## 1 Introduction  
In many coastal areas across the world, aquaculture represents the most rapidly expanding food sector. As per the Scientific, Technical and Economic Committee for Fisheries, aquaculture generated a turnover of EUR 4.1 billion from 1.2 million tons of produce in Europe during 2018 [1]. Croatian production volume increased from 13.92 tons (2012) to 24.05 tons in 2020 (73% increase) and from EUR 78 million to EUR 181 million in 2023 (142% increase). Fisheries play a significant role in the Republic of Croatia's agricultural product exports, contributing approximately 7% of the total export value. The Croatian fisheries sector faces several key challenges, including the need to enhance safety and working conditions, as well as improve shellfish quality and enhance catch monitoring [2]. In Croatia, the Mediterranean mussel (_Mytilus galloprovincialis_) and the European flat oyster (_Ostrea edulis_) are the predominant shellfish species, holding great importance for the country's shellfish industry [3; 4]. Scallops play a significant role in supplying the shellfish market and support both capture fisheries and aquaculture efforts [5]. The Mediterranean scallop _Pecten jacobaeus_ (L.), also known as \"capesanta\" in the local region, constitutes a considerable share of the market; however, overfishing has become a concern in the northern Adriatic Sea [6]. In contrast, the queen scallop _Aequipecten operularis_ (L.), locally referred to as \"canestrelle,\" is typically caught as bycatch during commercial fishing. For this reason, _A. opercularis_ represents a potentially interesting species for the development of shellfish aquaculture and for filling the market demand, by replacing the vulnerable and overfished species with alternative ones in the Mediterranean [5]. Since shellfish and other organisms that filter seawater are in direct contact with the marine environment, they are vulnerable to diseases [7; 8]. High stock density, improper feeding, seawater pollution, and pesticides containing agricultural drainage water affect the risk of bacterial, fungal, and viral diseases in marine shellfish [9]. In this century, the shellfish aquaculture industry has experienced significant and rapid development. Newly synthesized chemical compounds and antibiotics have had a negative impact on the environment, subsequently producing mutagenic microbial strains that adversely affect shellfish health, thus antibiotics are now discouraged for the prevention of disease outbreaks [10]. Conversely, probiotics, which refer to live microorganisms that can impart health benefits to the host when administered in the appropriate dosage, have found extensive application in aquaculture [11]. The gastrointestinal tract represents the primary site for probiotic isolation and action. The advancements in targeted delivery systems have facilitated the precise localization of probiotics to their intended destinations, enabling them to exert favorable effects [12]. Multiple studies have been carried out on cultured animals, substantiating the influence of diverse probiotics on the abundance of beneficial bacteria and the reduction in pathogenic burden [13; 14]. Employing probiotics has been demonstrated to be an effective approach in enhancing shellfish health and productivity [15; 16]. Various genera of bacteria are used as probiotics in aquaculture, including _Aeromonas_, _Atheromonas_, _Arthrobacter_, _Bifidobacterium_, _Bacillus_, _Clostridium_, _Enterococcus_, _Lactobacillus_, _Paenibacillus_, _Phaeobacter_, _Pseudoalteromonas_, _Pseudomonas_, _Rhodosporidium_, _Roseobacter_, _Streptomyces_, and _Vibrio_[14]. Lactic acid bacteria (LAB) have become increasingly used as probiotics in aquaculture over the past decade [17; 18; 19]. Most LAB are generally recognized as safe (GRAS) by the US Food and Drug Administration (FDA), i.e., they are listed as having a Qualified Presumption of Safety (QPS) by the European Food Safety Authority (EFSA).