RECOVERY OF HAEMAL LORDOSIS IN EUROPEAN SEA BASS Dicentrarchus labrax

Introduction

 Haemal lordosis is a frequent vertebral abnorm ality in finfish aquaculture , with significant and variable effects on fish externally morphology ( Fragkoulis et al. 2019, 2021 ).  In most  of the  fish species studied so far, haemal lordosis develops during the late metamorphosis and early juvenile period , mainly due to elevated fish swimming activity (Sfakianakis et al. 2006 , Palstra et al. 2020, Printzi et al. 2021 ). Despite the significance abnormalities for the product quality , it was  recently  shown that haemal lordosis can  substantially  recover during the growth of seabream in sea cages (Fragkoulis et al. 2019) .

In the present study we examined whether  haemal lordosis may recover in European sea bass , the species where swimming-induced lordosis was first described (Divanach et al. 1997).

Material and Methods

Fifty -six sea bass  juveniles  (77±6 mm standard length, SL)  with lordotic external morphology  (dorsally shifted caudal peduncle) were selected from one reared population . All fish were  anaesthetised, tagged electronically (FDX-B, Trovan Ltd, USA ), photographed  on their left side and returned to the sea cage until the end of the on-growing period (234±16 mm SL).  At that stage, fish were anaesthetised, photographed and tag-identified. Subsequently , a representative sample of fish with a recovered and lordotic external morphology was radiographically examined.

Results & Discussion

At  the end of the examined period (353 dpt) ,  the  57%  (32  out of 56 ) of the fish with severe lordotic morphology presented normal external phenotype, whereas 7%  (4  out of 56 )  presented light lordotic morphology. The rest 36% of the fish  (20  out of 56 ) continued presenting a lordotic external morphology . The r adiographic examination of the fish, verified  the macroscopically observed lordosis recovery. T he 32% of  specimens with recovered external morphology  (28 out of 41 ) presented a completely normal  vertebral column (Fig. 1Bi), whereas  the 18% presented minor abnormalities of individual centra ( Fig. 1Bii) . The rest 50%  of the recovered fish  presented a  light bending of the vertebral column ( Fig. 1Biii).

 Concerning  severely lordotic  fish  that turned in to  fish  with  light  lordotic external morphology, 40% presented light vertebral bending , whereas 60 %  presented  either  minor abnormalities of individual centra , severe lordosis ,  or  a counterbalancing kyphosis  anteriorly to  lordosis (Fig. 1 A, 1Bii-Bv).  Finally, all fish with severe lordotic external morphology, presented a severe vertebral bending  (Fig. 1Bv).

 In the present study we showed that the external morphology of lordotic sea bass juveniles  may  completely recover during the on-growing period .  The recorded recovery rate of lordosis in sea bass (57%) is higher than that previously reported for seabream ( 44%,  Fragkoulis et al. 2019) . Fragkoulis et al . (2019), assumed that the rearing in the sea cages, might be responsible for the recovery, due to the adaptability of the vertebrae in the less intense water current velocities, compared to conditions in the  tanks. Our results do not contradict this hypothesis, since the recovery was observed during the on-growing period. Similarly ,  our radiographic findings on the anatomy of the lordosis recovery are in total agreement with Fragkoulis et al. (2019), with the same phenotypes observed in both studies. Our results show that lordosis recovery is not species-specific and has to be taken into consideration during the quality control at the end of the hatchery phase. Further morphometric analysis  in the future, could provide useful indices for the accurate discrimination between abnormal fish, on the basis of their recovery potential.

Acknowledgments

This study was financially supported by EU (European Maritime and Fisheries Fund (EMFF) and national (Greek) funds (NSRF 2014-2020, call Novelty in Aquaculture, Project No. 5010952) of the Ministry of Rural Development and Food, Greece.

References

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Fragkoulis S, Printzi A, Geladakis G, Katrimpouzas N, Koumoundouros G (2019) Recovery of haemal lordosis in Gilthead seabream ( Sparus aurata  L.). Sci. Rep. 9, 9832.

Fragkoulis S, Koumoundouros G (2021) Simple morphometrics for predicting lordosis-induced deviations of body shape in reared Gilthead seabream (Sparus aurata L.). J Fish Dis. 2021; 00:1–3.

 Sfakianakis DG, Georgakopoulou E, Papadakis I, Divanach P, Kentouri M, Koumoundouros G (2006) Environmental determinants of hemal lordosis in European sea bass, Dicentrarchus labrax (Linnaeus, 1758). Aquaculture 254:54-64.

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