Morphology of fertilized eggs
In general, morphology of fertilized eggs is showed family or genus specificities although the size of fertilized eggs and adhesive property are different in fishes (Kim et al. 1996, 1999, 2005, 2009, 2013, Joo and Kim 2013, Choi et al. 2019). The fertilized egg of Norman’s lampeye was spherical, demersal, have a narrow perivitelline space and small oil droplets in the vitelline membrane (Fig. 1). The size of fertilized egg was about 1.49 ± 0.07 mm (n = 30) and a bundle of adhesive filaments was on one side of fertilized egg for attachment on the spawning place and length of a bundle of adhesive filaments with very high elasticity was about 1–3 mm. The adhesive property was only at a bundle of adhesive filaments and their adhesive property was maintained even after electron microscopic preparation. But the fertilized egg itself was not adhesive. The morphology of this fertilized egg is same as that of Melanotaenia praecox belongs to Melanotaeniidae under light microscope (Sohn and Kim, 2021).
In general, adhesive fertilized eggs have adhesive structures or not in teleost. The species belong to Cichlidae have adhesive reticular structure on the outer surface of fertilized egg (Deung et al. 1997, Kim et al. 2009). Although tomato clown anemonefish (Pomacentridae) and dark sleeper (Eleotrididae) are different family and habitat, the long elliptical fertilized eggs have same morphology with a bundle of adhesive filaments (Kim et al. 1998, 2002), but that of Norman’s lampeye was spherical in this study. Peculiarly enough, the adhesive property of fertilized eggs without adhesive structures from Ancistrus cirrhosus is known to disappeared after spawning although the adhesive property was maintained in some parts that fertilized egg contact with other egg or spawning place (Kim 2020). Also Corydoras adolfoi and C. sterbai belong to Callichthyidae have same adhesive protuberances structures on the fertilized eggs (Choi et al. 2019).
Structure of fertilized egg envelope
The outer surface of fertilized egg envelope was smooth without special structures. The adhesive filaments were distributed around the egg envelope (Fig. 2A) and each adhesive filaments were joined together to form a long bundle (Fig. 2B). In study on in Melanotaenia praecox belongs to Melanotaeniidae, the long and thick adhesive filaments were only at the area of animal pole, and short and thin adhesive filaments were around the long filament and long adhesive filaments were coiled together under the scanning electron microscope (Sohn and Kim, 2021).
The around micropyle was a peak shape with several straight wrinkles (Fig. 3). But accurate structure of micropyle could not be observed because it is in too deep. The micropyle of M. praecox was known to conical shaped with whippy adhesive filaments even if these fertilized eggs of M. praecox and Norman’s lampeye have same external morphology under the light microscope (Sohn and Kim, 2021). Also the total thickness of the egg envelope was approximately12.5-14.5 µm and the fertilized egg envelope consisted two layered, an outer electron dense layer and an inner lamellar layer consisting of 10 layers of differing thickness (Fig. 4A & 4B). The morphology of fertilized egg and section of fertilized egg envelope is very similar with that of M. praecox belong to other family, but the inner layer was different in both species (Sohn and Kim, 2021).
The adhesive or non-adhesive structures on the outer surface of fertilized egg envelope were showed species, genus or family specificities. Most species belong to Cyprinidae have different structures. That of fertilized egg envelope in Tanichthys alborubes have rod-like structures (Kim et al. 1998), that of Zacco platypus have Indian club-like structures (Deung et al. 2000), that of Hemibarbus longirostris have taste bud-like structures (Kim et al. 2001), and that of Danio rerio have knob-like structures (Joo and Kim 2013). That of Cichlidae have adhesive reticular structures (Deung et al. 1997), that of Belontiidae have a lot of grooves of envelopes covered by thin adhesive layer (Kim et al. 1999), that of Nothobranchiidae have many adhesive whip-like structures (Kwon et al. 2017), and that of Callichthyidae have adhesive protuberances (Choi et al. 2019). Therefore, these families were showed family specificity. Also these outer surface structures can be same or different even if the species belong to different genus (Kim et al. 1996, Kim et al. 2005). But tomato clown anemonefish belongs to Pomacentridae (Kim et al. 1998) and dark sleeper belongs to Eleotrididae (Kim et al. 2002) have same smooth outer surface of fertilized egg envelopes such as Norman’s lampeye.
The structure of a micropyle, passage of sperm without acrosome seems to be species, genus or family specificities such as structures on the outer egg envelope. The species belong to Characidae, Belontiidae and genus Nothobranchius have same micropyle structure in same family (Kim et al. 1996, 1999, Kwon et al. 2017, Chang et al. 2019). But morphology of micropyle is differ according to the species in Cyprinidae (Kim et al. 1998, 2001, Deung et al. 2000). The micropyle was not found on fertilized egg with a bundle of adhesive filaments (Kim et al. 1998, 2002) but that of Norman’s lampeye was found outside the bundle of filaments.
In general, fertilized egg envelope of fish eggs consisted of 2 or 3 layers. In particular, the inner layer has a layered structure consisting several layers, and even if it is same family, the number of layers consisting the inner layer vary depending upon the species (Kim et al. 1996, 2005, Chang et al. 2019, Sohn and Kim, 2021). But the section structure of fertilized egg envelope has same structure according to the family in all species belong to Belontiidae (Kim et al. 1999), Cichlidae (Deung et al. 1997, Kim et al. 2009), Callichthyidae (Choi et al. 2019), and Nothobranchiidae (Kwon et al. 2015, 2017). The thickness of fertilized egg and the number of layers consisting an inner layer can be measured differently without accurate cross section. Also the total number of fertilized egg envelope can be measured differently between transmission electron microscope and scanning electron microscope. This study on the one species are very limited, so more researches on other species are needed to get more information on the structure of fertilized egg and fertilized egg envelope in Poeciliidae.
Collectively, these morphological characteristics and adhesive property of fertilized egg with a bundle of adhesive filaments, and ultrastructures of outer surface and section of fertilized egg envelope are showed species specificity.