Metrarabdotos unguiculatum (Canu & Bassler, 1928)

Santana, Flávia T., Ramalho, Laís V. & Gumarães, Carmen P., 2009, A new species of Metrarabdotos (Bryozoa, Ascophora) from Brazil, Zootaxa 2222, pp. 57-65 : 59-61

Metrarabdotos unguiculatum (Canu & Bassler, 1928)

( Fig. 2 A–C)

Trigonopora unguiculata Canu & Bassler, 1928a: 128 ; Marcus 1955: 304.

Metrarabdotos unguiculatum: Canu & Bassler 1928b: 92 ; Cheetham et al. 2007: 62.

Material examined. Bahia State (18°06’ S, 36°04’ W), MNRJ-349 one colony, 3 May 1974, collector fishing vessel Riobaldo. Espírito Santo State (18°40’ S, 38°28’ W), MNRJ-400 one colony, 7 May 1974, collector fishing vessel Riobaldo.

Description. Colonies encrusting, unilaminate. Dark brown, to burgundy-coloured or reddish in alcohol. Autozooids disposed in quincunx, rectangular, longer than wide, 780–1110 (980) µm long x 520–650 (550) µm wide ( Fig. 2 A). Frontal skeletal wall (frontal shield) granular, with 18–22 areolar pores 63–94 (69) µm diameter, in a single row around the autozooid. Primary orifice slightly longer than wide, 190–260 (210) µm long x 130–190 (180) µm wide, with elevated peristome and a small, U-shaped sinus. Secondary orifice with a similar sinus, formed by two prominent denticles ( Fig. 2 B).

Ordinary avicularia present in most autozooids, 320–440 (360) µm long, situated proximolateral to the peristome, arising from one of the areolae, directed distally towards the orifice, slightly curved along and encroaching on the peristome; one avicularium located at the right or left side, sometimes absent ( Fig. 2 A-B). Pre-mandible region rounded; mandibular region elongate, pointed and curved; crossbar complete. Mandible conforming to the rostrum ( Figure 2 C).

Maternal autozooids bearing ovicells slightly larger than ordinary autozooids, 910–1170 (1150) µm long x 590–780 (680) µm wide; frontal skeletal wall crenulated with large areolar pores; orifice crescent to rectangular, 520–580 (540) µm wide. Ovicell wider than long, 390–580 (520) µm long x 520–770 (720) µm wide, with areolar pores forming up to three rows around the margin, the most external being larger than the most internal ( Fig. 2 A, B).

Remarks. Within the genus Metrarabdotos , only M. unguiculatum ( Canu & Bassler, 1928a), M. pacificum (Osburn, 1952) , and M. cookae Cheetham, 1968 have encrusting colonies, single adventitious avicularia directed distally (curved, with rostral tip applied to peristome rim), and a bidenticulate orifice.

Metrarabdotos pacificum differs from Brazilian M. unguiculatum in having smaller autozooids (730 µm long x 510 µm wide) and avicularia (250–260 µm long), shorter ovicell-bearing maternal zooids (340 µm wide), a perforate ovicell with ribs on the surface and smaller areolar pores.

Metrarabdotos cookae is similar to M. unguiculatum described here in the size of the autozooidal orifices (200 µm long x 170 µm wide) but has a greater number of areolae (24), autozooids (810 µm long x 570 µm wide) and avicularia (320 µm long) smaller in length, and larger ovicells (570 µm long x 700 µm wide) with ribs and small pores on the surface and smaller areolae.

The only encrusting Metrarabdotos recorded from Brazil is M. unguiculatum ( Canu & Bassler 1928a; Marcus 1955). The specimens described here are very similar to those described by Marcus (1955) and Cheetham et al. (2007) in the following features: number of pores (18–24; 21, respectively), size of the autozooids and maternal zooid orifice, size of ordinary zooids, and in the rarity of paired avicularia. Colonies described here have ovicells slightly larger than those of Marcus (1955) and Cheetham et al. (2007) (550 µm long x 720 µm wide; 540 µm long x 700 µm wide, respectively), and longer avicularia (Marcus: 350–370 µm long, Cheetham et al.: 250–350 µm long). These differences, however, are small and can be considered within the limits of intraspecific variation.

Geographic distribution. Yucatan, Gulf of Mexico (west of Florida) ( Cheetham et al. 2007). Brazil: Bahia and Espírito Santo states ( Canu & Bassler 1928b; Marcus 1955; present study).

Figures

FIGURE 2. Metrarabdotos unguiculatum (Canu & Bassler, 1928 a) (MNRJ- 349). A, Part of the colony with growing edge, autozooids and maternal zooids. B, Autozooids with ordinary avicularia and two maternal zooids. C, Detail of the mandible of an avicularium. Scalebars: A, 1 mm; B, 500 µm; C, 100 µm.