Intra- and interspecific differences: comparison with former studies on Pagellus species and other fish species
To understand the relationship between function, shape, and the environment, it is essential to include the morphological variability of otoliths, considering biological and environmental variability leads to otolith shape heterogeneity through morpho-functional adaptation to different habitats. Several authors have highlighted changes in otolith shape between species and, in many cases, among populations of the same species (e.g., herrings, salmonids, and lutjanids). The intra-specific variability of otolith morphology and shape are the basis of stock separation and assessment and is related, especially in sagittae, with environmental (e.g., water temperature, salinity, and depth) and biological factors (e.g., sex, ontogeny, and genetic variability)20.
The analysis of the three Pagellus species revealed that otolith morphology and morphometry did not follow those described in a previous study1 conducted in the western Mediterranean Sea and the Atlantic Ocean in term of rectangularity, circularity, sagitta aspect ratio and sagitta length to total fish length ratio. Although the images provided in our study closely resembled those from research in other geographical areas, the morphometric measures (obtained according to the procedures and methods described in the previous literature1,20,23,32) exhibited several differences. Considering the scale of our study compared to previous studies, it is difficult to provide an entirely valid comparison; the differences in sagitta morphology and morphometry could have been triggered by biotic and abiotic parameters (e.g., temperature, salinity, genotype, habitat type, differences in food quality and quantity)13,33,34,35. Such environmental and genetic factors may be primary drivers of otolith morphometry and morphology among fishes in different habitats. Therefore, detected shape differences are at the basis of fish stock differentiation36.
Our results indicate that the min–max circularity and rectangularity of P. bogaraveo from the Southern Tyrrhenian Sea differ from those calculated in a previous study1 in the western Mediterranean Sea, and the north and central-eastern Atlantic ocean. Moreover, the increase in circularity in larger specimens, confirms a greater tendency toward circular than elliptical otolith shape in southern Tyrrhenian Sea species compared to those in other Mediterranean and Atlantic areas.
Despite statistical differences and correlations in this study supported the hypothesis that some changes in sagitta morphology are related to fish size differences, several aspects and studies should be performed to better understand this relation. The negative correlation between the ratio of sulcus acusticus surface to the entire sagitta, rostral morphology, and the increase in specimen’s size was related to the expansion in the length and surface of the entire sagitta and rostral area in larger specimens. These features, with no statistical relevant increment in sulcus acusticus surface and increased rostrum length, could be correlated with more pronounced peripheral sagitta growth in this species. Sagitta, in fact, after fish pelagic phase, might increases its surface in the rostral area and the margins. Since the present study did not take into account ontogenetic stages and specimens age, it is hard to relate this result with sagitta and sulcus acusticus growth. But reading this increase by an ecological point of view, it could be related to the lifecycle of the species. During the juvenile stage, in the early stage of pelagic life, the species inhabits shallow water. Adults inhabit deep-water environments, migrating down the continental slope to a depth of 800 m after the juvenile stage. These changes in habitat might be the cause of morphological variations in the sagittae, highlighting the relationship between sagitta features and environmental and biological factors.
The P. acarne specimens demonstrated the highest number of morphometrical parameters that did not follow those of the same species described in a previous study (i.e., circularity, rectangularity, sagitta length to total fish length ratio, and sagitta aspect ratio)1. These morphometrical changes are reflected in otolith shape. The otoliths from specimens in our study were largely circular, with highly irregular margins and a rostrum that varied in length and width through the left and right sagitta, as indicated by the significant differences in rostrum aspect ratio values.
The morphometrical results in P. erythrinus revealed differences in circularity, rectangularity, sagitta length to total fish length ratio and sagitta aspect ratio compared to a previous study1 in the western Mediterranean Sea and north-central eastern Atlantic ocean.
The P. erythrinus specimens were characterized by a pentagonal otoliths shape, and increased circularity compared to the same species from other areas. The results also indicated small differences between the left and right sagitta. This small differences were previously described in other Mediterranean sub-areas, for example, otolith width values in P. erythrinus specimens collected in the Gulf of Tunisia37,38.
As said above for P. bogaraveo, it is hard to relate the differences between juveniles and adults with fish growth due to the absence in present paper of ontogenetic and age analysis. The higher width than length, demonstrated by min–max width values in Tables 1 and 2, in sagittae of adults P. erythrinus specimens could be correlated with an exponential increment in fish size compared to the sagittal length. Further analyses on ontogenetic development of this species are required to better define the sagitta growth related to fish growth.
The increase in sulcus acusticus surface exhibited in the adult specimens could be correlated with feeding habits; during its adult life, this species is a benthic feeder and inhabits deeper environments than juveniles28,29.
Although meaningful lateral dimorphism of the sagittae was detected only in flatfish, statistical analysis revealed several small differences between the left and right sagitta in P. erythrinus and P. acarne, as previously described in other round fish species, such as Chelon ramada (Risso, 1827)40, Diplodus annularis (Linnaeus, 1758)41, Diplodus puntazzo (Walbaum, 1792)42, Clupea harengus (Linnaeus, 1758)43, and Scomberomorus niphonius (Cuvier, 1832)44.
Our study confirmed slight differences between width values in left and right sagitta previously described in P. erythrinus and extend the differences to other parameters, such as circularity and rectangularity (Tables 1, 2). Concerning P. acarne, however, marginal differences between the left and right sagittae were observed for the first time.
This slight differences are supported by the literature concerning genetic and environmental stressors41. Since the functional morphology of otoliths is not completely understood, it is difficult to find a direct link between these small differences and the ecology of the species. However, several eco-functional factors, such as feeding behavior, deserve attention as fundamental for a better understanding of the relationship between otolith features and species habitat. For example, P. erythrinus largely preys on strictly benthic organisms, such as polychaetes, brachyuran crabs, and benthic crustaceans. Most of these species frequently escape predators by hiding under the sandy substrate. Other Sparidae (Lythognathus mormyrus, Linnaeus, 1758) feed on benthic fauna, engulfing sediment and filtering it in the buccal cavity, demonstrated by the high percentage of detritus and benthic remains (e.g., scales, urchin spines, and benthic foraminifers) in the gut and stomach contents29. To engulf sediment, P. erythrinus performs a particular movement with the head and body, laterally shifting and pushing forward, to dig the bottom sand and reach prey. This kind of behavior, common in all benthopelagic species with the same feeding habits, could influence the sagitta growth and morphology, triggering small differences between the left and right sagitta. Further studies on this and other species with this behavior (e.g., L. mormyrus) are necessary to confirm this hypothesis.
Concerning inter-specific differences in sagitta morphology among the three species, it is difficult to read the results obtained in this study eco-morphologically since an insufficient understanding of the functional morphology and physiology of otoliths prohibits a direct relationship, valid for all the species, between eco-functional features and otolith morphology. Nevertheless, as expected, the shape analysis (Fig. 1) revealed clear differences between the three congeneric species. Considering several ecological, functional, and biological features in each species, the results have demonstrated a sagitta morphology that could be in accordance with the ecology and lifestyle of these three congeneric seabreams.
Relationship between otolith morphology and ecology/lifestyle
The sagittae of P. acarne exhibited a shape resembling those in other pelagic species, with a long rostrum and the entire sagitta elongated and narrower than those in other two seabream species. The species that show the most pelagic habits, with largely planktivorous feeding at a small size, adapt also to benthopelagic feeding activity in adult life. The statistically relevant similarity found in P. bogaraveo could be proof of the ecomorphological adaptation of sagittae to pelagic and demersal environments. This hypothesis may be confirmed by marked differences in shape compared to those in P. erythrinus, which is the most benthic among the three species.
Pagellus erythrinus was the species with the shortest rostrum. It also has the most benthic habits, largely preying on epibenthic and infaunal species. Moreover, its ecology and life cycle differ among the three species under study since they are strictly related to the benthic environment. This lifestyle could be in accordance with the differences observed in the shape analysis results. The sagitta contours appeared more circular and wider than those in the other two species. The PCA and LDA also confirmed the most difference in shape among the three species.
The species with the most marked antirostrum and sagitta shape was P. bogaraveo, which is a cross between the other two congeneric species. Pagellus bogaraveo is a demersal species, which inhabits the deep biocenosis and feeds in both benthic and mesopelagic environments. Furthermore, the ecology of this species could support the sagitta shape described in our study27,30.
Otolith morphology and morphometry in congeneric Pagellus species described in this study has followed the relationship between sagittal parameters, habitat, and depth described in previous literature15. According to several authors, the percentage of species with large otoliths increases with depth, except for abyssal depth. The specimens of P. bogaraveo analyzed in this paper (especially adult individuals) had larger otoliths than the other two Pagellus species due to their demersal habits (they inhabit the continental slope to a depth of 800 m). A larger sagitta is essential in demersal environments to compensate for light reduction by providing improved acoustic communication, sound perception15,45, and a sense of equilibrium46.
Sulcus shape
Considering the sulcus acusticus, in the otolith atlas for the western Mediterranean Sea and Atlantic ocean1, studies describing and comparing otoliths10 and the diversity and variability of otoliths in teleost fishes9, the sulcus in P. bogaraveo, P. acarne and P. erythrinus was described as heterosulcoid, with an ostium shorter than the cauda and a long, narrowed rostrum, especially in adult P. bogaraveo and P. acarne individuals. Heterosulcoid otoliths were also observed in south Tyrrhenian Sea Pagellus individuals, with marked differences between juvenile and adult specimens. In a demersal species, such as P. bogaraveo, juveniles live in shallow, coastal water. Once adults, they inhabit deeper water (to a depth of 800 m). Changes in the crystalline and morphological structure of sulcus acusticus between juveniles and adults reflect this species’ need to adapt to deeper environments with less light.
The results indicate that in P. bogaraveo, the sulcus acusticus does not differ in surface between juvenile and adult specimens. This feature could be correlated with earlier sulcus acusticus development in this species, compared to P. erythrinus and P. acarne, emphasizing the role of the sulcus acusticus in this demersal species48,49. This might also confirm the strict correlation between biological and environmental factors and sagitta morphology in studied seabreams species.
Another morphological feature of the sulcus acusticus, which might support the ecology of the species, is the deep ostium and cauda. In adult specimens of P. bogaraveo and P. acarne, the sulcus structure deeply penetrated in the sagitta carbonate structure. Conversely, in adult P. erythrinus specimens, the sulcus did not penetrate as deeply as in the other two seabream species. This sulcal feature could correspond with the ecology and feeding behavior of P. erythrinus, which specializes in benthic strategies, including small differences between left and right sagitta and the absence of the notch and antirostrum in sagittae.
Although the deeper sulcus acusticus in P. bogaraveo and P. acarne might be linked to depth distribution, as in P. bogaraveo, it may also correspond with high mobility related to feeding behavior, as in both P. bogaraveo and P. acarne. The different depths of sulcus acusticus can change the thickness of the otolithic membrane, by varying the relative motion of otoliths with the macula sacculi2. As previously demonstrated50, the different thicknesses of the otolithic membrane induce differences in mechanical resistance between the otolith and sensory epithelium.
The differences in sulcus acusticus and otolith ratio between P. bogaraveo specimens and the other congeneric species, demonstrated by the results, might be also correlated to the differences in habitat, feeding habits, and soundscape.
Despite the lack of information concerning the physiological ear response related to variations in macula or sulcus size, the sensory hair cells in macula sacculi are likely to be affected by changes in sulcus depth, shape, 3D structure (planar vs. curved), and surface.
The significant difference in relative sulcus area may be due to typical alteration in this parameter concerning differences in the mobility patterns, food, feeding behavior, and spatial niche.
Higher relative sulcus area ratios have been observed in the deepest species or those with high mobility49. In our study, the morphometry results concerning the sulcus did not follow those in the previous literature, displaying higher values in P. erythrinus and P. acarne compared to P. bogaraveo, although the latter inhabits a deeper environment than the other congeneric species.
This higher relative sulcus acusticus surface and the larger, curved sulcus acusticus of P. acarne and P. erythrinus could be correlated with higher mobility in these species (especially P. acarne). In P. erythrinus, however, these features might be related to its benthic lifestyle.
As demonstrated by the PCA and LDA of sulcus acusticus parameters, P. erythrinus and P. acarne, which share similar depths and habitats, revealed marked similarities, whereas P. bogaraveo, which lives in the deepest strata of the water column, displayed the most different sulcus acusticus. However, PCA and LDA indicated that the otolith shape in the entire P. erythrinus sagitta was significantly different compared to those in P. acarne and P. bogaraveo.
These features could provide a reading key for sagitta and sulcus acusticus eco-morphology in the life cycle and environmental adaptation of fish.
The connection between the otoliths and the macula sacculi is fundamental for transducing environmental acoustic signals and for the relative motion of fish (balance). The sulcus acusticus is the area of the otoliths in which this connection occurs.
Features of the texture
Furthermore, the external textural organization23 changes between juveniles and adults or when environmental changes occur. The differences in the external textural organization found in juveniles and adults support those reported in the literature concerning other species39. Figures 3b, c and 5a–h, demonstrate that our study supported this prediction. However, P. bogaraveo and P. erythrinus juveniles, compared with other species (such as gurnards)23 displayed a more uniform, mineralized, external textural organization.
SEM images of the crystalline structure of P. bogaraveo, juveniles (a, b) and adults (c, d), and P. erythrinus, juveniles (e, f) and adults (g, h) sagittae.
According to previous literature8, improved hearing capabilities in a species are closely related to a higher value of relative sulcus area ratio. Habitat features, such as depth, feeding strategies, mobility, trophic distribution, and ontogeny, could also influence this ratio.
Hence, it may be concluded that morphological differences in sulcus acusticus shape and surface among species are important for comprehending the ecomorphological and eco-functional role of sagitta 2,48.
Comparing the intra-specific differences indicated by our results with those in the literature, discussing other populations, we cannot determine whether site-differences observed in sagitta shape are related to genetic evolution and/or adaptative response to environment. To make this distinction it would require a specific experiment in which offspring from different populations are raised in a controlled environment.
Furthermore, the knowledge about physiology and functional morphology is insufficient to provide a clear correlation between inter-specific differences among the three congeneric Pagellus species and their ecological and functional features. However, differences in sagitta morphology and morphometry among these three Pagellus species may be related to differences in lifestyle, ecology, and biology since they follow the ecomorphological features of sagittae and species ecology described in the literature.
Source: Ecology - nature.com
