A comparative analysis of osteocranium morphology in Lutjanus johnii (Bloch, 1792) and Lutjanus gibbus (Forsskal, 1775) inhabiting different habitats

The snapper species Lutjanus gibbus (L. gibbus) is commonly found in coastal areas characterized by coral reefs and clear water, while Lutjanus johnii (L. johnii) resides in mangrove waters with murky conditions and strong currents. The habitat plays a crucial role in influencing body shape and ossification patterns in fish. Therefore, this study aimed to examine the differences in the cranial bone morphology between the two Lutjanus fish species that resided in contrasting habitats. The fish samples were collected from local fishermen operating in the waters of Pulo Aceh, Aceh Besar, Indonesia. Following capture, the fish were transported and landed at Lampulo Fishing Port in Banda Aceh for further analysis. For each species, a total of 5 fish samples were randomly selected, and thorough cleaning of scales and muscles was performed before the drying of the bones. These dried cranial bones were then photographed, edited, and subjected to detailed analysis. The study showed that, of the 23 observed skull bone characters, 15 characters showed the differences in the cranial bones of the two snapper species, including the pre-ethmoidal, ethmoidal lateralis, prefrontal, supraorbital, nasal, ethmoidal, sphenoticum, crista occipital, lacrimal, inter-opercular, opercular, preopercular premaxilla, endopterygoideum, and hyomandibular. Significant differences were observed in the bones composing the eyes, nose, jaws, and ventral region of the head. In the mangrove snapper ( L. johnii ), the bones associated with the eyes (orbitospenoidal, supraorbital, and prefrontal) were less developed, while those related to the nose (nasal, ethmoidal, ethmoidal lateralis, pre-ethmoidal) showed well-developed characteristics. The coral snapper L. gibbus displayed a relatively small but thicker premaxilla bone in the jaw region, along with smaller bones in the ventral head area.

Several scientific reviews explored the relationship between fish morphology and their respective habitats, including the relationship between osteology and various environmental factors such as depth (Gerringer et al., 2021;McGonagle et al., 2023;Muir et al., 2014), water turbidity (Jalili and Eagderi, 2014), water currents (Cao et al., 2021;Souza and Pompeu, 2020;Lauder and Drucker, 2003), and feeding habits (Friedman et al., 2016: Wainwright, 2002;Winemiller et al., 1995;Muir et al., 2014).The cranium bones (Osteocranium) plays an important role in protecting the brain and nerves such as vision (sensory) and smell (alfactory).Specific studies examined the correlation between the bone shape and water depth in marine fish, such as investigations on snailfishes of the Liparidae family conducted by Gerringer (2021).These reviews consistently concluded that noticeable differences existed in the facial bones, particularly the opercular, hyoid, dentary, and angular bones.In species inhabiting deep waters, these bones tend to exhibit reduced size, along with a softer and thinner bone structure.
Previous studies also examined the habitat-related differences observed in cave-dwelling fish species such as Iranocypris typhlops and Garra lorestanensis.Significantly, it was observed that the lacrimal bones, contributing to the development of the eyes, were poorly developed in these fish (Jalili and Eagderi, 2014;Sabet and Eagderi, 2016).Several reviews have investigated the relationship between feeding habits and jawbone morphology, concluding that carnivorous fish generally exhibited well-developed dentary and angular bones compared to herbivorous fish and vice versa (Friedman et al., 2016;Wainwright, 2002;Gidmark et al., 2012); Wainwright and Richard, 1995).Cao et al. (2021) and Langerhans et al. (2004) indicated that water currents could influence the shape of the osteocranium in fish, with species inhabiting fast-flowing waters often possessing a low hump in the occipital region to reduce resistance while swimming against the current.Despite these previous investigations, studies examining the osteology of snapper species, particularly focusing on the head (osteocranium) based on their habitats, remain scarce.Therefore, this study aims to examine the differences in the osteocranium (head bone) between two Lutjanus species that occupy distinct habitats.These species include L. johnii (Bloch, 1792), inhabiting mangrove waters characterized by high turbidity and swift currents, and L. gibbus (Forsskal, 1775), dwelling in coral reefs with clear water and slow currents (Rome and Newman, 2010).Therefore, information on the osteocranium of these two species of snapper is crucial to differentiate these species based on the cranium bones due to adaptation to different environments and contributes to the ichthyological science as an alternative tool in fish taxonomy.

Time and site
This research was conducted from March to May 2022.Fish samples were caught by fishermen in the waters of Pulo Aceh Aceh Province, Indonesia, and landed at the Lampulo Fishing Port, Banda Aceh City, Aceh Province.The collected samples were analyzed at the Physiology Laboratory, Faculty of Mathematics and Natural Sciences, Syiah Kuala University, Banda Aceh.The object species in this study were L. gibbus and L. johnii.According to Rome and Newman (2010), these two fish live in two different habitats, namely L. gibbus in coral reefs, while L. johnii in mangrove ecosystems.This information was confirmed and validated by the fishermen who caught the fish sample.

Fish sample preparation and data analysis
A total of 5 fish samples per species, weighing between 1500 g to 2000 g, and total length of 30 -35 cm were randomly selected for the study.The samples were immediately placed in a styrofoam box and kept at a temperature of 4 o C to maintain their freshness during transportation to the laboratory.Upon arrival, the fish was carefully rinsed with fresh water to remove any external debris.Each fish sample was photographed using a Canon 1200D digital camera (Japan), with an example shown in Figure 1.Furthermore, hot water at 100 o C was poured over the entire body of each fish sample to facilitate the removal of scales and skin, which were delicately peeled off using a scalpel.The muscles were filleted from the dorsal to the median part on both sides of the body.Moreover, the head was separated from the body by cutting the neck bone, and any remaining muscles were extracted using tweezers.
All the remaining meat and fat were soaked in 70% alcohol for two days to clean the cranium.Following this process, the cranium bones underwent sun-drying for one day until achieving a white appearance.Any dislocated bones were reconstructed in their original positions using adhesive.Once the cranium was reconstructed, it was photographed and edited using Photoshop CS6 software.The identification of the different parts of the cranium was based on the references provided by Zulfahmi et al. (2022) and Rojo (2013).These references were consulted to ensure accurate identification and naming of the cranium parts.The data obtained from the analysis were presented in the form of images and analyzed descriptively.A comparison was made with relevant reports and references to ensure accuracy and provide a comprehensive description of the results.

Data analysis
Data were presented in the tables and figures and then analyzed descriptively by comparing the findings with previous studies or reports, and other relevant theories.

Description of pre-ethmoidal, ethmoidal lateralis, prefrontal, supraorbital, nasal, ethmoidal, sphenotic, and crista occipital bones
The pre-ethmoidal bones displayed lateral widening in L. johnii and anterior narrowing in L. gibbus.In L. johnii, the ethmoidal lateralis had a wide axis with a narrow end, whereas L gibbus exhibited a small axis with a short end.The prefrontal in L. johnii exhibited a wavy and narrow-angle, contrasting with the straight and wider angle observed in L. gibbus.In L. johnii, the supraorbital had a concave posterior angle, whereas in L gibbus, it had an anterior concave angle (Figure 3 and Table 2).The nasal foramen in L. johnii was small and narrow laterally with a long posterior, while in L. gibbus, it was large and wide laterally with a short posterior (Figure 4 and Table 2).Regarding the ethmoidal bone, L. johnii exhibited a slender and long shape, while L. gibbus had a wide and short shape.The sphenotic bone in L johnii showed slight indentation, whereas L. gibbus displayed a dorsal to a ventral concave shape.The crista occipital in L. johnii had a low angle directed posterior-ventrally, in contrast to the high angle dorsally observed in L. gibbus (Figure 5 and Table 2).

Lacrimal, inter-opercular, and opercular bones
The lacrimal bones in L. johnii exhibited a wide articulation site, with a blunt posterior that formed a small angle anteriorly.Meanwhile, the lacrimal in L. gibbus had a small articulation site, with a tapered ventral posterior and dorsal and ventral angles in the anterior (Figure 6 and Table 2).The inter-opercular bones in L. johnii had a blunt posterior and lacked anterior bony elevation, while it had a sharp posterior with anterior bony elevation in L. gibbus (Figure 7).In L. johnii, the opercular tended to be shallow, whereas in L. gibbus, it was thinner, and the dorsal notch was highly concave (Figure 8).

Preopercular, premaxilla, endopterygoid, and hyomandibular bones
The preopercular bone in L. johnii had a straight angle in the posterior-dorsal portion, while in L. gibbus, it displayed a curved shape (Figure 9).The premaxilla in L. johnii had a longer kinetmoid and lateral maxilla, but a smaller premaxilla.Meanwhile, it had a shorter kinetmoid and lateral premaxilla, but a wider premaxilla in L. gibbus (Figure 10 and Table 2).The endopterigoid bones in L. johnii exhibited a straight posterior with bony prominences on the ventral side, whereas, in L. gibbus, the posterior was blunt without bony prominences (Figure 11).In L. johnii, the hyomandibular bone had a wider dorsal side, while in L. gibbus, the dorsal part was small (Figure 12 Table 2).
In the case of L. johnii, the mangrove snapper, the orbit sphenoidal (eye holes) have appeared degraded, smaller, and oval-shaped, extending towards the posterior head.However, in L. gibbus, the orbit sphenoidal was rounded and centrally located on the head.The observation suggests that the eye structure of L. johnii may have undergone adaptations due to its habitat in murky waters, where a visual function is less dominant.This speculation finds support in the work of Schaduw (2018), who noted that mangrove waters exhibit high turbidity caused by silt and organic materials from the land.Consequently, snapper species inhabiting mangrove waters rely more on their olfactory function, which is indicated by the presence of large nasal bones, as observed in L. johnii.Nasri et al. (2016) and Jalili et al. (2015) reported that fish species with reduced reliance on their sense of smell tended to have smaller ethmoidal bones.Additionally, Akmal et al. (2020) Radhi et al. (2023) that ethmoidal bones functioned as chemosensors, housing receptors for olfactory nerves located in the nostrils.These results were consistent with the work of Eagderi and Adriaens (2010), who noted that the Hoplunnis punctata fish (Nettastomatidae), depending on olfactory for prey detection, possessed long and pointed ethmoidal bones.The results of this study showed that the hump bones (occipital), specifically the occipital crista, were not well-developed in the mangrove fish (L.johnii).A low hump is typically observed in fast-swimming fish (Cao et al., 2021), suggesting that despite inhabiting murky waters, L. johnii is capable of swimming at high speed due to its reduced hump structure.However, several cranium bones in L. gibbus appear to be degraded and smaller, such as the inter-opercular, opercular, preopercular, endopterygoid, and hyomandibular bones.The opercular and endopterygoid bones also show signs of thinning.This degradation is likely attributed to the fact that L. gibbus inhabits deeper waters compared to L. johnii.Previous studies reported that bone shape degradation could be linked to the depth of the water where fish reside (Gerringer et al., 2021;Rendal and Farrell, 1997).According to Rome and Newman (2010), L. gibbus can be found at depths of up to 50 m, while L. johnii inhabits even deeper waters, reaching depths of up to 80 m.
Further observations on the masticatory bone (premaxilla) in L. johnii show a long, thin premaxilla with fewer sharp teeth, whereas in L. gibbus, this bone is shorter, thicker, and the teeth are more pointed.Studies by Friedman et al. (2016) and Gidmark et al. (2012) suggested that differences in jaw bone morphology in fish could be influenced by variations in feeding habits.Although both species are carnivorous (Prihatiningsih et al., 2017), there are distinctions in their prey-capture strategies.L. johnii tends to adopt a passive approach, hiding and launching sudden high-speed attacks, while L. gibbus actively hunts for prey.The presence of a tall tail and a narrow or slender caudal peduncle base in L. gibbus supports this hunting behavior (Wainwright and Bellwood, 2002;Gonza´lez et al., 2022;Song et al., 2020).

Conclusions
In conclusion, this study identified distinct differences in the cranium bones of L. johnii and L. gibbus, specifically in the bones related to the eyes and nose.L. johnii exhibited underdeveloped eye bones but well-developed nasal bones compared to L. gibbus.However, L. gibbus displayed shorter and larger jaw bones, along with smaller ventral bones.

Figure 1 .
Figure 1.The sample of snappers used in this study.(a) L. johnii dan (b) L. Gibbus