1 Introduction

Mangrove habitats are renowned as one of the world's most productive ecosystems. However, they face significant challenges from human-made pollution, alterations in coastal structures, and global climate changes [1,2,3]. These mangrove ecosystems serve as crucial nursery grounds for numerous fish species, with cryptic gobies being particularly prevalent among the indigenous fish diversity in estuarine-based mangrove ecosystems. The presence of these gobies play a pivotal role in upholding the intricate and unique food chain within this ecosystem [4]. While India's coastline spans ~ 5000 km2 of mangrove vegetation, the majority of these mangroves are concentrated on the eastern coast. In contrast, the western coast only covers an area of about 1500 km2, with the state of Kerala (the focus of the current study) hosting the smallest mangrove patch, covering a mere 9 km2 [5]. Consequently, research on mangrove-associated gobies in this specific region of India is notably scarcer compared to other parts of the country [6].

According to the most recent compilation by Mishra et al. [7], a total of 83 gobioid species associated with mangrove habitats have been documented in India, with only 10 species reported in the state of Kerala under the Gobiiformes order. A more recent study by Sreeraj et al. [6] identified four gobioid species in the study area, including Acentrogobius viridipunctatus (Valenciennes, 1837), Mugilogobius tigrinus Larson, 2001, Oxyurichthys ophthalmonema (Bleeker, 1856), and Pseudogobius poicilosoma (Bleeker, 1849). Thus far, a total of 14 gobioid fish species have been documented in Kerala.

The genera Eugnathogobius Smith, 1931, and Mugilogobius Smitt, 1900, both falling within the Gobionellinae group, are widely recognized as two of the most prevalent genera of gobioid fishes in the Indo-Pacific region. Their ability to thrive in diverse salinity levels and exhibit various degrees of cryptic behaviour contributes to their ubiquity. However, these genera faced considerable confusion in their classification until the genus revision conducted by Larson [8, 9]. Genus Eugnathogobius has been represented by just one species, Eugnathogobius mas (Hora, 1923), recorded in India (Odisha and Tamil Nadu) as per Larson et al. [10]. Similarly, Mugilogobius has been represented by a single species, Mugilogobius tigrinus Larson, 2001, found in various regions of India, including the Andaman and Nicobar Islands, Andhra Pradesh, Tamil Nadu, and Kerala [6, 11].

Therefore, this present study aimed to explore the gobioid fauna within Kerala's mangroves, resulting in the identification of three previously undocumented species in the region, which are discussed in this communication.

2 Materials and methods

The fish specimens were collected as part of routine surveys of the "Kadalundi-Vallikkunnu Community Reserve" and from the Kallai area in Kozhikode in July 2023. The Kadalundi-Vallikkunnu Community Reserve spans an area of 1.5 km2, encompassing mangrove and estuarine habitats (Fig. 1). Kallai is a small town located along the banks of the Kallai River, which is situated within an estuarine region. The collection process involved visually searching for the fish and thereafter either handpicking them or using a small aquarium net to capture them from the intertidal zone. The collection coordinates are 11° 7′ 41.8944′′ N, 75° 49′ 58.1736′′ E and 11° 14′ 15.9792′′ N, 75° 0.78′ 6638′′ E. On-site, the captured specimens were photographed within a little aquarium (12 cm × 4.5 cm × 10 cm). Recently dead specimens were conserved by immersing them in a solution of formalin diluted to a concentration of 10% in saltwater. These specimens were then transferred to a solution of ethanol diluted to a concentration of 70% for long-term preservation. One sample of each species was also preserved directly in 100% ethanol for DNA studies. These preserved samples have been deposited in the National Zoological Collections of the Zoological Survey of India.

Fig. 1
figure 1

Map and image of the collection locality of Eugnathogobius mindora (Herre, 1945), Mugilogobius mertoni (Weber, 1911), Mugilogobius rambaiae (Smith, 1945)

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Identification of the animals was carried out using references by Larson [8, 9, 12]. Precise measurements were taken to the 0.1-mm level using a Digital Vernier Calliper. The nomenclature used in this study is based on Eschmeyer’s Catalog of Fishes, as documented in Fricke et al. [13].

3 Results

The details of the newly recorded species are given below:

Class: Actinopterygii.

Order: Gobiiformes.

Family: Gobiidae.

Subfamily: Gobionellinae.

Genus: Eugnathogobius Smith, 1931.

3.1 Eugnathogobius mindora (Herre, 1945)

1945. Vaimosa mindora Herre [A.W.C.T.] Proceedings of the Biological Society of Washington v. 58, 13p. (Brackish swamp on Hacienda Waterous, Mangarin, Mindoro, Philippines). Holotype: CAS-SU 36826.

3.1.1 Common name

Stripe-face Calamiana.

3.1.2 Synonyms

Vaimosa mindora Herre, 1945; Vaimosa zebrina Herre, 1950; Vaimosa zebrinus Herre, 1950.

3.2 Conservation status

Least Concern (IUCN Red List 3.1), date assessed 9 Aug, 2010.

3.2.1 Economic importance

Not used in any commercial means.

3.2.2 Material examined

India, Kerala: Kadalundi, Kozhikode, 11° 7′ 41.8944′′ N, 75° 49′ 58.1736′′ E, 1 ex., 03.VII.2023; Kallai, Kozhikode, 11° 14′ 16′′ N, 75° 47′ 12′′ E, 2exs, 20.VII.2023, Hussna Nazar (reg no. ZSI/WGRC/IR-V-3700, 3701).

3.2.3 Diagnostic characters

Second and third dorsal spine is moderately long. Second dorsal fin and anal fin moderately tall, short based, all rays are approximately with similar height although elongation in first three dorsal spine are seen. Pectoral fin slender, oval; pelvic fin short and oval. Caudal fin rounded; lower rays slightly shorter than upper.

Body slender and compressed. Head wider than the deep. Large eye; Mouth terminal, slightly oblique, jaw reaching to below mid eye. Lips smooth, fleshy and narrow. Snout rounded and short. Head pores are modally absent; interorbital space moderate. Body has mostly ctenoid scale. Anterior nostril is short tube like. Cheeks without scales; tongue tip blunt to rounded. Gill opening restricted, extending anteriorly to just ventral to opercle. Anterior nape scales are larger compared to others. Caudal peduncle compressed.

In a freshly collected specimen, there are small brown patches and lines distributed throughout the body, base body colouration is of a pale-yellow hue. The anal fin, pectoral fins, and pelvic regions are translucent. The dorsal and caudal fin rays exhibit alternating brown spots, with a higher concentration of brown dots along the lateral line.

In the preserved specimen, the subcuticular brown spots across the body, fins, and the stripes on the cheek remain intact (Fig. 2). Morphological characters of the collected specimen has been described in Table 1.

Fig. 2
figure 2

Eugnathogobius mindora (Herre, 1945). a Live specimen. b–d Preserved specimen head. b Lateral view. c Dorsal view. d Ventral view

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Table 1 Morphological characters of collected specimen
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3.2.4 Habitat

Mostly recorded from brackish water creeks and estuarine area, the present collection locality is also a brackish swamp of mangrove habitat.

3.2.5 Distribution

Worldwide—Western Pacific: Fiji, Australia (Queensland), the Philippines, Thailand, Irian Java, Indonesia, Papua New Guinea [12, 14, 15]. India—Pazhaverkaadu [= Pulicat] lagoon and Ennore estuary in South India [16]. Kerala- Kallai and Kadalundi from Kozhikode (New Record).

3.2.6 Remark

In India, the Eugnathogobius genus was previously represented by only three species: Eugnathogobius kabilia (Herre, 1940), Eugnathogobius mas (Hora, 1923), and E. mindora can be easily distinguished from its closest relatives by its distinctive feature of having five prominent brown stripes on the cheek and suborbital area. Initially, this species was classified under the genus Calamiana by Larson in [12] and later reclassified by Larson under the Eugnathogobius genus as part of a revision.

Previously, this species had only been reported in Tamil Nadu, India [16]. However, this represents the second documented occurrence of the species in India, marking the first-ever record of its presence in the state of Kerala and along the western coastline of India. This discovery adds to the ichthyofaunal diversity of Kerala.

Genus: Mugilogobius Smitt, 1900.

3.3 Mugilogobius mertoni (Weber, 1911)

1911. Gobius mertoni Weber [M.] Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft Frankfurt-a.-M. v. 34 (no. 1), 37p. (Aru Islands, Maluku, Indonesia). Lectotype—SMF 6699.

3.3.1 Common name

Chequered mangrove goby

3.3.2 Synonyms

Gobius mertoni Weber, 1911; Stigmatogobius inhacae Smith, 1959; Tamanka mindora Herre, 1945; Vaimosa layia Herre, 1953; Gobius durbanensis Barnard, 1927.

3.3.3 Conservation status

Least Concern (IUCN Red List 3.1), date assessed 29 Feb, 2020.

3.3.4 Economic importance

Not used in any commercial means.

3.3.5 Material examined

India, Kerala: Kadalundi, Kozhikode, 11° 7′ 41.8944′′ N, 75° 49′ 58.1736′′ E, 2 exs, 03.VII.2023; Kallai, Kozhikode, 11° 14′ 16′′ N, 75° 47′ 12′′ E, 6exs, 20.VII.2023, Hussna Nazar (reg no. ZSI/WGRC/IR-V-3688, 3689).

3.3.6 Diagnostic characters

First dorsal fin is shorter than the second dorsal fin. Anal fin and second dorsal fin are longer. The pectoral fin is rounded, and the central ray is the longest. Pelvic fin is rounded and short, jointed. Caudal fin rounded. Most predorsal scales are small, evenly sized, reaching to close behind eyes, with anterior-most scale enlarged. Small cycloid scales at operculum, naked cheek.

Overall compressed body, head is broad, round and wider than deep. Eyes small and positioned high on the head, forming a part of dorsal profile, upper part is more of flattened. Three black stripes are present from the head towards the mouth. Sub-terminal mouth, jaws reaching to below mid-eye. Lips are smooth and fleshy, snout rounded. Nostrils are located at anterior-most region, short tube-like structure. Fins are well developed, showing characteristic colour patterns. Body is mostly covered by cycloid scales but region near to belly has isolated patch of ctenoid scales. Gill opening extended below opercle region.

The live specimen displays a blend of black, brownish, and yellowish hues. Notably, it features alternating yellow spots that run almost centrally from the opercle to the base of the caudal fin. At the base of the caudal fin, there is a distinct subcuticular thick black vertical line. The first dorsal fin exhibits a blackish coloration covering approximately three-quarters of its surface, with a quarter displaying a light-yellow shade, and the remainder being transparent. The second dorsal fin displays vertical alternating bands of yellow and black. A prominent yellow line is also present at the base of the caudal fin, which itself has a dusky shade.

In preserved specimens, the yellow spots vanished, but the thick black vertical line at the base of the caudal fin and the black stripes near the eye remained consistently prominent. However, the coloration patterns observed in live specimens faded or disappeared entirely (Fig. 3). Morphological characters of the collected specimen has been described in Table 1.

Fig. 3
figure 3

Mugilogobius mertoni (Weber, 1911). a Live specimen. b–d Preserved specimen head. b Lateral view. c Dorsal view. d Ventral view

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3.3.7 Habitat

These samples were acquired from the intertidal zones within the mangrove region, specifically in the tidal-fed creeks. They were gathered in areas where the water depth is less than one foot during low tide. The substrate in this region consists of a mixture of muddy and clayey materials. The collection site is characterized by a narrow waterway that connects to the river and features a thriving mangrove ecosystem.

3.3.8 Distribution

Worldwide—From East Africa and Seychelles, Madagascar east to Philippines, Japan, Pakistan, Indo-Malaysian archipelago, Papua-New Guinea, New Caledonia, Vanuatu, Australia and Santa Cruz Islands, Sri-lanka, Brunei, Japan [8, 14, 15]. India—Maharashtra [8], Kerala (New Record).

3.3.9 Remarks

This species is known to inhabit various aquatic environments worldwide, including freshwater, marine, and brackish water areas. Mugilogobius mertoni closely resembles Mugilogobius filifer Larson, 2001. However, they can be differentiated based on certain characteristics. M. mertoni typically has a lateral scale count ranging from 26 to 35, with an average of 30, while M. filifer averages 35 (ranging from 29 to 40). Additionally, M. mertoni typically has 10–16 predorsal scales, whereas M. filifer has 13–21. Another distinguishing feature is that in M. mertoni, the first to fourth dorsal fin rays are the largest, whereas in M. filifer, the first dorsal fin ray is the largest.

Literature from the Indian region, previously only mentioned one species within the Mugilogobius genus, which was Mugilogobius tigrinus Larson, 2001, found from the Andaman and Nicobar Islands, Tamil Nadu, Andhra Pradesh, and Kerala [6, 11]. M. mertoni and M. tigrinus can be easily distinguished from each other. M. mertoni has two longitudinal stripes on the side of the head and a distinctive checkered pattern on its body, whereas M. tigrinus lacks these features and has a pointed snout and darker eyes compared to M. mertoni.

Although no prior Indian literature mentions the presence of M. mertoni from the Indian region; a revision by Larson [8] does note the collection of four specimens from Bandra Rocks in Mumbai, Maharashtra, during the German Indian Expedition in 1955. These specimens were deposited in the Zoologische Museum, Hamburg. Therefore, this study serves as definitive proof of the species' presence in India, 68 years after its initial discovery in the country. Additionally, a previous study by Sreeraj et al. [6] confirms that both M. tigrinus and M. mertoni inhabit the same mangrove patches in Kerala.

3.4 Mugilogobius rambaiae (Smith, 1945)

1945. Vaimosa rambaiae Smith [H. M.] Bulletin of the United States National Museum No. 188, 538, Pl. 9. (Canal in Bangkok, Thailand). Holotype: USNM 119646.

3.4.1 Common name

Queen of Siam goby.

3.4.2 Synonyms

Vaimosa rambaiae Smith, 1945.

3.4.3 Conservation status

Least Concern (IUCN Red List 3.1), date assessed 1 March, 2020.

3.4.4 Economic importance

Potential for usage as ornamental fish.

3.4.5 Material examined

India, Kerala: Kadalundi, Kozhikode, 11° 7′ 41.8944′′ N, 75° 49′ 58.1736′′ E, 2 exs, 03.VII.2023, Hussna Nazar (reg no. ZSI/WGRC/IR-V-3690).

3.4.6 Diagnostic characters

First dorsal fin is shorter than second dorsal. In male, first dorsal spine is longer than female. Anal fin and second dorsal fin are longer. The pectoral fin is rounded, and the central ray is the longest. Pelvic fin is rounded and jointed, may reach about halfway to anus. Caudal fin rounded. Most pre-dorsal scales are small, evenly sized, reaching to close behind eyes. Operculum with small cycloid scales, cheeks naked. Sensory papillae longitudinal distributed.

Body compressed and rounded, head wider than deep. Eyes small, lateral high on head forming part of dorsal profile; upper portion is flattened. Mouth sub-terminal, jaws reaching to below posterior half of eye. Smooth, fleshy lips and snout rounded, tongue tip blunt to rounded. Anteriorly located nostril, tubular and short type. Gill opening extending forward to upper opercle. Caudal peduncle compressed. Body is mostly covered by cycloid scales but region near to belly has isolated patch of ctenoid scales is present.

The live specimen has a predominantly blackish body coloration. About three-quarters of the dorsal fin is black, while a quarter of it is white, and the remainder is transparent. When viewed from the side (lateral view), the body lacks any discernible patterns or spots. At the base of the caudal fin, there is a distinct subcuticular black vertical line. The head region is characterized by 3 to 4 yellow circles surrounded by a black color. Irregular arrangements of small yellow and black circular patterns are also visible on the head. Additionally, two vertical stripes can be observed: one below the eye, extending towards the upper jaw, and the other above the eye, extending backward. The anal, pelvic, and pectoral fins do not display any specific color patterns. The caudal fin features a vertical zig-zag pattern. A thick black spot is present above the opercular region in this specimen.

In the preserved specimen, the subcuticular black vertical line and the black-and-white combination on the dorsal fin persists, while the rest of the body becomes predominantly blackish (Fig. 4). Morphological characters of the collected specimen has been described in Table 1.

Fig. 4
figure 4

Mugilogobius rambaiae (Smith, 1945). a Live specimen. b–d Preserved specimen head. b Lateral view. c Dorsal view. d Ventral view

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3.4.7 Habitat

Collected from the same area as M. mertoni, therefor same kind of habitat conditions are observed.

3.4.8 Distribution

Worldwide—Sri Lanka, Myanmar, Thailand, Malaysia, Singapore, Indonesia and Papua New Guinea, Burma [8, 14, 15]. India—New Record.

3.4.9 Remarks

M. rambaiae can be readily differentiated from the other two species of Mugilogobius found in the Indian region due to the absence of distinctive coloration patterns seen in M. tigrinus and M. mertoni. Furthermore, its vertical gill opening and sub-terminal mouth provide additional distinguishing characteristics for M. rambaiae. Although M. rambaiae bears a resemblance to Mugilogobius rivulus Larson, 2001, it can be easily distinguished from the latter by the presence of a pair of dusky spots and various other dark spots near the base of the caudal fin, as well as an elongated white first dorsal fin. None of these features are present in M. rambaiae.

Based on the review of the literature, it can be said that this species has only been documented in the eastern part of the Indian Ocean and the East China Sea. Therefore, this current study marks the first recorded instance of the species in India and the Arabian Sea.

4 Discussion

This study has unveiled one previously undocumented record from India and two fresh records from the state of Kerala, highlighting the diversity of Gobioid fish species in the region. The specimens were collected from the estuarine network associated with mangroves in Kerala, India. These collection sites indicate that the exploration of Gobioid fauna along the western coast of India has been inadequate. The presence of Eugnathogobius and Mugilogobius species in these areas underscores the potential for a substantial Gobioid fauna in this region. These fishes, being small and non-migratory, find refuge in mangroves. Even a relatively small 9 km2 mangrove patch in Kerala can support a rich variety of Gobioid species and serve as a crucial nursery ground for their lifecycle and future survival. Therefore, to fully understand the species richness in these mangrove areas along India's western coast, more ecological, genetic, and morphological study is required. The Indian data on gobioid diversity is rather modest when compared to the worldwide scenario, where the Gobiidae family is scattered pretty broadly, appearing in most tropical and subtropical locations with 263 genera and 1952 species [17]. Endemism and cryptism is the greatest characteristics of this group of fishes, thus protection of the microhabitats provided by the mangrove ecosystem is crucial for these fishes [18]. Moreover, the presence or absence of these fishes can serve as indicators of changes in the biotic or abiotic factors affecting the entire ecosystem [19].

The current documented range of E. mindora, M. mertoni, and M. rambaiae in the western Indian Ocean, which is different from their mostly uninterrupted range in the eastern Indian Ocean, has similarities with the fragmented distribution patterns of other species within these two genera [20]. This distribution may be due to the specific habitats these species prefer, and it could also be a result of incomplete sampling and insufficient surveys. Another potential cause for these observations might be the colonisation through ballast water, as observed in the case of other small marine fishes [21]. It is worth noting that the Cochin port in Kerala has been one of the busiest ports on the west coast of India for many years. Consequently, future research is essential to gain a deeper understanding of this relatively unexplored group of fishes in the region.