Length-Weight Relation of the Angular Rough Shark, Oxynotus centrina (Linnaeus, 1758) in the Mediterranean Sea

Oxynotus centrina is considered a rare or uncommon shark species throughout its distribution range. Because of this reason, it is one of the 46 uncommon elasmobranchs in the Mediterranean Sea, which requires filling the knowledge gaps, such as length-weight relations. Based on the analysis of 34 angular rough sharks, the length-weight relations (LWR) of males, females and combined sexes of O. centrina were described as W = 0.216L 2.686 , W = 0.134L 3.041 and W=0.102L 3.173 , respectively. Total lengths (TL) of females and combined sexes were strongly correlated with total weights (TW); while TL of males was moderately correlated with TW. The b values of LWRs of females and combined sexes (3.041 and 3.173, respectively) were not significantly different from 3.0 (t-test, p>0.10) and indicated positive allometric growth for the examined TL and TW range of O. centrina in the Mediterranean Sea.


Introduction
The angular rough shark, Oxynotus centrina (Linnaeus, 1758) is a bottom dwelling squaliform shark of the family Oxynotidae, inhabiting continental shelves and upper slope at depths of 50 to more than 700 m (Ebert & Stehmann, 2013).It is commonly found on coralline algal and muddy bottoms, mostly deeper than 100 m depth (Ebert & Stehmann, 2013).The distribution range of O. centrina extends from north-to-southeastern Atlantic, and throughout the Mediterranean Sea, where the northernmost range extends to the Sea of Marmara (Ebert & Stehmann, 2013;Turan et al., 2018;Serena et al., 2020;Kabasakal, 2020).
In a recent study, Tsikliras & Dimarchopoulou (2021) emphasized that Oxynotus centrina is one of the 46 uncommon elasmobranchs in the Mediterranean Sea which requires filling the basic knowledge gaps, such as length-weight relations.It is, therefore, the aim of the present paper to provide length-weight relation (LWR) along with the growth parameters of O. centrina, based on individuals collected from several localities throughout the Mediterranean Sea.

Materials and Method
Length-weight data of Oxynotus centrina (n=34; Figure 1), that were examined in the present study, were obtained from the following sources: (1) unpublished morphometric measurements of the O. centrina individuals (n=17), which were recorded during the 1960 expedition of the former Meat and Fish Institution of the Republic of Turkey and donated to first author (HK) for analysis by Turkish Marine Research Foundation (TÜDAV); (2) length-weight measurements of the Mediterranean specimens of O. centrina (n=16), which were published in relevant references (Megalofonou & Damalas, 2004;Dragičević et al., 2009;İşmen et al., 2009;Barría et al., 2015;Başusta et al., 2015;Kousteni & Megalofonou, 2016;Yığın et al., 2016;Koehler, 2018) and (3) unpublished length-weight data of a gravid female (n=1), which is kept in the archives of second author (EÖÖ).The geographical distribution of the examined individuals of O. centrina throughout the Mediterranean Sea is shown in Figure 2.
Figure 1.Gravid female angular rough shark, Oxynotus centrina, captured from Antalya Bay (specimen no 22 in Table 1)  (Ricker, 1975), where W is the weight of the fish, L is the total length, and a and b are constants.Regression analyses and ttest were computed on excel software analysis toolpak.

Results
Length and weight data of 34 individuals of Oxynotus centrina, captured in different regions of the Mediterranaean Sea, and adjacent seas (Adriatic, Aegean and Marmara Seas) were analysed.Eighteen out of 34 individuals (52.9%) were females and followed by 10 males (29.4%), while 6 individuals (17.6%) were sexed unknown.TL and TW data, sexes, capture localities of unpublished individuals together with same data of published individuals with relevant references, are presented in Table 1.Minimum, maximum and mean of TL and TW, and growth constants for males, females and combined sexes of the examined individuals of Oxynotus centrina are presented in Table 2.The b value of LWR of males (2.686) was significantly different than b values of females and combined sexes, and indicated negative allometric growth for the examined TL and TW range of males (t-test, p<0.05).The b values of LWR of females and combined sexes (3.041 and 3.173, respectively), were significantly larger from 3.0 (t-test, p<0.05) and indicated positive allometric growth for the examined TL and TW range of O. centrina.The LWRs of males, females and combined sexes of O. centrina were described as W = 0.216L 2.686 , W = 0.134L 3.041 and W=0.102L 3.173 , respectively.TLs of females and combined sexes were strongly correlated with TWs (correlation coefficients 0.936 and 0.923, respectively), while TL of males was moderately correlated with TW (correlation coefficient 0.534).LWR plots of males, females and both sexes are presented in Figure 3.

Discussion
In one of the remarkable meta-analyses of LWRs, which focused on the 3929 LWRs of 1773 species of fishes, Froese (2006) concluded that across species median b=3.03 is significantly larger than 3.0, which indicates a tendency towards slightly positive-allometric growth, increasing relative body thickness or plumpness.The author also concluded that the expected range of 2.5 < b < 3.5 is confirmed.The estimated b values of females and both sexes of the present study (3.041 and 3.173, respectively; Table 2), which were within the safe limits (Froese, 2006), indicate positive allometric growth for Oxynotus centrina.One of the well-known descriptive characteristics of O. centrina is "a species with high (or deep) and thick body" (Compagno, 1984;Quéro, 1984;Ebert & Stehmann, 2013), which is coinciding with the definition of positiveallometric growth (Froese, 2006).On the other hand, the b value of examined males (2.686) indicated a negative-allometric growth for the same species of shark (Table 2).According to Capapé et al. (1999), males of O. centrina reach sexual maturity between a TL range of 60 to 66 cm, while individuals smaller than this TL range are considered immature or maturing males.Therefore, TL ranges of the examined males in the present study (Table 2) coincide with immatures of O. centrina.
The LWR in fishes can be affected by a number of factors including season, habitat, gonad maturity, sex, stomach fullness and length ranges of the specimens captured (Kuriakose, 2017).Since fishes typically grow very rapidly in length in the first few months or years of life, until maturation (Moyle & Cech, 1988), the mature fish is typically heavier per unit of length than immature fish.The TL and TW ranges of the present males (Table 2) were restricted with the immature phase of Oxynotus centrina, which resulted in b lower than expected 3.0 and indicated a slimmer shark than the description of the species in this phase of its lifespan.This circumstance is also seen in the LWR plot of males (Figure 3), which depicts a slight inclination than the expected power graph of growth.However, the b values of the examined females and both sexes are slightly greater than 3.0 (Table 2), which is coincided with the "plumpness" of O. centrina, a well-known descriptive characteristic of the species.Furthermore, the resulted LWR plots of the examined females and combined sexes are representatives of the expected power graphs of growth (Figure 3).Therefore, LWR equations of the examined females and combined sexes (W = 0.134L 3.041 and W=0.102L 3.173 , respectively) provided more assuring equations.
In the literature, the paucity of available information on the LWR and growth parameters of Oxynotus centrina is obvious.Capapé et al. (1999) reported LWR for males and females of O. centrina,respectively, where EW is the eviscerated body weight of fish.Authors also reported that LWRs for both sexes did not show a significant difference in slopes between sexes, in waters of the western Mediterranean and eastern tropical Atlantic.In a recent study of LWRs for 10 shark species from Saros Bay (northern Aegean Sea), İşmen et al. (2009) reported on the TL and TW of 4 individuals, but they did not give LWR equation for O. centrina, due to low number of sample size.Based on TL and TW data of 4 individuals (TL range 53.3-79 cm), Tsikliras & Dimarchopoulou (2021) reported a and b values for O. centrina as 0.0087 (range 0.0080-0.0093)and 3.04, respectively.
Considering the limitations of the present study, such as the sampling locations that are distributed throughout very wide area in the Mediterranean Sea, a low number of examined samples and narrow TL range of males, the estimated LWR of Oxynotus centrina is far from being complete for the moment.But, despite all these weaknesses, the present study provides a valuable contribution to a better understanding of the LWR of O. centrina in the Mediterranean Sea.

Table 2 .
LWR parameters for Oxynotus centrina in the Mediterranean Sea Figure 3. LWR plots of Oxynotus centrina for males, females and both sexes