Introduction
Pteropods, commonly called ‘Sea Butterflies’, are holoplanktonic marine gastropods. These zooplankton are ubiquitous in the world's oceans, exhibiting vast vertical and geographical distributions (Byrne, Reference Byrne, Liu, Atkinson, Quiñones and Talaue-McManus2011; Ikeda, Reference Ikeda2014). Although they are generally epipelagic, occurring in the upper 200 m, some are mesopelagic species that undergo diurnal vertical migrations from the epipelagic to the mesopelagic zone (Myers, Reference Myers1968; Haagensen, Reference Haagensen1976; Bè and Gilmer, Reference Bé, Gilmer and Ramsay1977; Almogi-Labin et al., Reference Almogi-Labin, Hemleben and Meischner1998). These organisms are key contributors to the carbonate pump, owing to their aragonite shells that represent a significant source of calcium carbonate (CaCO3). Approximately 10–42% of the ocean's CaCO3 flux is contributed by pteropod shells, playing a crucial role in carbon cycling (Bednaršek et al., Reference Bednaršek, Tarling, Bakker, Fielding, Jones, Venables, Ward, Kuzirian, Lézé, Feely and Murphy2012b). They are ecologically essential because they are primary food sources for marine organisms such as fish, seabirds and whales (Fabry et al., Reference Fabry, Seibel, Feely and Orr2008). Being highly sensitive to ocean chemistry changes, pteropods are used as bioindicators of climate change and ocean acidification, emphasizing their importance in ecological and climate studies (Bednaršek et al., Reference Bednaršek, Feely, Reum, Peterson, Menkel, Alin and Hales2014). Pteropods are highly significant for paleoclimatic studies as they are the only living metazoan planktons with abundant fossil records (Bé and Gilmer, Reference Bé, Gilmer and Ramsay1977).
Pteropods of the family Cavoliniidae Gray, 1850 (1815) comprise uncoiled, non-tubular, globose Euthecosomes. Their shells are primarily triangular and oval. The presence of lateral spines in the caudal end is typical in this family. Cavoliniids prefer deeper waters and are found mainly in the upper mesopelagic zones (Bednarsek et al., Reference Bednaršek, Tarling, Bakker, Fielding and Feely2012a, Reference Bednaršek, Tarling, Bakker, Fielding, Jones, Venables, Ward, Kuzirian, Lézé, Feely and Murphy2012b; Wall-Palmer et al., Reference Wall-Palmer, Smart and Conversi A2014; Howes et al., Reference Howes, Bednaršek, Büdenbender, Comeau, Doubleday, Gallager, Hopcroft, Lischka, Maas, Bijma and Gattuso2015). These are rarely available in zooplankton samples, but the dead shells collected are relatively more numerous from the benthic sediment samples (Oakes et al., Reference Oakes, Peck, Manno and Bralower2019). Like other pteropods, highly diverse species of cavoliniids are present in tropical and subtropical regions than in polar waters (Lazzari et al., Reference Lazzari, Mattia, Solidoro, Salon, Crise, Zavatarelli, Oddo and Vichi2013; Howard et al., Reference Howard, Nash, Anthony, Schmutter, Bostock, Bromhead, Byrne, Currie, Diaz-Pulido, Eggins, Ellwood, Poloczanska, Hobday and Richardson2014; Burridge et al., Reference Burridge, Goetze, Wall-Palmer, Le Double, Huisman and Peijnenburg2016; Anglada-Ortiz et al., Reference Anglada-Ortiz, Zamelczyk, Meilland, Ziveri, Chierici, Fransson and Rasmussen2021, Reference Anglada-Ortiz, Meilland, Ziveri, Chierici, Fransson, Jones and Rasmussen2023). Cavoliniidae comprises 39 species belonging to four genera under three subfamilies (MolluscaBase, 2025). The subfamily Cavoliniinae comprises the genera Cavolinia, with nine species reported so far, and Diacavolinia, which has twenty species under this family (MolluscaBase, 2025). The subfamily Diacriniinae has two genera; among them, the genus Diacria has seven species, and the genus Teleodiacria has four species (MolluscaBase, 2025). Meanwhile, the newly introduced subfamily Vaginellinae (Janssen, Reference Janssen2020) has mostly fossil records, and no proper genus has been assigned to it yet.
The pteropod review by Siddique et al. (Reference Siddique, Purushothaman, Raghunathan, Chandra, Chandra, Raghunathan, Pillai, Purushothaman and Mondol2021) reveals that sixteen species of Cavoliniids are reported from the Indian subcontinent. Among these, the subfamily Cavoliniinae included four species of the genus Cavolinia (C. globulosa, C. inflexa, C. tridentata and C. uncinata) and eight species of the genus Diacavolinia (D. angulata, D. bicornis, D. flexipes, D. longirostris, D. soulyeti, D. striata, D. triangulata and D. vanutrechti) are available in the Indian waters so far. Whereas, from the subfamily Diacriniinae, only one species of the genus Diacria (D. trispinosa) and three species of the genus Telodiacria (T. costata, T. danae and T. quadridentata) are reported from India (Siddique et al., Reference Siddique, Purushothaman, Raghunathan, Chandra, Chandra, Raghunathan, Pillai, Purushothaman and Mondol2021). The study also suggests that the Cavoliniid species diversity is highest in the Bay of Bengal (10 species), followed by the Andaman Sea (8 species), the Arabian Sea (7 species) and finally the Laccadive Sea (3 species). Taxonomic and distributional studies of pteropods are scarce in India. Being in the tropical region, India is favourable for the availability of diverse pteropod species (Panchang et al., Reference Panchang, Nigam, Riedel, Janssen and Hla2007).
Our study revealed four species of the family Cavoliniidae that had never been recorded in the Indian waters before. Additionally, we found four other species in the Andaman Islands for the first time. In this paper, we discuss the global and Indian distribution of the species. We also describe the key features that identify these species and provide detailed morphological measurements and descriptions of their shells.
Oceanographic setting
The present study area includes the Andaman Islands, encompassing the Andaman Sea on the east coast and the Bay of Bengal on the west. This distinct oceanographic region of the northern Indian Ocean is characterized by unique hydrographic features. The semi-enclosed Andaman Sea is a tropical basin connected to the Bay of Bengal through channels. The circulation patterns in both regions are primarily influenced by the monsoon system, with the Southwest Monsoon Current (SMC) from June to September and the Northeast Monsoon Current (NMC) from November to February. These currents are crucial for water mass movement in this region (Kumar et al., Reference Kumar, Muraleedharan, Prasad, Gauns, Ramaiah, De Souza, Sardesai and Madhupratap2002). Furthermore, the Bay of Bengal is characterized by the East India Coastal Current (EICC) that flows northward in February–May and southward in October–December, influencing the mixing of water mass (Shankar et al., Reference Shankar, Vinayachandran and Unnikrishnan2002). This region is also under the influence of the Indonesian Throughflow (ITF), which aids in transporting Pacific Ocean waters through the Malacca Strait in the Andaman waters (Gordon et al., Reference Gordon, Huber, Metzger, Susanto, Hurlburt and Adi2012). The Equatorial Counter Current (ECC) also facilitates cross-basin mixing between the eastern and western Indian Ocean. The complex interactions of SMC, NMC, EICC, ITF, and ECC create multiple pathways for the mixing of marine organisms.
Materials and methods
Sampling
The Centre for Marine Living Resources and Ecology (CMLRE), Kochi, manages the Fishery Oceanography Research Vessel (FORV) Sagar Sampada. This multidisciplinary research vessel carries out research in fisheries, marine biology and oceanography. The seabed sediment samples from the littoral zones of the Andaman Islands were collected during the voyage of FORV Sagar Sampada (Cruise 355). The samples were collected from 16 December 2016, to 4 January 2017. Collection was carried out at different depths of 50, 100 and 200 m using a Smith McIntyre Grab sampler. The sampling stations included off Port Blair (PB), Rangat (RT), and Diglipur (DI) on the eastern coast, i.e. the Andaman Sea and off Little Andaman (LA), Mayabundar (MA), Rangat (RT), South Andaman (SA) and Wandoor (WA) in the western coast from the Bay of Bengal (Figures 1 and 2). The coordinates of the sampling stations from the Andaman Sea (Eastern coast) and the Bay of Bengal (Western coast) are provided in Tables 1 and 2, respectively.
Map of the study area.
Bathymetric map of the Andaman and Nicobar Islands.
Sampling locations in the Andaman Sea (Eastern coast)
Sampling locations in the Bay of Bengal (Western coast)
Sample preservation and analysis
The collected seabed sediment samples were preserved using a 4% neutral buffered formaldehyde solution and dried in a hot oven at 40°C overnight. The pteropods were arranged in a paleontological slide, and a stereo-binocular microscope (LEICA M205C) was used to sort, take morphometric measurements and photograph the Cavoliniid specimens. The keys of van der Spoel et al. (Reference Van der Spoel, Bleeker and Kobayasi1993) were used for preliminary identification, followed by the descriptions provided by d'Orbigny (Reference d’ Orbigny1836–1846), van der Spoel (Reference Van der Spoel1967, Reference Van der Spoel1973), Sakthivel (Reference Sakthivel1976), Janssen (Reference Janssen2012, Reference Janssen2020) and Janssen et al. (Reference Janssen, Bush and Bednaršek2019). The significant morphological features of Cavoliniid shells used for identification are provided in Figure 3.
Morphology of Cavoliniid shells in various orientations: (A) dorsal; (B) lateral; (C) ventral.
Results
Taxonomic description
Systematic account
Order Pteropoda
Suborder Euthecosomata
Family Cavoliniidae
Subfamily Cavoliniinae
Genus Cavolinia Abildgaard, Reference Abdilgaard1791
Cavolinia uncinata d'Orbigny, 1834
Type species: Hyalea uncinata d'Orbigny, 1834
Global distribution: Indo-Pacific region, Mediterranean Sea, Western Atlantic
Indian distribution: Arabian Sea, Bay of Bengal
Specimen details: Registration No.: ZSI/ANRC/M/30775; location: Andaman Sea (DI: 100 m)
Diagnosis
Dorsal: Dorsal side flat with 5 radial ribs that disappear towards the caudal end. Dorsal lip forms a hood over the aperture, lack ribs. Transverse growth lines present.
Lateral: Lateral spines short and not bent (Figure 4A).
Cavolinia uncinata: (A) lateral; (B) ventral.
Ventral: Globular in shape. Eight imbricate transverse striae near ventral lip. Ventral lip with median depression. Aperture wide. Caudal spine in the protoconch II area is elongated, strongly bent dorsally. Fine irregular transverse striations present on the caudal spine. Protoconch I is broken off from the caudal spine. Caudal fold straight. Shell colourless, with a mild brown hue near the ventral lip and caudal spine (Figure 4B).
Morphometric measurements: Length: 5.25 mm; width: 3.90 mm; aperture: 0.90 mm; caudal spine: 0.85 mm; caudal fold: 1.25 mm.
Genus Diacavolinia van der Spoel, 1987
Diacavolinia angulata Souleyet, 1852
Type species: Cavolinia angulosa Gray, 1850
Global distribution: Atlantic Ocean, Indian Ocean
Indian distribution: Laccadive Sea
Specimen details: Registration No.: ZSI/ANRC/M/30776; location: Andaman Sea (PB: 200 m)
Diagnosis
Dorsal: Dorsal side slightly convex with prominent central and laterodorsal ribs. Lock ribs small and thick. 11–14 transverse growth lines present. Hump and nose strongly developed. Dorsal lip strongly bent towards the ventral side. Brown colouration present on the hump and the lock ribs (Figure 5A).
Diacavolinia angulata: (A) dorsal; (B) lateral; (C) ventral.
Lateral: Flanks absent. Lateral spines hooked and unbent with a sharp tip. Gutter corners small. Brown colouration on the lateral spine surface area (Figure 5B).
Ventral: Ventral side globular. Eighteen thin line-shaped ventral ribs present. Ventral lip is moderate size. Median depression absent. Broad rostrum in the dorsal lip. Notch, gutter and constriction absent. Aperture narrow. Lip bellies and the lip shoulders mildly developed; lip flaps are absent. Protrude protoconch II area and caudal joint. Caudal joint extends far beyond the lateral spines. Caudal fold curved and moon-shaped. Brown hue present on the entire ventral surface (Figure 5C).
Morphometric measurements: Length: 3.45 mm; width: 2.75 mm; aperture: 0.25 mm; caudal joint: 0.80 mm; caudal fold: 0.45 mm.
Remarks: According to the description of Diacavolinia angulosa by van der Spoel et al. (Reference Van der Spoel, Bleeker and Kobayasi1993), the species had a clear median depression contrary to the absence of a median depression in the present species.
Diacavolinia aspina van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Type species: Diacavolinia aspina van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Global distribution: Indian Ocean (Mentawai Islands)
Indian distribution: Absent
Specimen details: Registration No.: ZSI/ANRC/M/30777; location: Andaman Sea (PB: 200 m)
Diagnosis
Dorsal: Dorsal side flattened with moderately developed central and laterodorsal ribs, and small lock ribs. Transverse growth lines present. Indication of hump. Nose absent. Dorsal lip slightly bent to the ventral side. Mostly colourless with a brownish hue near the hump of the dorsal lip (Figure 6A).
Diacavolinia aspina: (A) dorsal; (B) lateral; (C) ventral.
Lateral: Flanks faint. Lateral spines unbent, reduced and rounded. Small gutter corners present (Figure 6B).
Ventral: Globular ventral side with 12 line-shaped ventral ribs. Ventral lip normal-sized with a moderately visible median depression. Rostrum broad, constriction, notch and gutter absent. Aperture broad. Weak lip bellies and small lip shoulders present, lip flaps absent. Slightly projecting protoconch II and caudal joint. Caudal fold almost straight. Colourless ventral side (Figure 6C).
Morphometric measurements: Length: 3.61 mm; width: 2.89 mm; aperture: 0.93 mm; caudal joint: 0.92 mm; caudal fold: 0.53 mm.
Diacavolinia bandaensis van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Type species: Diacavolinia bandaensis van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Global distribution: Banda Sea, North-western Taiwan Strait
Indian distribution: Absent
Specimen details: Registration No.: ZSI/ANRC/M/30778; location: Andaman Sea (RT: 100 m)
Diagnosis
Dorsal: Flattened with highly developed central rib. Poorly developed laterodorsal ribs and lock ribs. Transverse growth lines faint. Strong hump present. Nose absent. Dorsal lip strongly bent towards the ventral side. Brown colouration on hump, lock ribs and caudal end.
Lateral: Well-developed flanks. Lateral spines reduced, not hooked and do not bend dorsally (Figure 7A).
Diacavolinia bandaensis: (A) lateral (B) ventral.
Ventral: Bulged ventral side with 14 line-shaped ventral ribs. Ventral lip huge and thickened with a distinct median depression. Broad rostrum on thickened dorsal lip. Notch and constriction absent. Small gutter present. Aperture remarkably narrow. Moderate lip bellies and large lip shoulder. Lip flaps absent. Moderately protruding protoconch II area. Caudal joint almost on the same level as lateral spine. Caudal fold moon-shaped. Brown colouration present along ventral lip rim and central globular region (Figure 7B).
Morphometric measurements: Length: 3.47 mm; width: 3.26 mm; aperture: 0.28 mm; caudal joint: 0.38 mm; caudal fold: 0.70 mm.
Diacavolinia bicornis van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Type species: Diacavolinia bicornis van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Global distribution: Indian Ocean, Western Atlantic Ocean
Indian distribution: Laccadive Sea
Specimen details: Registration No.: ZSI/ANRC/M/30779; location: Andaman Sea (LA: 200 m)
Diagnosis
Dorsal: Convex dorsal side with distinct central and lateral rib, moderate lock ribs. Prominent transverse growth lines. Hump and nose absent. Dorsal lip slightly bent towards the ventral side. Brown colour present on the dorsal lip, lock ribs and caudal end (Figure 8A).
Diacavolinia bicornis: (A) dorsal; (B) lateral; (C) ventral.
Lateral: Flanks underdeveloped. Lateral spines sharp, unbent and lance-shaped. Gutter corners small. Brownish hue on the lateral spine surface (Figure 8B).
Ventral: Globular with 8 line-shaped ventral ribs. Ventral lip rim thick and medium-sized, with a weak median depression. Dorsal lip with broad rostrum, deep gutter and prominent notch. Constriction absent. Aperture moderate. Slightly developed lip bellies and small lip shoulder. Lip flaps absent. Slightly projected protoconch II with small caudal joint. Caudal fold curved. Prominent brown colouration on rostrum, gutter, and edge of the ventral lip (Figure 8C).
Morphometric measurements: Length: 3.78 mm; width: 3.24 mm; aperture: 0.72 mm; caudal joint: 0.41 mm; caudal fold: 0.41 mm.
Remarks: The aperture in the present species is quite wide with respect to the size of the shell, and the lip bellies are somewhat developed. However, the species described by van der Spoel et al. (Reference Van der Spoel, Bleeker and Kobayasi1993) had a narrow aperture, whereas the lip bellies were well-developed. The shell size in their study was much bigger (7.84 mm) than the specimens encountered from the Andaman Islands.
Diacavolinia deblainvillei van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Type species: Diacavolinia deblainvillei van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Global distribution: Caribbean Sea, North Atlantic Ocean
Indian distribution: Absent
Specimen details: Registration No.: ZSI/ANRC/M/30780; location: Andaman Sea (PB: 200 m)
Diagnosis
Dorsal: Convex with well-developed central, lateral, and lock ribs. Prominent growth lines. Hump and nose absent. Dorsal lip with a slight bent on the ventral side. Brown colouration throughout the dorsal side, more prominent in the caudal end (Figure 9A).
Diacavolinia deblainvillei: (A) dorsal; (B) lateral; (C) ventral.
Lateral: Flanks prominent. Short, sharp, hooked lateral spines slightly bent to the dorsal side. Brown hue on the lateral spine surface (Figure 9B).
Ventral: Globular with 17 comb-shaped ventral ribs. Ventral lip normal in size, with a median depression. Dorsal lip with a broad rostrum, remarkable notch and shallow gutter. Constriction absent. Aperture moderately wide. Lip flaps absent, highly pronounced lip bellies, tiny lip shoulders. Protoconch II area and caudal joint not projecting. Caudal fold moon-shaped. Brown colouration on ventral bulged region, ventral lip and gutter (Figure 9C).
Morphometric measurements: Length: 3.05 mm; width: 2.98 mm; aperture: 0.68 mm; caudal joint: 0.71 mm; caudal fold: 0.55 mm.
Diacavolinia flexipes van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Type species: Diacavolinia flexipes van der Spoel, Bleeker and Kobayasi, Reference Van der Spoel, Bleeker and Kobayasi1993
Global distribution: Atlantic Ocean, North Indian Ocean, Pacific Ocean
Indian distribution: Arabian Sea, Bay of Bengal
Specimen details: Registration No.: ZSI/ANRC/M/30781; location: Andaman Sea (PB: 200 m)
Diagnosis
Dorsal: Highly convex dorsal side with strong central rib. Laterodorsal ribs small, lock ribs faint. Well-developed growth lines. Hump and nose absent. Dorsal lip strongly bent towards the ventral side. Very faint brownish hue at the caudal end (Figure 10A).
Diacavolinia flexipes: (A) dorsal; (B) lateral; (C) ventral.
Lateral: Flanks absent. Lateral spines hooked, sharp and strongly bent dorsally. Gutter corners prominent, groove present along the spine surface (Figure 10B).
Ventral: Bulged ventral surface with 15 line-shaped ventral ribs. Ventral lip medium, with prominent median depression. Dorsal lip gutter-shaped with a broad rostrum; constriction and notch absent. Aperture very wide. Strong lip bellies; lip shoulders and lip flaps absent. Slightly projecting protoconch II and caudal joint. Caudal fold straight. Slight brown tinge on the ventral lip (Figure 10C).
Morphometric measurements: Length: 3.66 mm; width: 3.57 mm; aperture: 1.20 mm; caudal joint: 0.95 mm; caudal fold: 0.49 mm.
Remarks: The species description by van der Spoel et al. (Reference Van der Spoel, Bleeker and Kobayasi1993) stated the absence of projection of the protoconch II area. They also described the caudal fold as having a moon shape. The present study from the Andaman Islands observed the presence of a mild projection of the protoconch II area and the caudal fold, which was straight, contrary to the moon shape of the Red Sea species.
Subfamily Diacriinae
Genus Diacria J. E. Gray, Reference Gray1840
Diacria erythra van der Spoel, 1971
Type species: Diacria quadridentata erythra van der Spoel, 1971
Global distribution: Western Indian Ocean
Indian distribution: Absent
Specimen details: Registration No.: ZSI/ANRC/M/30782, location: Andaman Sea (DI: 100 m)
Diagnosis
Dorsal: Flat with 5 strongly developed longitudinal dorsal ribs. Eight dorsal striae near the dorsal lip. Dorsal lip short, thickened and bends to form a hood over the aperture on the ventral side. Prominent reddish hue in the dorsal lip (Figure 11A).
Diacria erythra: (A) dorsal; (B) ventral.
Lateral: Lateral spines, extremely reduced, do not bend dorsally, not hooked.
Ventral: Sub-globose shape, strongly convex. Closely placed 17 imbricate ventral striae. Ventral lip thickened. Aperture broad. Protoconch does not protrude. Caudal joint elliptical. Caudal fold moon-shaped. Caudal spine absent. Reddish-brown hue in entire shell (Figure 11B).
Morphometric measurements: Length: 3.44 mm; width: 3.11 mm; aperture: 1.66 mm; caudal joint: 1.11 mm; caudal fold: 0.44 mm.
Remarks: Diacria erythra resembled Telodiacria quadridentata (Blainville, 1821), but the latter shows a strong folding of the dorsal lip to overhang in the aperture on the ventral side. The dorsal lip is highly thickened in T. quadridentata compared to D. erythra. The difference is also noted in the dorsal side as T. quadridentata consists of faint longitudinal ribs and 3 dorsal striae on the dorsal side. The aperture is also remarkably narrow in T. quadridentata.
Species distribution
The systematic identification of the family Cavoliniidae from the sediments of the Andaman Islands revealed seven species of the subfamily Cavoliniinae and one species of the subfamily Diacriniinae. The species identified from the subfamily Cavoliniinae included C. uncinata, Diacavolinia angulata, D. aspina, D. bandaensis, D. bicornis, D. deblainvillei and D. flexipes. The species C. uncinata was widespread and available in 11 stations (Tables 3 & 4), which included both the Andaman Sea (PB – 100 m, 200 m; DI – 100 m) and the Bay of Bengal (LA – 200 m; MA – 200 m; RT – 100 m, 200 m; SA – 50 m, 100 m; WA – 100 m, 200 m). Diacavolinia angulata was found in two stations of the Andaman Sea (PB – 200 m; DI – 100 m) and in one station of the Bay of Bengal (LA – 200 m). Diacavolinia aspina was found in eight stations (Tables 3 and 4) from various depths at 50 m (DI), 100 m (DI, SA) and 200 m (PB, MA, RT, SA, WA). Diacavolinia bandaensis was found in six stations at 50 m (DI), 100 m (PB, RT, DI) and 200 m (PB, LA). From the Bay of Bengal, this species was available only in LA (Table 4), whereas it was well distributed in the Andaman Sea (Table 3). Diacavolinia bicornis was present in three stations, exclusively at a depth of 200 m. These were available in the Andaman Sea from PB – 200 m (Table 3) and the Bay of Bengal from LA – 200 m and MA – 200 m (Table 4). Diacavolinia deblainvillei was absent in all the stations of the Bay of Bengal and was exclusively found in one station (PB – 200 m) of the Andaman Sea (Table 3). Diacavolinia flexipes were found in PB at 200 m in the Andaman Sea (Table 3), and MA – 200 m, and RT – 200 m in the Bay of Bengal (Table 4). The subfamily Diacriniinae comprised the species D. erythra, which was available in 12 stations (Tables 3 and 4) of varying depths, viz., 50 m (DI, WA), 100 m (RT, SA, WA) and 200 m (PB, LA, MA, RT, WA).
Presence of the studied Cavoliniids in the Andaman Sea (eastern coast)
Present: +, absent: −.
Presence of the studied Cavoliniids in the Bay of Bengal (western coast)
Present: +, absent: −.
Cavolinia uncinata, Diacavolinia angulata, D. aspina, D. bandaensis, D. bicornis, D. deblainvillei, D. flexipes and Diacria erythra are the eight species newly recorded from the Andaman waters. The first distributional record of Diacavolinia aspina, D. bandaensis, D. deblainvillei and Diacria erythra in the Indian EEZ was also marked during the present study.
Discussion
According to the results of the present study, it is observed that the Cavoliniids prefer deeper epipelagic zones. Species like Diacavolinia angulata, D. bicornis, D. deblainvillei and D. flexipes were exclusively found in depths beyond 100 m. Cavolinia uncinata, Diacavolinia aspina, D. bandaensis and Diacria erythra were well-distributed throughout the depths but dominated mainly beyond the 100 m. The studies of Wormelle (Reference Wormelle1962), van der Spoel (Reference Van der Spoel1967) and Bé and Gilmer (Reference Bé, Gilmer and Ramsay1977) also stated the similar occurrence of Cavoliniids at a depth of 200 m in tropical and subtropical regions.
The Cavoliniid assemblage in the present study showed similarities and dissimilarities with other regions of the Indian Ocean. The presence of Cavolinia uncinata across multiple depths aligns with its widespread distribution, as Siddique et al. (Reference Siddique, Purushothaman, Raghunathan, Chandra, Chandra, Raghunathan, Pillai, Purushothaman and Mondol2021) reported from the Arabian Sea and Bay of Bengal. However, the presence of Diacavolinia aspina, D. bandaensis, D. deblainvillei and Diacria erythra in the Andaman region, previously unknown from the Indian waters, suggested unique oceanographic conditions of the region. Several factors are likely involved in facilitating the distribution of these new reports. The location of the Andaman Sea at the confluence of major current systems, such as the Indian Ocean Equatorial Current and the ITF, created potential corridors for these species (Schott and McCreary, Reference Schott and McCreary2001).
Earlier, Diacavolinia aspina was reported in the Indian Ocean from the Mentawai Islands of Western Sumatra (van der Spoel et al., Reference Van der Spoel, Bleeker and Kobayasi1993). The combined effect of the South Java Current and the Equatorial Counter Current can be the plausible reason for transporting D. aspina from the Indian Ocean to the Andaman Islands (Table 5). This transportation mechanism is further proved by the studies of regional circulation patterns and their influence on plankton distribution (Kumar and Narvekar, Reference Kumar and Narvekar2015). Diacavolinia bandaensis seemed endemic to the Banda Sea till 1993 (van der Spoel et al., Reference Van der Spoel, Bleeker and Kobayasi1993) unless the study of pelagic pteropods by Chang and Hsueh (Reference Chang and Hsueh2005), where they reported it from the North-western Taiwan Strait owing to ITF. The occurrence of D. bandaensis from the present study in the Indian EEZ may be attributed to ITF, which facilitates connectivity between the Philippine Sea and the Indian Ocean (Gordon et al., Reference Gordon, Huber, Metzger, Susanto, Hurlburt and Adi2012). The species Diacavolinia deblainvillei had earlier been reported from the Caribbean Sea and parts of the Northern and Western Atlantic Ocean (van der Spoel et al., Reference Van der Spoel, Bleeker and Kobayasi1993). Its presence in the Andaman Sea is possibly due to the influence of the Agulhas Return Current (Beal et al., Reference Beal, De Ruijter, Biastoch and Zahn2011) and the West Wind Drift, which connects the Atlantic Ocean to the Indian Ocean (Table 5). More distributional studies of Diacavolinia from the nearby waterbodies are necessary to confirm the reason for their availability in the Andaman Sea. Understandably, the species from the Indo-Pacific region (Banda Sea, Mentawai Islands, Arabian Sea) and the Indian Ocean are available in the Andaman waters because of their immediate vicinity and connection through the Indian Ocean Current, Southwest Monsoon Current, Northeast Monsoon Current and the ITF.
Distribution of the family Cavoliniidae and the potential impact of ocean currents in the studied regions
The co-occurrence of these Cavoliniid species with different biogeography (Indo-Pacific, Atlantic and Western Indian Ocean) in the Andaman waters signifies a potential mixing zone for pteropods through surface and subsurface circulation patterns (Kumar and Narvekar, Reference Kumar and Narvekar2015). However, more research on the influence of ocean currents and the Cavoliniid distribution is required to justify the presence of these holoplanktonic species from the Atlantic Ocean in the Andaman waters.
Conclusion
This study records eight Cavoliniid species new to the Andaman Islands, four new in the Indian waters. Even though the significant features remained the same, slight variations of characters were observed during systematic identification in some species owing to the regional differences. Most species from the Andaman waters showed a smaller shell size, contrary to those encountered in the Atlantic Ocean and the Mediterranean Sea.
An attempt was made to justify the influence of global ocean currents in the distribution of newly reported species in the Andaman waters. Although this paper gives a preliminary insight into the species composition of Cavoliniids in the Andaman waters, there is a scope for further research pertaining to their diversity in this region. Since pteropods are bioindicators of ocean acidification, a thorough understanding of their taxonomy and biogeography will better aid the research on climate change and ocean acidification.
Acknowledgements
The authors thank the Centre for Marine Living Resources and Ecology (CMLRE), Kochi, for providing the opportunity to collect the samples from FORV Sagar Sampada (Cruise No. 355). The authors are also grateful to the Department of Ocean Studies and Marine Biology, Pondicherry University, for providing all the necessary laboratory facilities for conducting the research.
Author contributions
Conceptualization: K. S.; methodology: K. S.; validation: K. S.; investigation: K. S., P. M. M.; data curation: K. S., P. M. M.; resources: P. M. M.; writing – original draft: K. S.; writing – review and editing: P. M. M.
Financial support
This research received funding from the Ministry of Education, Government of India, under the Junior Research Fellowship (UGC-NET-JRF) scheme.
Competing interest
None.
Ethical standards
All sampling and data acquisition for this study were performed in accordance with local ordinances and standards for ethical research.
Data availability statement
All data are available upon request.
