Scutellastra H. Adams & A. Adams, 1854

Espinosa, F., Carballo, J. L., Bautista-Guerrero, E., Yáñez, B., García-Gómez, J. C. & Michel-Morfín, J. E., 2020, Redescription of the highly endangered species Scutellastra mexicana (Broderip & G. B. Sowerby I, 1829) (Mollusca, Gastropoda), Journal of Natural History 54 (15 - 16), pp. 991-1007 : 995-1003

Genus Scutellastra H. Adams & A. Adams, 1854

Scutellastra mexicana (Broderip & G.B. Sowerby I, 1829) Patella mexicana Broderip & G. B. Sowerby I, 1829 Patella gigantea Lesson, 1831 Patella maxima d’Orbigny, 1841 Ancistromesus mexicana Dall, 1871

Morphological description

The type locality is Mazatlán ( México), where some shells have recently been found ( Figure 2 ) . Examination of the shells revealed differences between the small and large-sized specimens, and separate descriptions are provided for each of the morphological forms.

Small-sized specimen shells and soft tissue parts

The small specimen LEB-ICML-UNAM-BR21 measured 11 cm in length and 9 cm in width showing a subcentral apex displaced to the anterior end ( Figure 3 (a)). The sculpture is formed by several broad primary radial ribs (up to 10) and many secondary ones between them, giving a wrinkled surface ( Figure 3 (b)). The figure of the shell opening is ovate but narrows near the anterior end with an irregular edge. The central area of the shell inside muscle scar white, and intermediate area between muscle scar and the margin creamy to brownish. Interior margin of the shell narrow and nacarated.

The foot is white to greyish with a brown line delimiting the margin of the mantle where two series of pallial tentacles can be observed. The smaller ones (secondary pallial tentacles) are located exteriorly, and the larger ones (primary pallial tentacles) are in a most delayed position. The head and cephalic tentacles are very dark, almost black ( Figure 3 (c)), with a distinctive scalloped edge of the mantle.

Large-sized specimen shells and soft tissue parts

The large-sized specimen LEB-ICML-UNAM-HH1 measured 20.1 cm in length and 16.9 cm in width also showing a subcentral apex displaced to the anterior end ( Figure 4 (a)). Although the shell opening has a similar shape, the edge is quite regular ( Figure 4 (b)), and the shell is completely eroded and encrusted by boring fauna (see Figure 4 (c)). The central area of the shell inside muscle scar white, and intermediate area between muscle scar and margin also white, but the whole internal shell area shows a spotted pinkish pattern ( Figure 4 (d)).

The foot is grey, and the margin of the mantle shows a most intense brown colour compared with the small specimens with similar dispositions. The head and cephalic tentacles are dark brown ( Figure 4 (e)) with a distinctive sculpture within the edge of the mantle similar to a pearl necklace that it is absent from small specimens.

Radula

Strong median tooth, fully developed and flanked by two pairs of central teeth; thus, there are five fully developed centrals in one horizontal series that are followed by a pair of large tetracuspid lateral teeth located between the series of central teeth ( Figure 5 (a–c)). Finally, three elongated marginal teeth are located on both sides (right and left) of each series ( Figure 5 (b)).

Habitat

This species usually occurs in shallow subtidal areas (up to 6 m depth) in zones of extreme wave action in which adult specimens defend their home scars, which are surrounded by a garden where they seem to graze.

Geographical distribution

The distribution ranged from Mexico to Ecuador and Peru in historic times (see Keen 1971; Alamo and Valdivieso 1997) but is presently restricted to the Pacific coast of Mexico.

Remarks

Specimens of the sympatric limpet Lottia discors (Philippi, 1849) are frequently observed to be very closed. However, this species is much smaller and possesses an ovate shell opening, a regular edge, and a finely ribbed sculptured shell. These two species also differ markedly in interior shell microstructure characters. The foot is orange, and the head and cephalic tentacles are white ( Figure 6 ). The small specimen described (LEB-ICML-UNAM- BR21 ) was dissected, and the external observation of the colour of the gonad and the histological analysis indicated that it was a male (August 2017) .

Molecular analysis

The phylogenetic position of S. mexicana was determined with two tree datasets that were constructed with two independent mitochondrial loci (COI: 55 taxa and 537 bp; 16S: 55 taxa

a

b

and 489 bp) using maximum likelihood and Bayesian methods. The ML and BI topologies for the 16S and COI datasets are presented in Figures 7 and 8 , respectively, with the corresponding support bootstrap values/posterior probabilities (ML/BI) for each node. Regardless of the gene (16S or COI) or method (ML or BI) employed, the tree topologies are consistent and revealed two mayor monophyletic clades, i.e. the families Patellidae and Nacellidae . A first subgroup clustered the species classified as Patellidae into the following genera: Scutellastra , Helcion , Patella , and Cymbula . A second clade included the species of Nacellidae , which were classified in the Nacella and Cellana genera. The members of Lottiidae were evidently divergent and distant from all other Patellidae and Nacellidae taxa.

Overall, the phylogenetic analysis with the 16S rRNA and COI yielded the most poorly resolved ML tree with weak bootstrap support values for most of the clades. The BI phylogenetic reconstruction using the 16S and COI datasets recovered the same two main clades ( Patellidae and Nacellidae ), which were supported by a high posterior probability. Specifically, all of the Mexican limpet specimens analysed in this study and their complete sequences matched with previously published sequences of S. mexicana ( AF058235 ), which confirmed the identity of all specimens collected. For S. mexicana , only one 16S sequence was previously available, and new sequences of the COI gene were provided in the present study.

Analyses of the independent COI and 16S datasets revealed a similar topology with a high degree of congruence at the family level with exceptions of some minute differences in the constructions of some clades. In the 16S tree, Scutellastra was resolved as a paraphyletic sister taxon to the rest of Scutellastra , Cymbula , and Helcion , whereas Patella was a monophyletic taxon. Specifically, S. mexicana formed a weakly supported clade (ML: 77, BI: 0.56) with S. optima , S. exuta , and S. flexuosa . The COI resolved to reveal some members of Scutellastra as a paraphyletic sister taxon to Helcion and Cymbula , whereas Patella was a monophyletic taxon. Interestingly, none of the sequences of S. mexicana had any close relationship with other members of Scutellastra ; thus, S. mexicana formed an independent and monophyletic clade relative to the other clades with high bootstrap support (ML: 100) and high posterior probabilities (BI: 1.00).

Figures

Figure 2. Three different views of the same shell of Scutellastra mexicana (A, B, and C) recently found on beaches from the type locality Mazatlán. Scale bar: 10 cms.

Figure 3. Small specimen BR21. (a) Dorsal view of the shell of the small specimen. (b) Ventral view of the shell of the small specimen. (c) Ventral view of the soft tissue parts showing a diagnostic feature of small specimens, i.e. a ‘scalloped’ edge of the mantle. Scale bar: 5 cms.

Figure 4. Large-sized specimen HH1. (a) Dorsal view of the shell of the large-sized specimen. (b) Ventral view of the shell of a large-sized specimen. Scale bar: 10 cms. (c) Detail of the shell surface. (d) Detail of the inner part of the edge. (e) Ventral view of the soft tissue parts showing a diagnostic feature of large-sized specimens, i.e. a ‘pearl necklace’ within the edge mantle (please note that this is a different specimen than HH1, but this specimen is from the same sampling site and is of a similar size).

Figure 5. (a) SEM photograph of a radula. Scale bar: 1 mm. (b) Detail of the pluricuspid lateral tooth (black arrow) and marginal teeth (white arrows). Scale bar:400 µm. (c) Optical microscope photograph of the radula. Scale bar: 1 mm.

Figure 6. Shells of Scutellastra mexicana (left) and Lottia discors (right). (a) Dorsal view. (b) Ventral view. Scale bar: 4 cms.

Figure 7. Consensus tree of the families Patellidae and Nacellidae with the position of S. mexicana using maximum likelihood and Bayesian methods based on partial 16S rRNA. The bootstrap values/ posterior probabilities (ML/BI) are indicated under the principal nodes. The taxon names are followed by their GenBank accession numbers (specimen AF058235 belonging to Scutellastra mexicana was also downloaded from GenBank, whereas the other S. mexicana specimens were sequenced in the present study). The shaded area indicates the position of S. mexicana.

Figure 8. Consensus tree of the families of Patellidae and Nacellidae with the position of S. mexicana using maximum likelihood and Bayesian methods based on the mitochondrial COI mtDNA gene. The bootstrap values/posterior probabilities (ML/BI) are indicated under the principal nodes. The taxon names are followed by their GenBank accession numbers (S. mexicana specimens were sequenced in the present study). The shaded area indicates the position of S. mexicana.