Porella acutirostris Smitt, 1868

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 : 3738-3742

Porella acutirostris Smitt, 1868 species complex

( Figure 14A–D)

Colony

Description. Unilaminar, encrusting, sheet-like, roughly circular, tightly adhering to hard substrates, light yellow to deep golden-yellow in colour, often with the margin more heavily pigmented; largest observed 5 cm across.

Zooids. Rectangular, barrel-shaped, or rounded hexagonal, sometimes tapering proximally; separated by a groove in which lies a narrow suture line between adjacent rows of areolae; 0.35–0.70 mm long (average 50.515 mm, n 521, 4) by 0.21–0.33 mm wide (average 50.260 mm, n 521, 4); interconnecting by a pair of multiporous septula in distal wall and a single elliptical multiporous septulum in each distolateral wall; basal wall completely calcified.

Frontal wall. Slightly to markedly convex; finely to coarsely granulated; imperforate, with 6–11 conspicuous areolar openings along each lateral margin, extending lateral to orifice, slit-like, oblong, circular, or irregular in shape.

Orifice. Primary orifice ( Figure 14D) semicircular, broader than long, 0.08–0.13 mm long (average 50.091 mm, n 520, 4) by 0.10–0.15 mm wide (average 50.115 mm, n 520, 4); proximal margin sometimes straight, but usually broadly convex, often rising to a low, broad, rounded lyrula; condyles variable within colonies, often minute, flattened, and scarcely evident, but raised, blunt, and conspicuous in some zooids. Peristome deep enough to obscure primary orifice; bounded proximally by avicularian chamber and laterally by a narrow flange on each side that is confluent distally with raised proximal lip of ovicell or raised proximal margin of succeeding zooid. Secondary orifice semicircular to roughly quadrate.

Spines. Five spines around distal orificial margin in zooids surrounding ancestrula; lacking outside zone of astogenetic change.

Avicularia. Each zooid has a median suboral avicularium ( Figure 14D); the chamber arises from an areolar pore on each side, is raised from frontal wall, broader than long, crescentic ( Figure 14C), 0.10–0.15 mm long (average 0.121 mm, n 520, 4) by 0.15–0.24 mm wide (average 50.199 mm, n 520, 4); rostrum partly within peristome, the hinge bar about level with top of peristomial flanges; rostrum orientated nearly vertically or angled to 45 ° in proximal direction, mandible a blunt equilateral triangle.

Ovicell ( Figure 14A, B). Raised, globose, 0.15–0.20 mm long (average 0.186 mm, n 520, 4) by 0.20–0.28 mm wide (average 50.237 mm, n 520, 4); imperforate, surface finely or coarsely granulated like frontal wall.

Ancestrula . Ancestrula proper was obscured in Ketchikan specimens, but budding pattern appears to be three zooids distally and distolaterally and a pair of larger zooids proximally. Periancestrular zooids are similar to later autozooids, but have five oral spines.

Remarks

Smitt (1868) originally described Porella acutirostris from the European Arctic, with the type locality near Spitsbergen Island (78 ° 109N, 13 ° 309E). Subsequently, it came to be considered a circumpolar, arctic-boreal species; see Kluge (1975) for many distributional records. The nominal species has been reported as far south as Wood’s Hole (Cape Cod) in the north-western Atlantic ( Osburn 1912), southern California in the north-eastern Pacific ( Osburn 1952), and middle Honshu, Japan in the north-western Pacific ( Mawatari and Mawatari 1981). We can find no record of this species from the boreal north-eastern Atlantic; in Europe, it has apparently not been reported south of the Lofoten Islands in the Norwegian Arctic ( Nordgaard 1918).

Hincks (1884) described from British Columbia a similar species, P. major , which Osburn (1952) listed as a junior synonym of P. acutirostris . However, Soule et al. (1995) resurrected P. major Hincks , speculating that this is a southern species, distinct from P. acutirostris and ranging from British Columbia to southern California in the eastern Pacific. They showed SEM illustrations of Hincks’s type of P. major , a young, periancestrular colony, and suggested that characters distinguishing P. major from P. acutirostris include flatter zooids; larger, more numerous areolae; weaker condyles; and a less bulbous avicularian chamber.

To try to clarify the differences between P. major and P. acutirostris , we examined Smitt’s type of P. acutirostris (SMNH Type-1752) ( Figure 14E–H), which includes two small fragments of P. acutirostris (about 3 and 7 mm 2) lacking ovicells, as well as a 2 mm 2 fragment of Porella peristomata ( Nordgaard, 1905) . The primary orifice in the type of P. acutirostris ( Figure 14F) is very similar to that in the type of P. major ( Soule et al. 1995, Plate 94B), with similar variation. Some zooids in the P. acutirostri s type have only a slightly convex proximal orificial margin, whereas others have a low, broad, rounded-truncate lyrula. The degree of development of the condyles is variable, ranging from weak and flattened to rounded and conspicuous. The shape of the avicularian chamber ( Figure 14E– H) is likewise variable; in some zooids it is bulbous, but in others it is narrow and sometimes crescentic, as in the type of P. major . Some zooids in the type of P. acutirostris have the avicularian rostrum narrower in proportion to length than in the type of P. major , with the mandible long-triangular; however, this is again variable, with other zooids having a wider, equilateral mandible. The types of P. acutirostris and P. major are similar in the structure of the secondary orifice and in the granulated texture of the frontal wall. Finally, zooid length is similar, 0.43–0.65 mm (n 520, 2) in the type of P. acutirostris compared to 0.45–0.60 mm given by Soule et al. (1995) for P. major .

There are also differences between the type of P. acutirostris and that of P. major . Porella acutirostris has fewer (four to seven on each side), slit-like areolae ( Figure 14E, G), whereas those of P. major are more numerous (6–10 on each side) and much larger, with the appressed lateral walls showing as a distinct suture line between rows of areolae ( Soule et al. 1995, Plate 94A, C, E). The avicularian chamber is proportionately larger in P. acutirostris ; its external area is generally equal to or greater than twice the area of the primary orifice. The chamber of P. major is less than twice the area of the primary orifice. In P. acutirostris , the avicularian rostrum tends to be set farther back on the chamber and is thus more separate from the peristome. Finally, an apparent difference is the flatness of the zooids of P. major compared to P. acutirostris . However, it is unclear whether this character is idiosyncratic to the type specimen of P. major , which is a young colony growing on a smooth shell, or an artefact resulting from the back-scattered electron imaging used to examine the uncoated specimen.

Our specimens from Ketchikan are similar to P. major in having larger, more numerous areolae with a narrow suture line between rows; an avicularian chamber smaller than twice the area of the primary orifice; and the rostrum tilted farther forward into the peristome, with an equilateral mandible. However, our zooids do not appear as flat as in the type of P. major . We cannot at present identify our material with either P. acutirostris or P. major with certainty. One reason is that there appears to be some clinal variation in relevant characters. A specimen we have from the Bering Sea, for example, shows areolae intermediate between P. acutirostris and P. major ; their number is low as in the former, but they are of larger size, as in the latter. By the same token, Osburn’s (1912, Plate 27, Figure 69) illustration of P. acutirostris from Wood’s Hole in the boreal north-western Atlantic shows 4–10 conspicuous areolae per side, much like those of P. major , but with a circular rather than a quadrate secondary orifice even in ovicellate zooids. This suggests either that there is clinal variation in areolae in P. acutirostris , or that boreal species distinct from arctic P. acutirostris exist in both the Atlantic (e.g. the Woods Hole population) and Pacific (e.g. P. major ). We note that although P. major has previously been reported from the Asian side ( Androsova 1958; Gontar 1980), Androsova’s species, with tiny, circular areolae and the suboral avicularium with a semicircular mandible, is likely not P. major Hincks , in light of the type figured by Soule et al. (1995).

Populations of nominal P. acutirostris previously identified around the Pacific rim, e.g. the Kurile Islands ( Mawatari 1956), Kodiak, Alaska ( Dick and Ross 1988), and Akkeshi, Hokkaido, Japan ( Mawatari and Mawatari 1981) have numerous, large areolae characteristic of P. major . However, they show variation in some of the same characters that vary within colonies in the types of P. acutirostris and P. major . For example, these Pacific populations tend to have a straight or broadly convex proximal orificial margin, without occurrence of a rounded-truncate lyrula. In P. acutirostris and P. major , the proximal orificial margin varies at the other end of a continuum, from being broadly convex to having a rounded-truncate lyrula. Compared to Ketchikan specimens, some specimens from the Bering Sea, Kodiak, and Akkeshi are more heavily tuberculate and have the avicularian rostrum suspended almost vertically in the peristome.

Porella columbiana O’Donoghue and O’Donoghue, 1923 is a nominal species closely related to P. major and P. acutirostris (compare Figures 13 and 14 herein with Soule et al. 1995, Plate 94). Like the latter two species, P. columbiana has a granulated frontal wall and ovicell; lacks oral spines outside the zone of astogenetic change; has a blunt-triangular mandible on the suboral avicularium; and has a somewhat quadrate secondary orifice. The areolae (5–11 per side, most frequently seven or eight) are much like those of P. major in size and number (6–10 per side, mode57); indeed, the two may be synonymous. However, compared to the type of P. major , P. columbiana has longer, more evenly granulated zooids and a narrower lyrula.

Porella acutirostris , rather than being a circumpolar, arctic-boreal species, likely comprises a complex of morphologically similar species, among which P. major and P. columbiana may be valid species. Integrated studies using morphological and molecular data will be necessary to adequately resolve this group; until this is done, we simply refer specimens from Ketchikan to the species complex, and include neither synonymies nor distributional data.

Figures

Figure 13. Porella columbiana O’Donoghue and O’Donoghue, paralectotype, BMNH 1964.1.2.15; bleached: (A) group of autozooids; (B) zooids near colony margin showing stages in formation of suboral avicularian chamber; (C) group of ovicellate zooids; (D) enlargement of autozooidal orifice showing condyles and weak lyrula. Scale bars: 500 Mm (A–C); 200 Mm (D).