Recently, the species name of oyster Crassostrea gigas has been changed to Magallana gigas [16]. Taking into account that the renaming was objected by 27 scientists and is urged to be reconsidered [17], the genus change does not seem final. Prior to a final decision being achieved by scientific community, we prefer to follow the traditional name of this mollusc—C. gigas.
Using COI DNA barcoding, we highlighted that the wild oyster collected in four localities of Primorye belongs to C. gigas. We also tested if sperms of C. gigas would help discriminate this oyster from other Ostreidae. We found that in each locality of the Peter the Great Bay the sperms of C. gigas appear with compact heads tailed with thin flagella, and hence correspond to “primitive spermatozoa”, known to be well adapted for external fertilisation in sea water [18,19,20].
Previous data concerning the oysters’ male gametes has been mostly obtained by TEM. Interspecific similarity of sperm ultrastructure was registered by this method in several reports [21,22,23,24,25,26]. Some authors speculated that minor intra-sperm differences may be used as interspecific signs to discriminate the oyster species [26, 27]. Since the data appears controversial, more detailed research seemed necessary to clarify if species specific traits could be found in oyster spermatozoa. Therefore, here, we focused on an in-depth study of C. gigas sperm to find out if its structure is similar or different regarding sperm of other oyster species previously described in the literature.
Our own results obtained by TEM were not definitive. On the one hand, the ultrastructural features of C. gigas habituating in the Peter the Great Bay are more likely typical for common oyster sperm patterns [28, 29]. On the other hand, a study of the anterior–posterior sperm head projections enabled discrimination of at least three morphs based on the ultrastructure of acrosomes. Based on TEM images, we have suggested that intraspecific sperm variation exists in C. gigas habituating in Primorye.
However, due to the randomness of the anterior–posterior projection finding, as well as restricted spatial possibility of the ultrathin sections, we could not guarantee convincing quantitative analyses of sperm morphs using TEM only. Taking into account that interesting results concerning interspecific and intraspecific differences have been obtained by studying the external morphology of male gametes [30,31,32,33], we also investigated sperm exteriors using SEM. Surprisingly, as shown above, we found six regularly repeated sperm morphs that were normally produced by testes of each sample collected.
The heteromorphy is a characteristic species specific feature of the C. gigas sperm in Primorye. Unfortunately, based on the data published, we were unable to find any other reports concerning sperm heteromorphy in Ostreidae. Therefore, we could not determine if the heterogeneity of sperm is a unique feature of C. gigas in Primorye or if the same phenomenon exists for C. gigas habituating other locations and for other oyster species. Further work will be needed to test if sperm plasticity could exist in other oysters. Answering this question is crucial to finding out if an interspecific comparison based on sperm morphology could be effective in Ostreidae.
Sperm plasticity may be assigned to environmental factors
The sperm plasticity found in C. gigas is not an exceptional phenomenon. Intraspecific variations distinct from any abnormal sperm types but related to heterogeneity of normal gametes is a usual feature described in both invertebrates and vertebrates [31, 34,35,36]. In marine invertebrates, intraspecific sperm variability has been discovered as a phenomenon concomitant to reproductive character displacement that entails genetic divergence of species [30, 37, 38]. Environmental factors could also influence sperm morphology [35]. Taking into account that oyster morphological plasticity is tightly connected to environmental conditions [9], the reasons for plasticity of oyster sperm might also be related to environmental factors.
It should be stressed that in Primorye the geographical locations are unique regarding the ecological conditions. Location 1, Amursky Bay, is highly contaminated due to the inflow of sewage from industrial areas. Pollution affects the physiological conditions of the bottom marine organisms in this geographical area [39, 40]. The poor physiological condition of males is usually accompanied by raising more sperm morphs [32]. The five abundant sperm morphs (more than 10% for each morph) were found in this area. The sixth pattern appears very minor and does not belong to the abundant morphs. It could be suggested that the presence of so many abundant sperm morphs is a result of a poor physiological oyster state induced by the permanent effects of chemical contaminations.
Location 2 is a peninsula situated in the Eastern Bosphorus Strait between the island and continent. This area undergoes a constant inflow of water, having no contamination and very stable temperature, salinity and current intensity [40, 41]. We found that oysters collected from this location have only one abundant sperm pattern although the other five are present in minor quantities. It could be proposed that the dominance of only one sperm morph may be connected to the normal physiological state of the oysters living in a stable environment.
Location 3 is in Ussuriisky Bay. The environmental conditions lack any serious contamination and are described as relatively stable for the bivalve molluscs’ life [1, 2]. However, the oyster collection was performed in the shore area that undergoes constant and strong surf. Intensive water turbulence is a characteristic feature of this area. The turbulent water movement can reduce the probability of reproductive success during external fertilisation [42]. Therefore, the sperm variations could be necessary to increase the fertilisation probability in these difficult conditions. Some sperm has higher speeds and shorter longevity, and some are slower but have higher longevity [43]. It seems possible that the three abundant sperm morphs typical for the oysters living in this area might have been adopted to cope with variation of water turbulence.
Location 4, is in Vostok Bay, situated far away from the industrial areas and is relatively independent regarding antropogenic activity [44]. However, it is characterised by a very unstable natural environment. The temperature and salinity undergo frequent changes defined by the flowing of streams that impound the bay due to rains and exchange with the open part of the Peter the Great Bay [40, 45]. C. gigas inhabiting this area produce six abundant sperm morphs. It seems likely that the maximum amount of abundant sperm morphs is connected to the very stressful environment of this location.
After investigating the oyster sperms in four geographical locations of Primorye, it seems possible that the amount of abundant sperm patterns is lower in locations with the most stable environments. A larger amount of abundant sperm patterns is typical for locations that are affected by a stronger influence of anthropogenic or natural factors. We suggest that the increase of the abundant morph variants may help to provide reproductive success of C. gigas during fertilisation in extreme aquatic conditions. Also, it should be stressed that there is a dominant sperm pattern that is most common in each of four studied geographical location of C. gigas in Primorye (Fig. 7a–d). Probably each of these morphs is most adapted to the specific environment of the corresponding location.
Sperm plasticity is a choice from six morphs
It is noteworthy that, regardless of the extreme conditions increasing the amount of abundant sperm patterns, the general amount of oyster sperm morphs never exceeded six patterns. Probably, these morphologically stable variants are genetically determined. This finding is consonant with the data of Moy et al. [46] who described the diversity of reproductive protein (bindin) isoforms in the sperm’s acrosomes in C. gigas. The amount of sperm morphs (six) found by us in Pacific oyster is comparable to the amount of bindins (five) discovered for this species. The oyster’s sperms contain bindin proteins of 35, 48, 63, 75 and 88 kDa. Besides, the formation of more bindin variations is possible due to recombination and splicing of the bindin gene [46]. It seems reasonable to suggest that specific bindin isoforms may correspond to definite oyster sperm morphs. This hypothesis could be checked using immunomicroscopy.
Practical implications from the study of C. gigas in Primorye
It seems possible that the assesment of sperm plasticity characteristic for local oysters might be considered as a possible biomarker of ecological monitoring of the marine areas targeted for farming. Indeed, lower or larger amounts of abundant sperm morphs in the testing samples may help to assess if a water area is comfortable or stressful for potential oyster plantation. Also, taking into account that only one sperm pattern is dominant in each geographical location, oyster intraspecies geo-authentification seems possible. Indeed, globalisation of the food supply in the world, and the growing complexity of the supply chains made it difficult to trace the origin of seafood. The methods of evidence that the product originates in the area are very valuable in this regard [47]. Identification of the dominant sperm pattern could help identify the oysters’ geographical origin. The supplement of mature testes preserved by fixation in alcohol or formalin followed by their testing in electron microscopy lab may be recommended for oyster producers.