Membranobalanus Hoek, 1913

Hosie, Andrew M., Fromont, Jane, Munyard, Kylie & Jones, Diana S., 2019, Description of a new species of Membranobalanus (Crustacea, Cirripedia) from southern Australia, ZooKeys 873, pp. 25-42 : 27-28

Membranobalanus Hoek, 1913

Type species.

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

Species composition.

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.

Nomen dubium.

M. orcuttiformis ( Kolosváry, 1941).

Diagnosis.

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.

Remarks.

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on the parietal wall, a character no other Membranobalanus possess and in reference to a cladistic analysis in an unpublished thesis.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.