Porella Gray, 1848

Dick, Matthew H., Grischenko, Andrei V. & Mawatari, Shunsuke F., 2005, Intertidal Bryozoa (Cheilostomata) of Ketchikan, Alaska, Journal of Natural History 39 (43), pp. 3687-3784 : 3732-3734

Genus Porella Gray, 1848

Porella alba Nordgaard, 1906

( Figure 12A, B)

Porella alba Nordgaard 1906, p 25 , Plate 3, Figures 43–45.

Porella alba: Hayward 1979, p 77 , Figure 2d–f; Hayward and Ryland 1999, p 156, Figure 54; Dick and Ross 1986, p 89; 1988, p 64, Plate 8B, C; Grischenko 1997, p 178, 2004, p 40.

Porella minuta :? Osburn 1953, p 780, Plate 81, Figure 9 ; Kluge 1975, p 552, Figure 302.

Description

Colony. Unilaminar, tightly adhering, glistening white in colour, forming circular to irregular patches on hard substrates; largest observed 1.5 cm across.

Zooids. Distinct, separated by a groove flanked by areolar pores; barrel- to spindle-shaped, often tapering proximally, 0.33–0.53 mm long (average 50.420 mm, n 515, 3) by 0.23– 0.30 mm wide (average 50.263 mm, n 515, 3).

Frontal wall. Moderately to markedly convex; heavily calcified at all stages, increasingly thick with age; imperforate except for 7–11 conspicuous, often enlarged, areolae along each lateral margin, frequently with conspicuous ridges or buttresses between them; often with a low, blunt suboral umbo covering avicularian chamber.

Orifice. Primary orifice ( Figure 12A) visible only on marginal zooids, rapidly becoming deeply submerged in peristome; semicircular, broader than long, 0.08– 0.10 mm long (average 50.086 mm, n 515, 3) by 0.08–0.13 mm wide (average5 0.104 mm, n 515, 3), proximal margin straight, condyles minute, lyrula lacking. Secondary orifice circular, oval, or rounded-quadrangular in outline, surrounded by a thickened rim.

Spines. Lacking.

Avicularia. A median suboral avicularium ( Figure 12A) lies at the proximal edge of the peristome in young zooids and within the peristome in older zooids; rostrum nearly circular, angled slightly proximally; mandible semicircular. Avicularian chamber small, crescent-shaped or occasionally globose, originating from a marginal pore on each side; rapidly obscured by increasing calcification.

Ovicell. Initially raised, but immersed with age ( Figure 12B) and often indistinguishable from frontal wall of next distal zooid; opening at bottom of peristome, not closed by operculum; imperforate, globose, 0.15–0.23 mm long (average 50.183 mm, n 515, 3) by 0.18–0.25 mm wide (average 50.214 mm, n 515, 3); proximal margin usually raised and continuous with peristomial rim.

Ancestrula . Not observed.

Remarks

Porella alba is superficially similar to P. donoghueorum in having similar colony colour and form, the rostrum and mandible of the suboral avicularium similarly shaped, immersed ovicells, and small zooids. Porella alba differs in having roughly twice the number of areolar pores, a pronounced tendency for inter-areolar ridges or buttresses, and a straight rather than a convex, weakly lyrulate proximal orificial margin. Porella alba lacks distal spines altogether, whereas these are typically evident in marginal zooids of P. donoghueorum ; it also lacks the pores in the avicularian chamber that are typical of the latter.

Distribution

Previously recorded at Kodiak, Alaska by Dick and Ross (1988), this is considered a circumpolar, arctic-boreal species ( Hayward and Ryland 1999). Ketchikan represents the southern limit of the known range in the north-eastern Pacific. In the north-western Pacific, it has been found as far south as the Commander Islands ( Grischenko 1997, 2004) and the Pacific coast of Kamchatka ( Kubanin 1997).

Figures

Figure 9. Celleporella hyalina (L.) species complex, showing variation between two colonies encrusting a single mussel (Mytilus edulis) individual collected subtidally; (A, B) are from one colony, (C, D) are from the other; both specimens bleached. (A) Autozooids near colony margin; (B) autozooids in primary layer (large primary orifices), a frontally budded male zooid (small primary orifice, at left), and frontally budded ovicellate zooids; (C) autozooids near colony margin; (D) autozooids in primary layer (large primary orifices), frontally budded male zooids (small primary orifices), and frontally budded ovicellate zooids. Scale bars: 500 Mm.