B. humeralis (Valenciennes 1837), also known as dark sleeper or olive flathead-gudgeon, was previously reported as B. melanostigma (Bleeker 1874) from Parangipettai (Porto Novo) coastal waters, Tamil Nadu (Ramaiyan et al. 1986). It was also known from Indian Sundarbans and Digha coast, West Bengal (Chatterjee et al. 2013; Mishra and Gopi 2017; Yennawar et al. 2015) and coastal and estuarine waters of Tamil Nadu (Jeyaseelen and Krishnamurthy 1980). Previous reports from Kerala have described three species of Butis; but the probable existence of other congeners is scanty, possibly due to little commercial interest in these species. The current study revealed the occurrence of B. humeralis in Kerala waters, which is the first report of its existence along west-coast of India. Although the information related to the early developmental stages of B. humeralis’ are lacking, the present study finds relevance as it details the egg morphology, hatching and development of preflexion larva utpo 144 HPH by monitoring the changes of eyes, pigmentation, fins, yolk sac size, oil globules and mouth.
According to Koumans (1953) and Miller et al. (1989), the habitat of B. humeralis and B. koilomatodon varies from marine to freshwater. In the present study, the brackish water environment where B. humeralis eggs were found was influenced by tidal incursion with marked fluctuations in salinity ranging from 0 to 20 ppt. According to Hui et al. (2010), though B. humeralis may survive in freshwater, they may not breed successfully if access to the marine environment is restricted.
B. humeralis shares similarity in egg morphology with that of the two sleeper gobies of Eleotridae, Eleotris fusca and E. acanthopoma, as well as with an indigenous Hawaiin goby, Lenticeps concolor. The eggs of these gobies appear pear-shaped and adhere to the substrate using a bundle of adhesive filaments at their proximal end, and even interconnect with the neighbouring eggs to form a monolayer. (Lindstorm 1998; Maeda et al. 2008). The average vertical and horizontal diameters of the eggs of B. humeralis are also comparable to those of E. fusca and E. acanthopoma.
Majority of gobiids have an iteroparous pattern of reproduction, with females releasing eggs on vegetation or substrates, while males assist in their post-fertilization care. The lunar cycle is crucial for their spawning and larval recruitment in estuaries (Miller 1984; Thresher 1984; Berra 2001; Dinh et al. 2016). In this study, the matured eggs of B. humeralis was collected five days prior to the new moon from CE. Eggs attached to substrate spawns during morning hours and are less vulnerable to predation due to parental care and their placement in protected areas (Brinley 1939; Myrberg Jr et al. 1967; Goulet 1995; Anil et al. 2012; Rohini Krishna et al. 2016) than the pelagic eggs, which is more prone to high mortality (Hirst and Lopez-Urrutia 2006). Gobies lay pear-shaped eggs in demersal habitats, where they attach to surfaces in single layers (Russell, 1976). A clutch of pear-shaped eggs (7,680 eggs / 61.80 cm2) connected to the glass substrate in the present study suggests that B. humeralis is benthic and iteroparous species exhibiting external fertilization and parental care, offering better protection to eggs.
Despite the fact that most gobioid fish deposits eggs on submerged objects and were cared by male (Miller 1984, Kinzie 1997, Keith 2003), B. humeralis had an egg mass of sparsely distributed tiny eggs similar to E. oxycephala (Dotu and Fujita 1959), E. fusca and E. acanthopoma (Maeda et al. 2008). This helped to distinguish their egg masses with those of other species.
The embryonic head of B. humeralis was located at the distal part of the egg before hatching which is observed to be common in many gobioid fishes while in O. mormoratus, the embryonic head was located at basal part of the egg and has been referred to as “agrippa egg” (Shinomiya et al. 1981 a, b; Suzuki et al. 1988; Suzuki et al. 1989). The embryonic head or tail of B. humeralis emerged out at the distal end of the egg capsule upon hatching.
For many gobies, hatching usually happens after eye pigmentation (Shinomiya et al. 1981 a,b; Suzuki et al. 1988; Suzuki et al. 1989). Like E. fusca and E. acanthopoma, eye pigmentation in B. humeralis started at 24-hour post-hatch and at 72 HPH, the eyes became densely pigmented (Dotu and Fujita 1959; Maeda et al. 2008). However, O. mormoratus hatched before, during and after eye pigmentation with ideal hatching stage occurred just before and after the slight pigmentation of embryonic eyes (Tan and Lam 1973; Senoo et al. 1994).
Eggs and newly hatched larvae of B. humeralis were very small with 0.41 mm diameter and 0.97–1.09 mm NL respectively, characterized by lack of pigmented eyes, mouth, and pectoral fins. However, the eggs and newly hatched larvae of all Eleotris species like E. oxycephala (Dotu and Fujita 1959; Dotsu et al. 2004); E. sandwicensis (Lindstrom 1999), described to date, were smaller in size and similar morphologies. Reports state that morphologically similar and smaller eggs and larvae were observed in a number of other eleotrid fish species, including Hypseleotris spp., Ophieleotris aporos, and Dormitator latifrons, as well as sicydiine gobies and their relatives, including Sicyopterus spp., Sicydium punctatum, Stiphodon percnopterygionus, Lentipes concolor, Awaous spp., and Stenogobius hawaiiensis. (Dotsu et al. 1998, 2000; Lindstrom 1999; Yamasaki and Tachihara 2006, 2007). Hence the reproductive strategy of B. humeralis characterized by numerous smaller eggs and smaller newly hatched larvae are shared with several eleotrid fishes and sicydiine gobies, as described earlier.
B. humeralis larvae were poorly developed at time of hatching and lacked a functional mouth, pigmented eye and differentiated fins due to the smaller size of eggs with narrow perivitelline space and yolk diameters less than 1 mm (Table 1). The presence and position of oil globules in yolk-sac larvae varies among fishes. Yolk sac stage starts at hatching and ends when the yolk is absorbed (Kendall 1984). In the present study, the absorption times of the yolk and oil globules were observed at 96 HPH as in Dormitator latrifrons of Eleotridae (Reyes-Mero et al. 2022). During the period of yolk and oil globule exhaustion, the larvae of B. humeralis developed pigmented eyes, functional mouth and differentiated fins enabling them to survive progressive starvation during the switch from endogenous to exogenous feeding.