Balanus declivis Darwin, 1854: 275, pl. 7 fig. 4a–d; by subsequent designation (Pilsbry 1916: 229).

M. brachialis (Rosell, 1972); M. costatus Zullo & Standing, 1983; M. cuneiformis (Hiro, 1936); M. declivis (Darwin, 1854); M. koreanus Kim & Kim, 1983; M. longirostrum (Hoek, 1913); M. nebrias (Zullo & Beach, 1973); M. orcutti (Pilsbry, 1907); M. porphyrophilus sp. nov.; M. robinae Van Syoc, 1988.

M. orcuttiformis (Kolosváry, 1941).

Parietes solid, unornamented, weakly articulated, basis membranous. Rostrum scoop or boat-shaped, often elongate relative to other parietes. Tergum with spur furrow open. Cirrus IV with erect spines, with or without recurved teeth on anterior ramus.

With the addition of the below described species, there are now 10 species included within Membranobalanus. Utinomi (1968) synonymised the taxa Balanus (Membranobalanus) longirostrum var. krusadaiensis Daniel, 1955, B. (M.) basicupula Suhaimi, 1966, and B. (M.) roonwali Prem-Kumar & Daniel, 1968 under M. longirostrum, proposing that the differences observed are within the bounds of intraspecific variability. Recently, van Syoc et al. (2015) transferred Acasta acuta (Kolbasov, 1993) out of Membranobalanus based primarily on the presence of calcareous spines on the parietal wall, a character no other Membranobalanus possess and in reference to a cladistic analysis in an unpublished thesis. The fact that A. acuta is found in sponges of the family Petrosiidae (order Haplosclerida), not the Clionaidae as reported for the remaining members of the genus, separates this species ecologically from Membranobalanus, was used as further justification. The general appearance, membranous basis and elongated rostrum of A. acuta are typical features of Membranobalanus, however. While we treat this reassignment with caution, we have no evidence with which to dispute it.

Kolosváry (1941) described Balanus (M.) orcuttiformis based on the parietes of a single empty specimen. The locality details of the specimen are vague, only given as “India Orient.”, but presumably meaning eastern India. No detail regarding a host was given. The description is very brief, giving very few clues to the identification of this species, and the only illustration of the specimen, in lateral view, could belong to a number of genera, but not Membranobalanus as currently defined. Most notable is the absence of any elongation of the rostrum and the largely horizontal basal rim of the parietal wall gives the appearance that it was attached to, rather than embedded within, a substrate. Additionally, the exceptionally broad alae and absent radii are reminiscent of the Pachylasmatoidea, which possess solid parietes and often have a membranous base (see Jones 2000). Membranobalanus orcuttiformis has not been recorded since its description and unfortunately the specimens are missing from the Museo di Storia Naturale dell’Università di Firenze, Italy and thus cannot be reexamined (Innocenti 2006). For these reasons, this species is considered herein a nomen dubium and has been excluded from the key below.

The remaining Membranobalanus species can be separated into two morphological lineages, approximating an American centred group and an Indo-West Pacific group. The former have recurved teeth, similar to those present in some members of the Acastinae, as well as erect spines on cirrus IV, smooth growth lines on the scutum and the articular ridge and groove of the scutum is prominent, extending well beyond the articular margin, with a correspondingly wide articular groove on the tergum. The latter group bears only the erect spines on cirrus IV, finely striated growth lines, and has relatively weak articular structures on the opercular plates. From a biogeographic perspective one species disrupts this pattern: Membranobalanus koreanus from the waters around the Korean Peninsula. As described and figured by Kim and Kim (1983), M. koreanus has recurved teeth and a large articular ridge on the scutum. The records of M. orcutti by Barnard (1924) and Rosell (1973, 1975) from South Africa and the Sulu Archipelago, respectively, were considered suspect by Zullo and Beach (1973) and Van Syoc and Winther (1999). The Sulu Archipelago specimens lack the recurved teeth on cirrus IV of the specimens described from Catalina Island by Zullo and Beach (1973), and while Barnard’s description specifically mentions recurved teeth on cirrus IV, most of his description is deferred back to either Pilsbry’s (1907, 1916) descriptions of M. orcutti or Hoek’s (1913) description of M. longirostrum. Of particular note in Barnard’s description is that the scutum has an external, setose membrane, a character seen in some species of the Acastinae, but not Membranobalanus. Both reports should be considered species inquirenda, but potentially represent previously undescribed species.

Holotype. AUSTRALIA • WAM C66803, 1 hermaphrodite; 9 mm rostro-carinal diameter; Western Australia SE of Rottnest Island, Wallace Island, The Count; 32°0.89'S, 115°33.53'E; 12 m; coll. A.M. Hosie; 23 Feb 2017; host: WAM Z86929, Spheciospongia purpurea.

Paratypes. AUSTRALIA • WAM C71852, 1 hermaphrodite; 8 mm rostro-carinal diameter; empty shell; same as data as for holotype. • WAM C71853, 1 hermaphrodite; same as data as for holotype. • WAM C71881, 1 hermaphrodite; same as data as for holotype. • SAMA C12706, 1 hermaphrodite; South Australia, Kangaroo Island, off Second Gully between Western River Cove and Snug Cove; 32 m; coll. J. Thiselton; 19 Nov 2002; host: SAMA S2910, S. purpurea. • SAMA C12707, 1 hermaphrodite; same data as for previous. • SAMA C12708, 1 hermaphrodite; 14 mm rostro-carinal diameter; same data as for previous.

Shell wall robust, cylindrical, growth ridges weak; orifice toothed, large; rostrum basal margin broadly rounded, extending below basal plane of remaining parietes. Tergum narrow, beaked, spur narrow, separated from basiscutal angle by half its own width; scutum with faint, external longitudinal striations; basitergal angle broadly rounded. Cirri III and IV with row of strong, erect spines on anterodistal margin of anterior ramus; cirrus IV pedicel without erect spines; cirri IV and V with row of stout spines on posterior margin of anterior ramus basal segment.

All shell plates, prosoma, and internal organs stained purple in vivo, otherwise white. Shell walls (Figs 1, 2) parallel, except bowed rostrum; parietes externally with horizontal growth lines raised, ridge-like, giving shell a roughened appearance; radii prominent, summits oblique, sutural edges roughened, alae wide, summits oblique. Internally parietes smooth, sheath occupying approximately half of shell height, with horizontal growth lines, basal margin adpressed. Carina approximately as wide as lateral plate, carinolateral narrowest plate, approximately one-third width of lateral plate. Rostrum elongate, extending below basal margin of other parietes, almost twice length and twice width of lateral plate, basal margin broadly rounded below basal margin of latera. Basal margins of carina and carinolatera more or less perpendicular to shell vertical axis, latera basal margins curving basally to form contiguous rim connecting latera with rostrum. Basis membranous, follows contour of basal rim, not depending below parietes.

Scutum (Fig. 3A–D, I, J) triangular, height 1.3–1.8 times width, growth lines prominent with faint longitudinal striations; basal and tergal margins separated by broadly rounded basitergal angle. Internal surface, slightly ridged apically near tergal margin, otherwise smooth; adductor muscle pit distinct; lateral depressor muscle pit distinct, without depressor muscle crests, extending one-third distance to beginning of articular ridge bounded by low, rounded adductor muscle ridge. Articular groove deep, narrow; articular ridge barely projecting beyond articular margin, basal margin curved.

Tergum (Fig. 3E–H, K, L) narrow, height more than twice width, articular and carinal margins arcuate with beaked apex; external growth lines conspicuous, but less raised than those of scutum; spur furrow indicated by shift in growth lines and slight depression; basiscutal angle sloping into spur; spur narrow, separated from basiscutal angle by half its width, basally truncate. Internally smooth, depressor muscle crests weak (may be absent in small specimens), articular groove wide, open, shallow; articular ridge low, becoming confluent with scutal margin basally.

Labrum (Fig. 4A–F) bilobed, with deep medial notch bounded by rounded crests, each with two or three marginal teeth and numerous fine setae.

Mandibular palp (Fig. 4A, C, E, G, H) rhomboid, apex obliquely truncate, anterior margin concave, posterior margin straight; setae (Fig. 4F) heavily serrulate, becoming longer and denser distally.

Mandible (Fig. 4I–K) with four distinct teeth, 2^nd and 3^rd tooth bifid, 4^th tooth much smaller than preceding three, 5^th tooth obsolescent, confluent with molariform inferior angle; short setae covering inner and outer faces, longer fine setae on inferior and superior margins.

Maxillule (Fig. 4l, M) with 10 robust setae on cutting margin, first, second, and ninth longer and more robust than remaining setae, inferior angle with several short robust setae. Cutting margin straight, with very slight notch below second seta. Dense, short setae regularly spaced on inner and outer faces, longer fine setae on inferior and superior margin

Maxilla (Fig. 4N, O) bilobed, basal lobe ovate, serrulate setae arranged on anterior margin; distal lobe elongate, serrulate setae on anterior margin becoming more dense at apex, longer than those on basal lobe.

Cirrus I (Fig. 5A) with unequal rami, anterior ramus twice length of posterior ramus, both rami bearing serrulate and simple setae. Posterior ramus segments with protuberant anterior margins, more densely setose than anterior ramus, arranged in tufts on anterior margins.

Cirrus II (Fig. 5B) shorter than other cirri, rami subequal; anterior margins of both rami slightly protuberant with serrulate and simple setae.

Cirrus III (Fig. 5C, D) pedicel with plumose setae on anterior and posterior margins of both segments; rami equal; anterior ramus with erect spines and ctenoid scales on anterodistal portion of medial segments, tuft of long serrulate setae on rounded anterior margin up to approximately three times as long as segment, tuft of serrulate setae as long as segment at posterodistal angle.

Cirrus IV (Fig. 5E–L) pedicel without erect teeth on anterodistal margins, pedicel with numerous small denticles on posterior and anterior margins of mesial face, tuft of short setae at posterodistal angle, small ctenoid scale-like denticles at anterodistal margin. Rami subequal in length; basal-most segment with up to 13 stout spines along posterior margin (holotype with six), basal segments with one or two stout spines at posterodistal angle. Anterior ramus intermediate segments with row of erect spines on anterodistal portion of all but most distal segments, much reduced in posterior ramus, without recurved teeth on anterior faces, posterodistal angles with tuft of setae.

Cirrus V (Fig. 6A–C) characters intermediate between cirri IV and VI; pedicel with numerous small denticles on posterior and anterior margins of mesial face, tuft of short setae at posterodistal angle, small ctenoid scale-like denticles on anterodistal margin. Rami equal, segments becoming elongate distally from half as long as wide to three times longer than wide, both rami with four pairs of serrulate setae on anterior margin, distal most pair longest, approximately 3 times length of segment; tuft of simple setae at anterodistal angle, setae up to half length of segment; erect teeth on anterior margins of intermediate segments less pronounced than those on cirrus IV; stout spines on posterior margins of rami similar to cirrus IV.

Cirrus VI (Fig. 6D–G) rami equal, pedicel with numerous small denticles on posteromesial face, tuft of simple setae at posterodistal angle; intermediate segments of both rami becoming elongate distally from 1.5 times wider than long to three times longer than wide, both rami with four pairs of serrulate setae on anterior margin, distal most pair longest, approximately three times the length of segment, basal pair 0.3 times length of segment; tuft of simple setae at anterodistal angle, setae up to 0.5 length of segment; anterior margins of all segments without erect teeth, posterior margins without stout spines.

Cirral segment counts as follows (anterior ramus, posterior ramus):

Penis longer than CVI, annulated, sparsely setose along length; basidorsal point (Fig. 6H) prominent, triangular, height twice width.

Southern Australia, from Perth to Adelaide.

From Greek porphyra, purple and philos, indicating a love of; gender masculine. In reference to the vibrant purple colour of the only known host.

Membranobalanus porphyrophilus sp. nov. is most readily distinguished from its congeners by the narrow, beaked tergum. The absence of recurved teeth on cirrus IV, the finely striated scutal growth lines, and the weak articular structure of the opercular plates further separates the newly described species from the predominantly American group of species, viz. M. costatus, M. declivis, M. koreanus, M. nebrias, M. orcutti, and M. robinae. The remaining species in the genus, M. brachialis, M. cuneiformis, and M. longirostrum all bear prominent, erect spines on the pedicel of cirrus IV and lack the stout spines on the posterior margins of the rami of cirri IV–VI. This is only the second member of the genus reported from Australian waters. The other, M. cuneiformis, is known from near Darwin and was reported by Hiro (1936) as commensal with an unidentified species of Cliona that was bioeroding the shell of Pinctada maxima (Jameson, 1901). Externally, M. cuneiformis can be separated from M. porphyrophilus by the conical shape of the shell, resulting in a small orifice and absent radii.