## 1 Introduction  
Tumours of a neoplastic nature occur across vertebrates, and fish are no exception. The neoplasms in cold-blooded species were early recognised as fundamentally similar in structure and biologic behaviour to the matching tumours in warm-blooded animals [1]. The histological diagnostic criteria defined for benign/malignant neoplasms in mammals stand as guidelines. A lipoma is defined as a benign and slow-growing mesenchymal tumour consisting of well-differentiated mature white adipocytes, typically with minimal connective tissue stroma within [1, 2, 3]. In humans, this neoplasm is the most common soft tissue tumour, often subcutaneous in location, with an (under)estimated prevalence of 1% [2, 3]. This neoplasm is rare and has only been diagnosed in a few fish species, typically appearing as isolated cases. The tumour was either encapsulated [4, 5, 6, 7, 8] or nonencapsulated and with the fat cells even merging with the surrounding tissue [9, 10].  
As it has been described for humans [11], and therefore suggesting phylogenetic conservation, most of the diagnosed lipomas in marine and freshwater fish were localised as dermal or hypodermal masses with visible skin protrusions [4, 5, 6, 8, 12, 13, 14, 15, 16, 17], sometimes invading the underlying musculature [18, 19]. Diagnes have been grounded on the lesion's histological features. Contrary to major advances in diagnosing human lipomas, there is not one established histochemical, molecular or cytogenetic biomarker for any fish lipoma. In addition, there is no published data on the prevalence of lipomas in fish populations.  
Despite the fact that lipomas rarely appear in other locations in fish, they were detected in the body cavity of yellow-eye mullet (_Aldrichetta forsteri_) [20], the mesentery of the Senegal seabream (_Diplodus bellotti_) [21], the musculature around the vertebral column of the southern bluefin tuna (_Thunnus maccopyuin_) [20] and northern bluefin tuna (_Thunnus thynnus_) [7], the stomach of the Indian oil sardine (_Sardinella longiceps_) [22] and the spleen of the largemouth bass (_Microperus salmonics_) [23] and perch (_Perca fluviatilis_) [24]. Regarding liver lipoma, as far as we know, it has been described once, and with very limited imagery, in the marine coly (also known as the coalfish, saithe and Atlantic pollock) (_Pollachius virens_) almost a century ago [25].  
When diagnosing a liver lipoma in fish, it is relevant to have in mind that there are species that naturally have non-hepatic cells inside the organ. Many fish have exocrine pancreatic acini within the liver (around blood vessels spreading into the organ from the hilum) [26]. Still, adipocytes are not seen intrahepatically, from the embryo (adipogenesis) to the adult, even though visceral adipose tissue is often found extrahepatically, at the hilum [26; 27; 28; 29]. Therefore, accumulations of adipocytes inside the liver should be regarded a priori as abnormal features.  
This study describes and presumptively diagnoses a microscopic liver lipoma in the black barbel (_Barbus hulcanicus_) while creating the first 3D reconstruction of such nodule from serial histological sections. This finding seems to be the first report in this fish species and is discussed in the light of the literature, drawing attention to a rare and overlooked lesion.