The hinge line is straight without a notch. The interarea and the triangular posterior part of the dorsal valve form a sharp angle at the level of the hinge line (Pl. 1, Figs. 1a–b, 2a–b) but the value of this angle is highly variable (Pl. 1, Fig. 2d; Pl. 2, Fig. 1c).

The dorsal valve, much smaller than the ventral valve, has a lid-shaped outline, subcircular to elongate oval and it is sometimes slightly emarginate (if a faint sulcus is developed on the anterior part of the ventral valve). The weak convexity of this valve culminates posteriorly with the protegulum. The surface of the dorsal valve is smooth except for the development of irregular growth lines. The protegulum is circular, always very distinct and its external surface is covered with numerous pustules (Pl. 1, Figs. 1c, 2c; Pl. 2, Fig. 2f). The posterior part of the dorsal valve is obtusely triangular with a flat striated surface.

Most of the morphological characters increase regularly and proportionally during growth as indicated by the linear relations observed between width, length, length of dorsal valve and width of the hinge (Text-Fig. 3). However, the thickness of the shell follows a sigmoid development. This is due to the fact that juveniles are extremely flat and the thickness is increasing very slowly at the beginning of growth. When the shell length reaches 2 mm, the thickness suddenly increases very rapidly. This type of growth is commonly observed in the development of Thecidellina species (Text-Fig. 3).

Internal shell characters. The ventral valve has an internal subcircular or sometimes subcordiform outline (Pl. 3, Fig. 2a). A ventrally concave, narrow, smooth peripheral rim is developed along the commissure (Pl. 3, Fig. 2a, 2c–d). Along the internal side of this rim, a row of regular tubercles is developed (Pl. 3, Fig. 2a–c).

The ventral valve floor, without a differentiated median ridge, is strongly tuberculate (with tubercles of regular size) except inside the two very deep oval pits where the gonads are situated (Pl. 3, Fig. 2a, 2c, 2g). The gonad pits are relatively large in this species, their length being frequently up to 40% of internal valve length (Pl. 3, Fig. 2a). The endopunctation is clearly visible between the tubercles of the valve floor.

The hemispondylium is made of two parallel, narrow plates with long pointed curved prongs attached to the posterior part of the valve and also to the valve floor for a very short distance (Pl. 3, Fig. 2e). The delthyrial cavity extends under the planodeltidium and has a wide dorsal ridge forming, in anterior view, the form of a letter “M” (Pl. 3, Fig. 2e). Large, oval kidney-shaped lateral adductor muscle scars are developed on either side of the hemispondylium and teeth (Pl. 3, Fig. 2f).

The cyrtomatodont, robust and short teeth are covered with secondary shell material and are obtusely pointed (Pl. 3, Fig. 2a, 2d–e).

The dorsal valve is slightly convex, subcircular with a cordiform anterior outline. A flat, relatively smooth flange is externally visible along the commissure. The external side of the peripheral ridge is ornamented with thick tubercles and its crest shows regular tubercles till its junction with the brachial bridge (Pl. 3, Fig. 1h). A clearly defined peripheral rim is not present.

In lateral profile the internal structures are raised towards the posterior part (Pl. 3, Fig. 1h). The angle formed between the anterior limit of the valve, the posterior part of the peripheral ridge and the top of the brachial bridge has a value of 17.5 degrees (Pl. 3, Fig. 1j). The median septum is straight, wide anteriorly, tapering posteriorly, and very thin in its posterior part. In lateral view the median septum is ventrally convex. The crest of the septum is covered with one row of tubercles decreasing in size from the anterior base to the tip (Pl. 3, Fig. 1g).

The intrabrachial ridge is clearly defined and is roughly tuberculate (Pl. 3, Fig. 1h). Marsupial orifices are situated in the posterior part of the intrabrachial ridge, on either side of the septal tip (Pl. 3, Fig. 1a, 1g). These orifices are small sub-circular holes (Pl. 5, Fig. 2a, 2e) confined by an extremely thin and fragile calcitic rim on their ventral side. For this reason, they appear often broken as an open circle (Pl. 3, Fig. 1a–b; Pl. 5, Figs. 5a, 8a–b). The lophophore groove is relatively deep and wide (Pl. 3, Fig. 1a). In the posterior part of the valve, two large lateral visceral gaps are situated between the intrabrachial ridge and the brachial bridge (“vg”, Pl. 3, Fig. 1h). The posterior visceral gap between the brachial bridge and the intrabrachial ridge is relatively narrow (Pl. 3, Fig. 1a).

The brachial bridge has a finely denticulated ventral crest (Pl. 3, Fig. 1j–k). The internal side of the brachial bridge exhibits numerous lophophore muscle scars. Clearly defined, posterior wide outgrowths firmly connect the inner side of the brachial bridge and the posterior side of the intrabrachial ridge in adult specimens (“og”, Pl. 3, Fig. 1e).

The cardinal process is short, massive, trilobate and dorsally curved (“cpr”, Pl. 3, Fig. 1a, 1j–l; Pl. 5, Figs. 6, 7, 8a).

At the posterior part of the cardinal process, the diductor muscle scars have a triangular outline (“did”, Pl. 3, Fig. 1k). Inner socket ridges are thickened and quite strong (Pl. 3, Fig. 1i–j). Outer socket ridges are flat and thin. The sockets are moderately deep. Placed on either side of the base of the inner socket ridges, the lateral adductor muscle scars are widely developed (Pl. 3, Fig. 1j–l).

The anterior side of the calcitic pole is narrow and has two wide lateral expansions (“Exp”, Pl. 3, Fig. 1f). These expansions are fused with the brachial bridge at maturity (Pl. 5, Figs. 6, 7, 8a–b). Seen from behind, the calcitic pole is thick, fused with the dorsal valve floor at the anterior part of the low but long and posteriorly pointed spur of the cardinal process (Pl. 3, Fig. 1k–l).

The brachial cavities (brood pouches in female specimens) are deep and covered with spiculate canopies on their half anterior part. Canopying spicules are thin, irregular in shape and interconnected, producing a quite strong cover protecting the embryos. Several small holes occur in the canopy structure (Pl. 3, Fig. 1a). These holes are variable in number and size. The posterior parts of brachial cavities remain uncovered.

Canopy spicules have a fine granulated structure ornamenting their ventral side (Pl. 3, Fig. 1a–b). This granulation, observed with high SEM magnification, consists of pointed subpyramidal prisms with acute tips (Pl. 3, Fig. 1c–d). The canopies are built with spicules situated on the external margins of the brachial cavities (Pl. 3, Fig. 1a). However, spicules situated on the internal margins can also participate in the erection of canopies (Pl. 5, Figs. 3, 5a, 7). This latest development of the spicules could be related to the growth stage and appearing only in fully mature specimens.

Shell ontogeny. At its youngest stage of growth, the ventral valve presents all the characteristics of its adult morphology: the flat triangular planodeltidium, the hinge structure, the rough internal valve floor and the bifid hemispondylium. In the contrast, the dorsal valve undergoes a great transformation giving rise to numerous morphological characters of generic and specific value.

The youngest dorsal valve stage of growth found in our material (Pl. 4, Fig. 1a–d) shows strong socket ridges and a developed, bilobed, cardinal process. A clear peripheral rim is visible in the posterolateral parts of the valve and the peribrachial ridge is delimited by a dozen small dispersed tubercles. The brachial bridge is already completely developed. Two separate spikes are erected in the centre of the valve floor: they are flat laterally but their tips are pointed and oriented backwards. The adductor muscle scars are also clearly visible (Pl. 4, Fig. 1d).

The next stage of growth (Pl. 4, Fig. 2a–c) shows a cardinal process with a smooth median spur. The lateral lobes of the cardinal process are covered with secondary fibers. All the muscle scars are situated on a triangular platform at the posterior end of the cardinal process. Note also that the structure of the hemispondylium is completely developed (Pl. 4, Fig. 2a–b). It is made of two separate long, pointed claws which for only a short part of their length are attached to the ventral valve floor.

The peribrachial ridge is more developed with an increasing number of tubercles. The two spikes in the center of the dorsal valve floor are now connected with the emergent septum which is still rather short.

The intrabrachial ridge begins its development by the appearance of lateral extension(s) growing from the base of the original spikes to the lateral parts of the shell floor (Pl. 4, Figs. 2a, 2c, 3b).

The development of these posterior parts of the intrabrachial ridge is better illustrated in Pl. 4, Fig. 3a–c where symmetrical progressive outgrowths are visible. The central spikes become longer increasing in length posteriorly. The median septum is slightly higher and more developed.

At this stage of development two small prominent outgrowths produced just at the base of the inner side of the brachial bridge first appear; they increase in length anteriorly (Pl. 4, Fig. 3a, 3e). These outgrowths later connect the brachial bridge and the intrabrachial ridge together. In Thecidellina mawaliana sp. nov. they appear very early in ontogeny when compared with other species of Thecidellina . Nevertheless, in some cases (for unknown reasons) the development of these outgrowths could be variable during ontogeny appearing later in development, as seen in one illustrated specimen (Pl. 5, Fig. 1a), or they remain very small. In this case, the two spikes at the beginning of ontogeny that are now at the tip of the posterior part of the median septum, continue to grow posteriorly to the peribrachial ridge and become thickened (Pl. 5, Fig. 1e). Finally, they join the small outgrowths from the brachial bridge connecting it to the peribrachial ridge (pl. 5, Fig. 3).

The intrabrachial ridge is under construction with the emergence of small tubercles that are quite discrete at the beginning (Pl. 4, Fig. 3a–d) but which increase in thickness and height progressively (Pl. 4, Fig. 4a–c). No calcitic pole is developed at this stage of growth.

Afterwards the median septum increases in length and joins the anterior part of the peribrachial ridge where it becomes clearly wider (Pl. 5, Fig. 1a–b, 1e). At this stage, this median septum remains relatively thin but its height reaches its maximum development at its posterior end where it is fused with the intrabrachial ridge (Pl. 5, Fig. 1c). A thin, lamellar calcitic pole is produced at this stage of growth. It develops in length more in its posterior part than in its anterior part (Pl. 5, Fig. 1f–g). The tubercles which initiated the construction of the peribrachial ridge are now fused together building the brachial cavities. Canopying spicules emerge progressively. This process begins in the posterior portion of the peribrachial ridge (Pl. 5, Fig. 1b–c, 1e), continues through its lateral parts (Pl. 5, Fig. 4) and is finally achieved in its anterior part (Pl. 5, Figs. 5a, 6–7, 8a).

The next step is the interconnection between the base of the brachial bridge and the posterior side of the peribrachial ridge. This interconnection is completed generally by the development of the posterior prominent outgrowths (Pl. 5, Fig. 2a, 2e). These outgrowths are also connected now with the anterior lateral expansions of the calcitic pole (Pl. 5, Fig. 2e). From these interconnections between brachial bridge and intrabrachial ridge appear three visceral gaps: two elongated lateral gaps and one posterior visceral gap. All these connections build a strong and resistant shell structure which is important for the protection of the brood pouches. The calcitic pole is also thickened. At this stage of development, the posterior spur of the calcitic pole is not fused with the base of the cardinal process (pl. 5, Fig. 2d). The median septum achieves its development being thicker and canaliculated on its ventral side. Several small tubercles are developed on the tip of the septum and along its ventral lateral sides (Pl. 5, Fig. 2a–c). The brachial cavities (brood pouches in females) are now built up by the development and fusion of the canopying spicules (Pl. 5, Fig. 2a, 2c–d). The ventral surface of the anterior part of the peribrachial ridge is ornamented by strong subspherical tubercles (pl. 5, Fig. 2b).

The dorsal valve has now reached the mature stage of development is now reached. The canopying spicules cover most of the brachial cavities (or brood pouches) and they fuse together progressively (Pl. 5, Figs. 3, 4, 5a, 6– 8a). The fusion of the spicules is variable but several holes always remain present. The ventral surface of the canopying spicules is ornamented with a regular granulation (Pl. 5, Fig. 5a–b). The calcitic pole is now fused with the ventral surface of the cardinal floor.

Development of the lophophore. The most juvenile specimen found has a lophophore at the trocholophe stage, consisting of 20 tentacles. Posteriorly, the lophophore is firmly applied against the brachial bridge and attached by the central spikes at the valve floor. The lateral adductor muscles are visible. The peribrachial ridge is poorly developed and only a very few small tubercles are visible (Pl. 6, Fig. 1).

A more developed step shows an early schizolophous lophophore made of 40 tentacles. The lophophore groove is already well developed. The lateral adductor and the diductor muscles are visible (Pl. 6, Fig. 2). A young specimen with a more regular early schizolophous lophophore consisting of 60 tentacles is illustrated in Pl. 6, Fig. 3.

The adult stage of development is illustrated in Pl. 6, Fig. 4. This specimen has a schizolophous lophophore consisting of 65 tentacles covering already developed brachial cavities covered by their canopies. Another fully adult specimen is illustrated in Pl. 6, Fig. 5a–e. It has a schizolophous lophophore also made of 65–70 tentacles (the tentacles along the inner sides of the brachial cavities are difficult to count precisely). The surface of the tentacles is densely ciliated (Pl. 6, Fig. 5g).

Lateral adductor muscles and median adductor muscles are clearly visible in pl. 6, Fig. 5f. The diductor muscles are mostly not visible as the posterior part of the cardinal process is broken.

When the lophophore is cut out, the SEM observation of the brachial cavities (or brood pouches) shows a thin membrane covering them completely. The specimen illustrated Pl. 6, Fig. 7 shows this membrane remaining on the right brachial cavity and removed by dissection on the left brachial cavity (the canopying spicules are visible in this case).

TEXT-FIGURE 3. Scatterplots of morphometric measurements of Thecidellina mawaliana sp. nov. specimens. Abbreviations: L, length; W, width; LDV, length of dorsal valve; T, thickness; WH, width of hinge line. Relationships between ratios L/W and width; LDV/W and width; WH/W and width; T/W and width. Linear regression applied when significant and regression coefficient (R²) indicated.

TEXT-FIGURE 4. Size frequency diagrams of living Thecidellina mawaliana sp. nov. found attached on Neopycnodonte shells in the Mawali wreck off Lembeh (depth - 25m). The width (W), the length (L), the width of the hinge (WH) and the thickness (T) of the shells were analyzed. For the width, the length and the width of hinge the distribution is bimodal. However, for the thickness the distribution is completely different (see explanation in text).

Reproduction. T. mawaliana sp. nov. is gonochoristic. Distinct female and male specimens have their gonads situated in the ventral valve floor in two deep pits placed on each side of the median floor crest (Pl. 3, Fig. 2a, 2c). The gonads are covered by a thin epithelium but viewing them using SEM after critical-point drying was not successful. However, the gonads are clearly visible with a normal stereomicroscope using a light placed under the valve. Due to the transparent membrane, the orange female gonads easily show the spherical gametes, which are not numerous (3–5 per gonad in the specimens observed). An example is illustrated in Pl. 6, Fig. 6.

The male gonads are placed similarly in male specimens but their structure remains undifferentiated.

The embryos are placed in the brachial cavities that are brood pouches. In males the brachial cavities remain empty. After incubation of the larvae they are released at maturity being expelled through the marsupial orifices situated at the posterior part of the brachial cavities.

This reproductive process protects the larvae during its development. The larvae are spawned when fully mature, allowing them to fix directly to a suitable substrate, improving their chances of survival and reducing the amount of energy expended in producing large numbers of larvae.

Population structure. The material collected in the Mawali wreck was sufficiently abundant to establish the population structure for T. mawaliana sp. nov. (Text-Fig. 4). The size frequency histograms for living specimens, based on different morphological characters such as length, width, and width of the hinge, tend to have a bimodal distribution. These characters follow a linear progression during growth.

This bimodal structure indicates that older, fully adult specimens are most abundant whereas younger generation(s), separated by a hiatus has (have) their own distinct distribution. Early juveniles grow rapidly at the beginning and then more progressively in order to reach the adult form. The second maximum on the right represents the accumulation of individuals from successive generations. It is not possible to evaluate the number of generations present with such a single sampling.

However, if the thickness is considered, the bimodal structure of the population does not appear. This is due to the sigmoidal function observed for this morphological character (Text-Fig. 3). Juveniles have extra-flat shells and they increase the thickness slowly. At a particular point this character undergoes a strong acceleration. This gives the size frequency histograms their sub-parabolic aspect.

This illustrates the fact that the understanding of the population structure depends on the character chosen for study. When the growth of a character is not linear, the size frequency diagrams obtained could be not useful for establishing a population structure.