The bubble snails (Gastropoda, Heterobranchia) of Mozambique: an overlooked biodiversity hotspot

This first account, dedicated to the shallow water marine heterobranch gastropods of Mozambique is presented with a focus on the clades Acteonoidea and Cephalaspidea. Specimens were obtained as a result of sporadic sampling and two dedicated field campaigns between the years of 2012 and 2015, conducted along the northern and southern coasts of Mozambique. Specimens were collected by hand in the intertidal and subtidal reefs by snorkelling or SCUBA diving down to a depth of 33 m. Thirty-two species were found, of which 22 are new records to Mozambique and five are new for the Western Indian Ocean. This account raises the total number of shallow water Acteonoidea and Cephalaspidea known in Mozambique to 39 species, which represents approximately 50 % of the Indian Ocean diversity and 83 % of the diversity of these molluscs found in the Red Sea. A gap in sampling was identified in the central swamp/mangrove bio-region of Mozambique, and therefore, we suggest that future research efforts concentrate on or at least consider this region.

Mozambique has the third largest coastline among eastern African countries, stretching over 2,400 km across subtropical and tropical latitudes. Few works have reported the occurrence of marine heterobranch gastropod species in the country and no dedicated account is available. The latter are either generalistic faunistic reports (Bergh 1900;MacNae andKalk 1958, 1962), field guides covering broader geographical regions (Branch et al. 2008;Gosliner et al. 2008Gosliner et al. , 2015King and Fraser 2014), or systematic revisions of certain genera (Malaquias and Reid 2008;Price et al. 2011;Carmona et al. 2014a). Gosliner et al. (2008), in their book about the shallowwater sea slugs (Heterobranchia in part) of the tropical and adjacent temperate areas of the Indo-West Pacific region (IWP), have cited the occurrence of 1397 species, but of those Communicated by V. Urgorri only 16 were specifically reported to occur in Mozambique, representing little more than 1 % of the known diversity of the entire IWP. This low number of species is in sharp contrast to the richness of marine life in IWP countries and the diversity of ecosystems in Mozambique where three major bio-regions are recognized: (1) the subtropical submerged rocky reefs on the south between Ponta do Ouro on the border with South Africa (26°51′S) and the Bazaruto archipelago (22°05′S), (2) the central mangrove/swamp region between Bazaruto and Pebane (17°05′S), and (3) the tropical coral reefs in the north between Pebane and the Rovuma River on the border with Tanzania (10°28′S) (Ministry for the Coordination of Environmental Affairs 1997; Pereira et al. 2014). Moreover, the Mozambique Channel is an extremely productive area due to a complex upwelling pattern and converging eddy dipoles (Pereira et al. 2014;Ternon et al. 2014). This diversity of biota and high oceanic productivity contributes to an explanation of the recognized high marine biodiversity of the country reported for some taxonomic groups; for example, Mozambique is known to have the second highest diversity of corals in the entire Indo-Pacific (Obura 2012).
The marine heterobranch gastropods contain some of the most spectacular and colourful gastropods of our seas, of which the nudibranchs are a prime example (Gosliner et al. 2008(Gosliner et al. , 2015. The aesthetic value of these animals makes them one of the most appealing groups for recreational divers and has triggered the development of a dedicated industry of safaris and cruises (e.g. Nudibranch cruises in Indonesia [http:// www.thearenui.com/VanessaKnutson2016Komodo.html] and the Nudibranch Safari in Norway at Gulen [http://www. scubapixel.com/blog/20715-nudibranch-safari-2016]).
The systematics of heterobranch gastropods went through dramatic rearrangements in recent years. The traditional clade Opisthobranchia, previously regarded as one of the three subclasses of Gastropoda (Kay et al. 1998;Burn and Thompson 1998), is now considered polyphyletic, and the relationships and composition of several of its previous lineages have suffered striking reorganisations (Jörger et al. 2010;Schrödl 2014;Oskars et al. 2015). The formal use of the name Opisthobranchia has been abandoned in recent works (e. g. Schrödl 2014;Caballer et al. 2015); for example, the order Cephalaspidea (bubble snails), which previously included the acteonids (Acteon, Pupa), ringiculids (Ringicula), aplustrids (Hydatina, Micromelo), runcinids (Runcina), plus all other traditional cephalaspids (e.g. aglajids, bullids, diaphanids, haminoeids, etc.) (see Burn and Thompson 1998), has been split into three independent lineages of equal rank, namely the Acteonoidea (now part of the so called BLower Heterobranchs^), the Runcinacea, and the Cephalaspidea (Mikkelsen 1996;Vonnemann et al. 2005;). The latter two clades are included in a larger group termed Euopisthobranchia (Jörger et al. 2010;Schrödl 2014). This paper provides the first dedicated account on the marine heterobranchs clades Acteonoidea and Cephalaspidea of Mozambique, based on a review of the literature and newly collected material. These two groups are commonly designated as bubble snails because of the presence of a rounded shell with an involute or short spire in many genera; they occur mostly in shallow coastal areas in soft substrates of mud, sand, and seagrass, but there are exceptions with some genera (e.g. Philine, Scaphander) extending their bathymetric distribution into the deep sea, as well as a single genus that is specialized in intertidal and shallow subtidal rocky-shores (Smaragdinella) (Gosliner et al. 2008;Oskars et al. 2015).

Material and methods
Specimens were collected as a result of sporadic sampling along the coast of Mozambique ( Fig. 1) during 4 years between 2012 and 2015 and during two dedicated field campaigns: 23 Jan-03 Feb 2014 in Inhambane Province, southern Mozambique and 12-28 May 2015 around Pemba and Vamizi Island, northern Mozambique. Specimens were collected by snorkelling and SCUBA diving between the tidal zone and up to a depth of 33 m. Several subtropical and tropical habitats were surveyed including tidal and subtidal reefs, fringing coral reefs, submerged rocky reefs, seagrass, sand flats, and estuaries (Table 1). All specimens were collected by hand, measured (total length BTL^), and photographed. Specimens were frozen in seawater overnight, defrosted, and transferred to 96 % ethanol.
The classification and nomenclature adopted in this work mostly follows the World Register of Marine Species (WoRMS Editorial Board 2015), but for higher ranks we adopted Schrödl (2014). Voucher specimens were deposited at the Department of Natural History of the University Museum of Bergen, Norway (ZMBN), and at the Zavora Marine Laboratory, A.C.C.M., Mozambique (YT). Duplicates of these collections are deposited at the Natural History Museum of Maputo, Mozambique (MHN).

Results
In total, 32 species were recorded during our surveys; four species of Acteonoidea and 28 species of Cephalaspidea. Eight families were sampled, with Aglajidae being the most diverse with 13 species, followed by Haminoeidae (seven species), Philinidae and Aplustridae (three species each), Bullidae and Gastropteridae (two species each), and Acteonidae and Colpodaspididae (one each).

Genus Hydatina Schumacher, 1817
Hydatina physis (Linnaeus, 1758)  Ecology: Found actively crawling during day and night in shallow and intertidal waters between 0.2 and 4 m deep. Mating and spawning recorded in May and December. The egg mass has a white flower-shaped structure, measuring approximately 80 × 30 mm (see Fig. 2d). Observed feeding upon marine worms.

Remarks:
The systematics of the genus Hydatina is in need of a comprehensive revision using morphological and DNA characters. Specimens similar to the one illustrated here (Fig. 2c) from tropical and sub-tropical regions across the globe have often been named Hydatina physis or Hydatina vesicaria (Voskuil et al. 1995;Wirtz 1999). All specimens observed in Mozambique had the typical color morph observed in the Indo-Pacific, with a bluish colour on the mantle edge instead of the whitish colouration more common in Atlantic Ocean specimens (Wirtz 1999). Ecology: Found actively crawling during day and at night on the reef and sand between 0.2 and 3 m deep.
Ecology: Observed crawling on sand in shallow waters close to the reef around 10 m deep.

Remarks: The species is invasive in the Mediterranean
Sea where it is now established (Malaquias et al. 2016). Ecology: Crawling on rocks in tidal reefs at 0.2 m deep. One specimen (ZMBN 105152) was found on algal mats near mangroves.
Remarks: First record for Mozambique. This species is similar to some colour morphs of C. sandrana (sometimes designated as C. tsurugensis; see Remarks of C. sandrana). C. punctata is not only bigger, but has larger orange-white pigment on the edge of the parapodial lobes and rear of cephalic shield, as both lobes of the tail are conspicuously developed and pointed, whereas in the C. tsurugensis-sandrana species complex the right lobe is short and stubby.   (Yonow 1994), Chagos (Yonow 1994;Yonow et al. 2002), and Red Sea (Yonow 1994(Yonow , 2008. WIO: Aldabra (Gosliner et al. 2008), Tanzania (Rudman 1973), Madagascar (Gosliner et al. 2008), and Mozambique (present study).
Remarks: Chelidonura sandrana is a highly variable species depicting a considerable range of colour patterns (see Gosliner et al. 2008Gosliner et al. , 2015Camacho-García et al. 2014). In Fig. 3e, we depict a pattern often referred to as C. tsurugensis Baba andAbe, 1964. Turner andWilson (2012) after preforming mating experiments across various colour morphs of the Chelidonura tsurugensis-sandrana species complex have suggested that until a systematic revision of Chelidonura is available Bthe use of the name C. tsurugensis should be restricted to Japan, and elsewhere the yellow-spotted colour form should be referred to as C. sandrana^. Additionally, Camacho- García et al. (2014) provided phylogenetic evidence for the existence of several species within the C. tsurugensis-sandrana species complex and have showed the difficulties of a taxonomy based in colour morphs in this group, as specimens ranging from those having dark background with yellow spots to those completely black, or nearly white with scattered orange dots can be, molecularly, completely the same.
Genus Odontoglaja Rudman, 1978 Odontoglaja mosaica Gosliner, 2011 (Fig. 3f Remarks: First record to Mozambique. Odontoglaja mosaica was described from Madagascar by Gosliner (2011) and is chromatically very similar to O. guamensis. Both species are difficult to distinguish based on their external appearance, but Gosliner (2011) suggested that O. mosaica is the only species present in the Western Indian Ocean, whereas O. guamensis is restricted to the Western Pacific.
Genus Philinopsis Pease, 1860 Philinopsis orientalis (Baba, 1949)  Remarks: First record for Mozambique. This is a variable species depicting a range of colour patterns varying from darker forms with a thick, dark reticulate line delimiting whitish polygons (Fig. 4a) to lighter forms with a mushy, finer reticulated dark line (Fig. 4b).
Remarks: According to Gosliner et al. (2008) the records from western Australia and the Pacific Ocean require further confirmation. Our specimen released a yellow fluid when disturbed.
Philinopsis speciosa Pease, 1860 ( Fig. 4d- (Yonow 1993), and Red Sea (Yonow 1994(Yonow , 2008. WIO: Tanzania (Rudman 1972a Remarks: Gosliner et al. (2008Gosliner et al. ( : 39, 2015 considered this species to be highly variable in colour with several distinct morphs regarded as conspecific. Von Martens (1879) has described two species from Mozambique, namely Doridium cyaneum, regarded presently as a synonym of P. speciosa (Gosliner et al. 2008;Yonow 2012;Bouchet 2015b) and Doridium nigrum. The latter species is hardly mentioned in the literature and is not even listed in the World Register of Marine Species (Bouchet 2015a;Gofas and Bouchet 2015). However, the total length of the type specimen, length of the head-shield, and colour pattern with a dark background, yellow and orange blotches and lines scattered over the body, and indigo blue margins (von Martens 1879: 738) matches one of the known morphotypes of P. speciosa (see Fig. 4d; Gosliner et al. 2008), and therefore, we regard the name Doridium nigrum as a junior synonym of P. speciosa.
Yet, we do not discard that a future systematic review of the genus Philinopsis, integrating morphological and molecular data, might reveal the presence of several species within the P. speciosa colour morphs. Several recent works using molecular phylogenetics have showed that species of sea slugs regarded as highly variable in colour were in fact complexes of several species (e.g. Ornelas-Gatdula and Valdés 2012 [Philinopsis]; Carmona et al. 2014b [Spurilla]), but there are also opposite examples; for instance, Malaquias et al. (2016) have confirmed that distinct colour morphs of Chelidonura fulvipunctata are conspecific.
Remarks: This species shows chromatic variability that is particularly noticeable on the cephalic shield. This region of the body can have different intensities of brownish pigment and patches of small white dots, but typically depicts a wide W-shaped mark between the eyes and the anterior margin (see Fig. 5a-c; Malaquias and Reid 2008). Another distinguishable feature of B. ampulla are the much shorter and less rolled cephalic tentacles that look almost absent when compared to any other species in the genus (see for comparison Fig. 5a-c B. ampulla and Fig. 5d-f B. orientalis). Ecology: Tropical and subtropical reefs often on rocks covered with green algae; found both crawling at night and in the daytime.
Remarks: This IWP species has two main morphotypes; one with a dotted shell occurring in the West Pacific and the other with a shell mottled with brown-reddish dots restricted to the Indian Ocean. It can be easily confused with Bulla vernicosa, but the latter species is only present in the western and central regions of the Pacific Ocean (see Malaquias and Reid 2008 for a discussion).
The specimen illustrated in Fig. 5g is chromatically distinct and the shell is more elongated and lighter than B. orientalis. It is not possible at this point to tease apart whether these differences reflect a distinct taxonomic identity or are ontogenetic since the specimen is small and likely a juvenile.
Ecology: Found crawling on a rock at 18 m deep.

Remarks
Remarks: First record for Mozambique.
Ecology: Found under a rock at 1 m deep. Remarks: This animal appears to correspond to an undescribed species, but additional specimens are necessary to soundly compare with other congeneric species from the Indo-Pacific.
Remarks: Preliminary molecular studies (Oskars and Malaquias, work in progress) indicate that most likely there are several species in the IWP Bhidden^under the name H. natalensis. Typically, specimens mottled with brownish/ greenish dots in the region are ascribed to this species (e.g. Gosliner 1987;Gosliner et al. 2008). Figure 6e and f depict two different colour morphos; whether this is related to different ontogenetic stages, different food sources, or different species, it remains to be thoroughly tested with morphological and DNA data. Distribution: Hawaiian Is (Moretzsohn and Kay 1995) and Mozambique (present study).
Remarks: First record for Mozambique. The lighter colour pattern of the specimen depicted in Fig. 7b is likely due to its smaller size. Yonow (2000;2008) has identified similar specimens from the Red Sea as P. olivaceus (Yonow 2000(Yonow , 2008, whereas all previous records of this morphotype in the Western Indian Ocean (Gosliner 1987;Gosliner et al. 2008) were identified as P. smaragdinus. This disagreement is not surprising since, as highlighted above, the taxonomy and nomenclature of this genus is little understood and hampered by poor original descriptions.
Ecology: Found active at night and in the daytime in tidal reefs between 0.2 and 2 m deep.
Remarks: Based on the general colour pattern we tentatively identified our specimens as P. cf. albocollaris, but typically this species has a conspicuous inverted BW^-mark on the rear part of the cephalic shield that is not visible in the specimens from Mozambique. First record for Mozambique and WIO.
Ecology: Found at 1 m deep crawling on sand.
Distribution: Mozambique, likely more widespread through the Indo-Pacific.
Remarks: The lack of a systematic revision of the genus Phanerophthalmus makes it difficult to ascertain the taxonomic value of small differences in the colour pattern of these slugs. Our specimen matches the one depicted by Gosliner et al. (2008: 29;: 32, for two localities along the IWP) and by Nakano (2004: 29 for Japan), both identified to genus only, but they could also be juveniles of P. cylindricus (see Figs. 6g and 7d). Distribution: Between Mozambique and South Africa (Branch et al. 2008;Price et al. 2011).
Remarks: This species was until recently regarded as broadly distributed across the IWP, but based on the study of the anatomy of specimens from several localities Price et al. (2011) concluded that P. aperta is restricted to the Western Indian Ocean countries of Mozambique and South Africa.  (Price et al. 2011;Gosliner et al. 2008), and Papua New Guinea (Gosliner et al. 2008). WIO: Mozambique (present study).
Remarks: The species P. elegans was until now only known from the western Pacific. Our specimens resemble those illustrated by Gosliner et al. (2008: 35;2015: 40) and Price et al. (2011: 6, Fig. 2d), but are also similar to those depicted by Gosliner et al. (2008: 37, Philine spc. 3 and2015: 42 as Philine spc. 4) from the Philippines. Many IWP species of Philine have been poorly described based only on conchological features, creating much confusion in the literature (Price et al. 2011). Thus, we do not discard that future studies will reassign our specimens to a different taxon. A sound taxonomic revision of these snails is lacking, and recently the systematics of philinid heterobranchs went through dramatic changes at both generic and familial levels (Oskars et al. 2015).
Ecology: Found on the green algae Ulva sp. at 12 m deep.
Remarks: Our specimen is lighter in colour compared with the more typical reddish colour pattern of P. rubrata (see Gosliner et al. 2008: 36).

Conclusions
Knowledge of marine heterobranchs from Mozambique was limited, with only 16 species mentioned in specialized literature and of those only seven were not collected during our surveys (see Table 2). This low figure reflected not only the extent of the diversity of this faunal group, but a general lack of baseline faunistic and taxonomic research on marine invertebrates in Mozambique. Marine research in Mozambique has been mainly concentrated on fisheries or on the conservation of emblematic organisms like corals, turtles, fish, and mammals (see Bandeira et al. 2002 for a synopsis). Gosliner et al. (2008) reported the occurrence of 228 shallow-water species of Acteonoidea and Cephalaspidea in the tropical IWP, the most diverse marine biogeographic region of the world (Paulay and Meyer 2002;Williams and Reid 2004;Frey and Vermeij 2008;Gosliner et al. 2008Gosliner et al. , 2015); 79 of those species are present in the Indian Ocean and Yonow (2008) listed 47 species inhabiting the Red Sea. In Mozambique alone 39 species are now reported, of which more than half (22 species) result from the present contribution; five of those are new records for the Western Indian Ocean.
The diversity of Acteonoidea and Cephalaspidea in Mozambique represents approximately 50 % of the total number of species of these molluscs known in the entire Indian Ocean and 83 % of those known in the Red Sea. Yet, it is certainly an underestimation of the global diversity, as habitats such as sandy substrates and mangrove areas have been hardly sampled for these molluscs and will certainly yield new taxa. Previous reports and our sampling efforts were concentrated in the northern and southern areas of the country, whereas the central mangrove/swamp region, almost one-third of the coastline, was not covered (see Fig. 1). Thus, we suggest that future sampling efforts concentrate or at least consider this region.