Hippothoa, Lamouroux, 1821

Dick, Matthew H., Ngai, Nguyen Danh & Doan, Hung Dinh, 2020, Taxonomy and diversity of coelobite bryozoans from drift coral cobbles on Co To Island, northern Vietnam, Zootaxa 4747 (2), pp. 201-252 : 212-214

Hippothoa sp.

( Fig. 5 D–F)

Material examined. VNMN-0217 (CT-11; with Antropora minor ), VNMN-0219 (CT-37; with Antropora minor ), on SEM stubs. Material was limited, consisting of a single autozooid from one colony and six autozooids from another.

Measurements. AzL (dilatation), 0.27–0.30 (0.288 ± 0.012); AzW, 0.16–0.19 (0.176 ± 0.013) (n = 7, 2). Cauda length, 0.394 –0.527 (0.441 ± 0.075) (n = 3, 1). OrL, 0.059 –0.071 (0.065 ± 0.008); OrW, 0.048 –0.056 (0.052 ± 0.006) (n = 2, 1).

Description. Colony encrusting, uniserial, branching. Branching pattern cruciform, with zooids giving rise to daughter zooids distally and laterally; daughter zooids arise laterally close to middle of dilatation or between middle and level of proximal edge of orifice.

Zooids comprising long cauda expanding to form dilatation; cauda length 1.5–1.8 times length of dilatation. Dilatation highly transversely convex, oval in outline. Frontal wall gymnocystal, with faint transverse striation and slight median keel. Orifice nearly terminal, raised, oriented parallel to frontal plane, longer than broad, with deep, evenly U-shaped sinus flanked by sharp, bracket-like condyles; small pit at base of each condyle; no pit proximal to sinus.

Spines, avicularia lacking. One zooeciule seen, arising proximolaterally from dilatation, unpaired, elongate opening comprising more than half zooeciule length (zooeciule incompletely developed?).

Ancestrula, ooecium not observed.

Remarks. This species was probably more abundant than our samples indicate, but was likely prone to loss by abrasion due to its small, widely separated zooids typically encrusting flat, open patches of substrate. Several species of uniserial Hippothoa having zooids with a long, narrow cauda occur in tropical and subtropical reef-rubble habitats, and the Co To specimens are not clearly identifiable with any of them. In Hippothoa flagellum Manzoni, 1870 , zooids have only one lateral pore chamber and thus produces zooeciules in place of daughter zooids ( Ryland & Gordon 1977), rather than additionally and proximolaterally. Hippothoa divaricata Lamouroux, 1821 var. pacifica Gordon, 1984 has the orifice tilted down distally rather than oriented in the frontal plane; there is a conspicuous median keel; and zooeciules are elongate, arising either laterally in place of daughter zooids, or proximolaterally. Hippothoa calciophila Gordon, 1984 is similar to the Co To specimens, but has a broader proximal orificial sinus with a pit proximal to the sinus, and larger condyles. Hippothoa petrophila Dick & Grischenko, 2017 is also similar to the Co To specimens, but has much longer caudae; more-conspicuous, rounded condyles; a pit proximal to the orificial sinus; and zooeciules with a small, circular opening. Key features we could not observe include the ancestrula and its budding pattern, the distribution of pores and pore chambers in the lateral and transverse walls, and the form of female zooids and ooecia. Identification of this species will require more material from the Co To vicinity.

Figures

FIGURE 5. A–C. Smittipora harmeriana (Canu & Bassler, 1929). A. Colony, VNMN-0223. B. Autozooids and interzooidal avicularia, with arrowhead indicating vestigial ooecium; VNMN-0223. C. Ancestrula (asterisk) and periancestrular zooids, along with Plesiocleidochasma porcellaniforme, VNMN-0256. D–F. Hippothoa sp. D. Parts of uniserial colony on bivalve shell, VNMN-0219. E. Autozooidal dilatation, showing cruciform branching and one proximolateral zooeciule, VNMN-0219. F. Autozooidal dilatation, showing orifice shape and slight frontal keel, VNMN-0217. Scale bars: A, 1 mm; B–D, 500 µm; E, F, 100 µm.