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

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Hippomonavella charrua

n. sp.

Hippomonavella charrua   n. sp.

( Fig. 2 View FIGURE 2 ; Table 1 View 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. GoogleMaps   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 View Materials . 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 View Materials .A bilaminar fragment. Same details as previous specimen. GoogleMaps  

Description. Colony encrusting unilaminar or erect bilaminar. Zooids ordered in quincunx, twice as long as wide, delimited by well-marked borders ( Fig. 2 View FIGURE 2 A–C). Frontal shield smooth in zooids near the colony margin ( Fig. 2A View FIGURE 2 ), tuberculated in older areas of the colony ( Fig. 2C View FIGURE 2 ), slightly convex, with a row of 12 to 20 (median: 16, depending on zooid size) conspicuous marginal areolae separated by ridges ( Fig. 2 View FIGURE 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 View FIGURE 2 ). One pair of blunt, robust condyles angled proximally, close to the proximal margin of the orifice ( Fig. 2C View FIGURE 2 ). A pair of cylindrical, disto-lateral oral spines, not observed in ovicelled zooids ( Fig. 2A, C View FIGURE 2 ). 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 View FIGURE 2 ); a pair of associated distolateral pores is located in the angle formed by the avicularium and the peristome ( Fig. 2B View FIGURE 2 ). Interzooidal communication by multiporous pore plates in the lateral walls ( Fig. 2D View FIGURE 2 ). 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 View FIGURE 2 ). Ancestrula tatiform (length ~ 220 µm), with 9 delicate, cylindrical spines around a circular opesia and a band of proximal cryptocyst ( Fig. 2E View FIGURE 2 ). 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 View FIGURE 2 ).

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 View FIGURE 1 ). Its known bathymetric range is 119– 128 m.