Characodoma strangulatum ( Calvet, 1906 )

Berning, Björn, 2013, New and little-known Cheilostomata (Bryozoa, Gymnolaemata) from the NE Atlantic, European Journal of Taxonomy 44, pp. 1-25 : 16-21

Characodoma strangulatum ( Calvet, 1906)

Fig. 5, Table 5

Myriozoum strangulatum Calvet, 1906: 158 .

Myriozoum strangulatum – Calvet 1907: 427, pl. 26, figs 11-12.

Cleidochasma strangulatum – Harmelin 1977: 1067, text-fig. 25, pl. 2, figs 1, 4, 7. — Harmelin & d‘Hondt 1992: 29.

Characodoma strangulatum – Rosso 1999: 429.

Material examined

Syntype series

MNHN 492, a single colony fragment on slide, Travailleur Dr. 49, 7 Aug. 1882, Canary Islands, 29° N – 16°08’ W, 3700 m; MNHN 493, a single colony fragment on slide, Travailleur Dr. 38, 30 Jul. 1882, NW Morocco, 34°13’30’’ N – 07°43’ W, 636 m; MNHN 947, a single colony fragment on slide, Travailleur Dr. 38, 30 Jul. 1882, NW Morocco, 34°13’30’’ N – 07°43’ W, 636 m; MNHN 2471, a single colony fragment on slide, Talisman Dr. 96, 15 Jul. 1883, off Cap d’Arguin ( Mauritania), 19°18’ N – 18°01’ W, 2330 m.

Other material

MNHN 15487, several colony fragments, Balgim DW07, 29 May 1984, SW Portugal, 36°46.1’ N – 9°27’ W, 1139-1144 m; MNHN 19804, several colony fragments, Balgim CP92, 8 Jun. 1984, NW Morocco, 34°24.3’ N – 7°30.3’ W, 1182 m.

While clearly stating that the original material comprised specimens from the Travailleur (stations Dr. 38, 49) and Talisman (station Dr. 96) cruises, Calvet (1906, 1907) did not indicate type specimens when introducing Myriozoum strangulatum . The listing of MNHN 2471 as “ lectotype ” by Tricart & d’Hondt (2009) was obviously for mere curatorial purposes and violates Article 74.7.3 (see also Declaration 44) of the ICZN Code. This designation is thus considered invalid. For reasons given below, I refrain here from choosing as lectotype a specimen from the syntype series.

Sampling positions of the Talisman and Travailleur cruises, the longitudes of which were initially measured with reference to the Paris meridian, have here been corrected to the Greenwich meridian by subtracting 2°20’14’’ from the longitude originally given (cf. Ryland 1969: 238).

Description

Colony presumably cellarinelliform (sensu Rosso 1992). Rods up to about 1 cm in length, cylindrical to oval in cross-section with constrictions at irregular intervals (Fig. 5A-E), which result from breakage and regeneration of the colony, or from a reduction in the number of distally budded zooids; ramifications or rhizoids were not observed; in fully developed colony parts, 3 zooids are simultaneously budded distally, and the rods are composed of an abfrontal side devoid of orifices on about one-fourth of the total perimeter (Fig. 5H), with the zooids opening at the latero-frontal sides (Fig. 5E) and being arranged in a plaited manner along the rod axis, i.e., zooids aligned in 3 alternating longitudinal series on each side of a median frontal (zigzag) line; all zooids inclined at about 45° to rod axis, pointing towards the median frontal line (Fig. 5E, G, J). The zooids in these 3 series are polymorphic and decrease in size towards the median frontal line: the abfrontal side is exclusively composed of the extremely elongated proximal parts of the latero-abfrontally positioned series of zooecia of each side (Fig. 5H), while the distal parts of these zooecia are bent around the rod axis and open along the lateral sides; zooecium shape very elongated hexagonal; the intermediate lateral series consist of distinctly shorter subrhomboidal zooecia (Fig. 5E, G, J), abutting distally against a zooecium of the median series from the other side of the median frontal line; zooecia of the median series even shorter and subhexagonal (Fig. 5E, G), opening at an angle of almost 90° with respect to the latero-abfrontal zooecia. In between zooids at the median line, an orbicular opening is occasionally present that presumably marks the heterozooid from which a rhizoid is produced for colony support (Fig. 5J).

All zooecia separated by shallow grooves and indistinct meandering sutures owing to secondary calcification during later ontogeny; frontal shield slightly convex, distally forming a swollen, salient rim around distolateral orifice, surface distinctly nodular (Fig. 5G); few areolar pores of variable shape, usually situated in zooecial corners. Primary orifice somewhat immersed, situated at quite a distance to the distal zooecial margin (Fig. 5G), cleithridiate, slightly longer than wide, with a large suborbicular poster comprising about three quarters of a full circle and a very broadly U-shaped anter of about onethird of total orifice length and over half of orifice width, delimited by very short proximomedially directed condyles (Fig. 5F); orifice dimensions smaller in zooids of the median series than in those of the lateral and latero-abfrontal series.

Ovicells were not observed.

Avicularia adventitious, small, transversely elliptical, usually two per zooecium (Fig. 5E, G, I-J): one in the distolateral corner that is closer to the median frontal line, and another one usually at the proximolateral zooecial margin; the latero-abfrontal zooids may have another avicularium at the very proximal zooecial margin; cystid slightly raised during early ontogeny, later levelled by secondary calcification. Rostrum much wider than long, semi-elliptical, with a slightly raised smooth distal rim, directed distolaterally (distal avicularia) or proximolaterally (proximal avicularia); crossbar complete, without columella; proximal uncalcified area transversely oval.

Fig. 5 (page 18). Characodoma strangulatum ( Calvet, 1906) . A. Latero-abfrontal view of MNHN 492 (Canary Islands) from the syntype series, which is affected by Bynesian decay. B. Lateral view of MNHN 493 (NW Morocco) from the syntype series, a mature colony in which the zooecia are frontally thickened by secondary calcification; the abfrontal part (to the left) and the autozooecia from the opposite side cannot be seen in this view. C. Lateral view of MNHN 2471 ( Mauritania) from the syntype series; the constriction in this colony was caused by zooid regeneration after damage and does not mark a conventional constriction in growth as seen in other specimens (photo: P. Kuklinski). D. A long rod with reversed polarity growth after damage at the colony centre, the lower half representing the primary colony from which the upper part was budded; in both parts growth constrictions occur that did not result from colony breakage (MNHN 19804, NW Morocco; photo: J. Souto). E. Close-up of the same specimen in lateral view showing ontogenetically young zooids that lack secondary calcification; the three differently positioned, polymorphic autozooids are indicated as “la” (elongated latero-abfrontal zooids), “l” (lateral zooids of intermediate size), and “m” (small median zooids) (MNHN 19804, NW Morocco; photo: J. Souto). F. Orifice (MNHN 492, Canary Islands); the calcium acetate crystals growing on the skeletal surface are the result of Bynesian decay owing to the specimen having been kept enclosed in a wooden slide in relatively humid conditions. G. Late ontogenetic polymorphic autoozooids (MNHN 493, NW Morocco); see E for abbreviations. H. The colony growth margin of another colony of sample MNHN 19804 (NW Morocco) with two zooids having been budded after a normal constriction in colony diameter; view of the abfrontal side of the rod with the extremely elongated latero-abfrontal zooid; the orifice is to the left whereas its proximal margin is at the very right (arrow) (photo: J. Souto). I. Close up of two avicularia (MNHN 19804, NW Morocco; photo: J. Souto). J. Yet another colony of sample MNHN 19804 (NW Morocco) showing the approximate median line at which zooids from both colony sides meet; note the orbicular opening of a kenozooid at the median line from which a rhizoid for colony attachment presumably originates (arrow) (photo: J. Souto). Scale bars: A-C, E, H, J = 200 µm; D = 400 µm; F = 30 µm; G = 100 µm; I = 50 µm.

Early astogenetic stages absent in the examined material.

Remarks

In a revision of the Mediterranean species of the genus Characodoma Maplestone, 1900 , Rosso (1999: 429) first assigned Myriozoum strangulatum to this genus when describing the closely related Pleistocene species Characodoma reclinatum Rosso, 1999 . Although the characteristic ovicell could not be observed on any of the available material, the general zooidal and zoarial features of C. strangulatum correspond to those of Characodoma .

However, there are serious problems concerning the specific taxonomic identity of C. strangulatum that arise from the specimens of the syntype series. 1) The four syntypes are small colony fragments that are very poorly preserved. Specimen MNHN 947 is strongly affected by Bynesian decay and could not be studied in detail. MNHN 492 and 493 are also affected to varying degrees but some characters are still visible, while in MNHN 2471 all openings are filled with sediment. 2) Important species-specific characters are either entirely lacking in these specimens (e.g., ovicells) or are mostly obscured, such as the primary orifices, which are concealed by opercula in MNHN 493, or filled with sediment in MNHN 2471. Cleaning or bleaching of the specimens is presumably impossible without damaging or destroying the fragile colonies. Moreover, as they are glued onto slides, only a restricted number of measurements could be taken (cf. Table 5, and see below). 3) The specimens are from widely distant locations, with a distance of over 800 km between each of the three sampling regions ( Mauritania, Canary Islands, Gulf of Cádiz). 4) Of the four specimens of the syntype series, two (MNHN 492 and 2471) resemble the original figure ( Calvet 1907: pl. 26, fig. 11) owing to the presence of constrictions typical for the species. Neither of these two colonies can be unequivocally identified as the figured specimen, maybe because the colony surface depicted by Calvet cannot be seen in the mounted specimens, i.e., they were glued onto the slides with the wrong side facing upwards.

Thus, beyond the absence of ovicells and early astogenetic stages from all material assigned to C. strangulatum , the few small and poorly preserved specimens of the syntype series do not allow this species to be precisely defined. Moreover, the great geographic range of distribution and distance between sampling regions raises the question of whether all syntype specimens do indeed belong to the same species. For instance, the lateral zooids in specimen MNHN 493 have slightly smaller orifices (mean OL 124, OW 113, N = 3) than the remaining syntype specimens (mean OL 136, OW 122, N = 11), although the number of measurements is clearly too small to be statistically reliable. In contrast, the specimens reported from NW Morocco by Harmelin & d’Hondt (1992; MNHN 19804), i.e., from the same region as MNHN 493 and MNHN 497, differ from the original suite of C. strangulatum in having distinctly larger orifices (mean OL 168 µm, OW 146 µm, N = 10), both measured in lateral and lateroabfrontal zooids (MNHN 19804, Fig. 5D, E, H, J).

Therefore, I consider the specimens of the syntype series as insufficiently preserved and lacking important diagnostic characters for the species to be precisely defined, and it cannot be ruled out that the syntype series comprises more than one distinct species. Hence, I will not designate a lectotype from the syntype series but suggest instead that a neotype should be chosen as soon as newly sampled material from either one of the three original sampling sites becomes available. In the present study, C. strangulatum is considered sensu lato, as is its geographic range and depth distribution.

Another incongruity concerns the avicularium: in the original figure of C. strangulatum ( Calvet, 1907: pl. 26, fig. 12) the avicularia are depicted as having elongated and rather triangular rostra, instead of the extremely short and broad, semi-elliptical rostra reported in other specimens assigned to this species. Harmelin (1977) ascribed this difference to a possible intraspecific variability between populations inhabiting different environments. However, in none of the specimens of the “ syntype series” are crossbars visible, either because they are broken and lost, or because the mandibles are still in place, concealing the skeletal structures underneath. Another possibility is that they are truly absent. In any case, it seems likely that, in the absence of any visible crossbars, Calvet decided that they should be, as in most other bryozoans, perpendicular to the longest avicularian dimension, and haphazardly added crossbars to the figure. That the avicularium is, indeed, broader than long in C. strangulatum is indicated by the slightly raised distal margin of the rostrum, while simple crossbars spanning the longest dimension are present in well-preserved specimens from central and northwestern Morocco that were assigned to C. strangulatum by Harmelin (1977) and Harmelin & d’Hondt (1992), respectively (Fig. 5I).

Characodoma strangulatum has been recorded from depths between 600 and 3700 m and its geographic range is provisionally considered to stretch from southern Portugal along the Moroccan shelf, and via the Canary Islands to the Mauritanian slope, i.e., some 2000 km in N-S direction. The species occurs on muddy substrata and is, as other congeneric species (cf. Rosso 1999), likely to be anchored in the fine sediment by rhizoids. The rhizoids themselves were never observed, not even in well-preserved specimens from the Gulf of Cádiz recovered during the Balgim cruise ( Harmelin & d’Hondt 1992). This may not be surprising, as Hirose (2011) recently showed that, in certain species, rhizoids may be translucent and extremely delicate structures, and may thus be among the first tissue to decay after sampling. However, the orbicular kenozooidal openings situated along the frontal midline of the C. strangulatum colonies do suggest this mode of attachment (Fig. 5J). This means that the tentacles of the median zooids face towards the substratum when everted, which has already been inferred for other species of this genus ( Rosso 1999). The relative proximity of the median zooids to the substratum may also explain the smaller orifice dimensions (i.e., smaller tentacles, if these traits are interrelated) in comparison with zooids from the lateral and latero-abfrontal series (cf. Table 5). On the other hand, the small orifices may simply be correlated with the smaller zooid size of the median series owing to constructional constraints of the colony.

Undamaged zooecia with one or two intramural buds, recognisable by the presence of multiple orifice rims within the primary orifice (cf. Berning 2008), were present in all of the studied colonies. Reparative growth within primary zooids may either be triggered by partial predation ( Berning 2008) or, in (seasonally) oligotrophic conditions, by partial starvation (J.-G. Harmelin pers. comm. 2008). The typical constrictions within colonies of C. strangulatum (Fig. 5D), from which the specific epithet derives, may also be a response to seasonal reduction in food supply during which colony growth slows down or comes to a halt. The paucity of ovicells even in long rods, in concert with frequent signs of breakage and reparative growth at colony constrictions, may reflect another adaptation to unfavourable conditions. Instead of investing energy in embryos, propagation may more often proceed by fragmentation of the colonies at these constrictions (Fig. 5C, D). However, ovicells are commonly present in a very closely related, hitherto undescribed species from the Great Meteor Bank at similar depths (pers. observation). This seamount is situated in the central North Atlantic in certainly more oligotrophic conditions than the populations of C. strangulatum from the continental shelf and slope.