Lernanthropus is the most common genus of parasitic copepods. There are more than 100 species described from gills of different marine fish (Toksen, 2007). The current investigation revealed hemorrhagic areas on the body surface with excessive mucous secretion and marbling appearance of the gills of infected M. labrax with L. kroyeri. These lesions could be attributed to attachment of the parasites by their rigid claws, feeding activity, severe irritation caused by parasitic movement, and mucous increase as a defense mechanism from the host to overcome the infection as reported by Heba Abdel-Mawla and El-Ekiaby (2012).
The present study recorded isolation of L. kroyeri from gills of M. labrax. Likewise, Tosken et al. (2008), Henery et al. (2009), and Essa et al. (2012) were isolated the same parasite from the same host and the same site. Meanwhile Noor El-Deen et al., (2013) and Dawlat Hassanin (2016) isolated L. kroyeri from the gills of other fish species such as Mugil cephalus and Moolgarda seheli.
In the current prospective, the prevalence of L. kroyeri was 81%, Concurrent with a previous study; Aneesh et al. (2014) recorded 81.4% infection of Strongylura strongylura by L. kroyeri. Additionally, Toksen (2007) reported a higher infection rate (100%) by L. kroyeri in Dicentrarchus labrax. Nevertheless, Manera and Dezfuli, (2003) obtained a lower infection rate (35%) with L. kroyeri in D. labrax. Our paper reported that L. kroyeri infection was highest during Spring (94%) followed by Summer (90%) then Autumn (78%) and finally Winter (31%). This sequence is nearly in agreement with Eissa et al. (2017) who also reported that the infection rate with L. kroyeri reached their maximum rate during Spring and Summer, while the lowest infection was recorded during Autumn. These results were inconsistent with Ola Abu Samak and Ashraf (2008) who reported that the infection rates with the same parasite reached their maximum rates in Autumn and Winter (42.5% and 35%) respectively, while their minimum value was 7.5% in Spring. These variances in the total infection and seasonal dynamics could be returned to the difference of fish species and the difference of locality of fish collection.
Certain fish parasites can accumulate heavy metals at concentrations significantly higher than those in host tissues or the environment (Sures, 2001, 2003, 2004; Sures et al., 2003; Schludermann et al., 2003; Thielen et al., 2004; Tekin-Ozan and Kir, 2005). The data of our study revealed that there was a high concentration of Zn in the collected samples. While the concentrations of Cu, Cd and Co were under detection limit. In general, the accumulation of Zn was significantly higher in the non-infested tissue in comparison to the infested tissue samples. It is opined that L. kroyeri can absorb Zn from the fish tissue through its alimentary canal and accumulates it in the parasite tissue, this finding was verified by analysis of Zn in the parasite tissue. On the same manner, a recent study by Hassanine and Al-Hasawi, (2021) reported that acanthocephalan accumulates higher concentrations of heavy metals. Concurrent with other study, Szefer et al. (1998) suggested that the bioaccumulation of parasites may reflect the higher ability of host to clear heavy metals. Also, Sures and Siddall (1999), Taraschewski (2000) and Malek et al. (2007) considered the parasites beneficial and might act as a heavy metal sanitizer for the host. Gills were accumulated higher Zn value compared to the edible part of its fish host. The low ratio of Zn concentration in host muscle could be returned to the longer exposure time as metal uptake occurs faster in parasites as opined by Sures (2001).
Considering histopathological findings, we illustrated sections of L. kroyeri distributed in the gills. Similarly, a recent study by Eissa et al. (2020) reported occurrence of L. kroyeri fragments in the gills of D. labrax. Destruction of secondary lamellar epithelium, goblet cell metaplasia with hemorrhage and excess mucous secretion could be induced as a tissue reaction to decrease the irritation against the infestation. Concurrent with previous studies, Heba et al. (2015), Lester and Hayward (2006), Ragias et al. (2004), and Manera and Dezfuli (2003) reported extensive hemorrhage duo to the feeding activity of this parasite. Lymphocytes and eosinophils were found in gill filaments and arches, these outcomes were in accordance to Manera and Dezfuli (2003), Korun and Tepecik (2005), Toksen (2007), Jithendran et al. (2008), Yardimci and Pekmezci (2012). Also, erosion of gill raker beside necrosis of muscles were seen, likewise, Vinoth et al. (2010) reported pale gills induced by copepod parasites duo to loss of gill raker.
Our investigation concluded that, although L. kroyeri has a bad effect on the infected M. labrax, it also plays an important role in elimination of heavy metals from the tissue of the infected fish through its ability to accumulate heavy metals in its body; and this can be advantageous for the infected hosts, allowing them to tolerate much higher concentrations of certain metals. The present results also confirmed that L. kroyeri seem to be good indicators of environmental pollutions.