Hippomonavella charrua, López-Gappa & Liuzzi & Pereyra, 2020

López-Gappa, J., Liuzzi, M. G. & Pereyra, C., 2020, A new species of Hippomonavella (Bryozoa: Cheilostomata) from the Holocene and Recent of Argentina and Uruguay (Southwest Atlantic), Zootaxa 4728 (1), pp. 143-148 : 145-147

Hippomonavella charrua n. sp.

( Fig. 2 ; Table 1 )

Etymology. The species honours the memory of the charruan people, the aboriginal inhabitants of Uruguay and part of Argentina.

Material examined. Holotype: MACN-In 15933–1. A bilaminar fragment. Undine, 34º38’ S, 52º15’ W, 119– 128 m, leg. Capt. C. Alexandersson, 24 July 1925. Paratype: MACN-In 15933–2. A bilaminar fragment, formerly growing on a hydrozoan stem. Same details as holotype. Other material examined: MACN-In 42241. Several encrusting colonies on Arachnopusia sp. Labeled “Quequén”, but lacking further information. The accompanying fauna suggests that this material might have been collected by a trawler landing its cargo at Quequén Harbour. It is quite possible that it comes from the continental shelf off Buenos Aires Province. MLP 36237. A bilaminar colony fragment, 35°58’18.20” S, 57°26’57.88” W, mid-Holocene, on the left margin of Canal 15. MLP 36238.A bilaminar fragment. Same details as previous specimen.

Description. Colony encrusting unilaminar or erect bilaminar. Zooids ordered in quincunx, twice as long as wide, delimited by well-marked borders ( Fig. 2 A–C). Frontal shield smooth in zooids near the colony margin ( Fig. 2A ), tuberculated in older areas of the colony ( Fig. 2C ), slightly convex, with a row of 12 to 20 (median: 16, depending on zooid size) conspicuous marginal areolae separated by ridges ( Fig. 2 A–C). Orifice roundly subquadrate, with a convex distal margin and a straight to convex proximal margin, surrounded by a low peristome with lateral and proximal flaps ( Fig. 2B ). One pair of blunt, robust condyles angled proximally, close to the proximal margin of the orifice ( Fig. 2C ). A pair of cylindrical, disto-lateral oral spines, not observed in ovicelled zooids ( Fig. 2A, C ). Avicularia median, suboral, directed proximally, close to the orifice, subtriangular, with a rounded apex and a complete cross-bar without ligula, protruding over the frontal shield ( Fig. 2A, B ); a pair of associated distolateral pores is located in the angle formed by the avicularium and the peristome ( Fig. 2B ). Interzooidal communication by multiporous pore plates in the lateral walls ( Fig. 2D ). The distal and proximal walls have uniporous pore plates. Ovicell hyperstomial, not closed by the operculum (acleithral), initially spherical and protruding, then subimmersed in the distal zooid, its margins covered by the frontal walls of the neighbouring zooids, pierced by about 40 pseudopores distributed throughout its surface ( Fig. 2C ). Ancestrula tatiform (length ~ 220 µm), with 9 delicate, cylindrical spines around a circular opesia and a band of proximal cryptocyst ( Fig. 2E ). The early astogeny is symmetrical. The first 3 zooids budded from the ancestrula (one distal, two distolateral) have 5 spines. In the zone of astogenetic change, zooids in successive whorls increase in size, but the number of oral spines decreases. Most autozooids of the encrusting phase have suboral avicularia. In the erect portions, however, their occurrence is variable: in certain areas they are frequent (about 50% of the zooids), whereas in other ones they are lacking. For instance, in the holotype fragment, just 8% of the zooids have suboral avicularia.

Remarks. The bilaminar colonies of the Recent material were growing on hydrozoan stems. The encrusting colonies were growing on Arachnopusia sp. and the ancestrula was found on an oyster shell. Although most of the bryozoan colonies in the Holocene material occurred on mollusc shells, the fossil specimens of H. charrua n. sp. were found isolated, fragmented and eroded ( Fig. 2F, G ).

Geographic and bathymetric distribution. The species has been found on the continental shelf off Uruguay and perhaps also off Buenos Aires Province ( Argentina), and in mid-Holocene deposits of Canal de las Escobas Formation, Destacamento Río Salado Member ( Fig. 1 ). Its known bathymetric range is 119– 128 m.

Figures

FIGURE 1. Study area. 1: Type locality, MACN-In 15933. 2: Mid-Holocene material, Destacamento Río Salado Member, MLP 36237.

FIGURE 2. Hippomonavella charrua n. sp. A. General aspect. B. Detail of avicularia; the arrow shows the pore associated with the avicularium. C. Ovicelled zooids; the arrows show the spine bases. D. Detail of pore plates in a lateral wall. E.Ancestrula (a) and early astogeny. F. General aspect of a mid-Holocene specimen. G. Zooids and avicularia in the mid-Holocene material. A, B, D: holotype; C: paratype; E: MACN-In 42241; F, G: MLP 36237. Scale bars: A, 500 µm; B–E, G, 200 µm; F, 1 mm.