Non-indigenous cephalopods in the Mediterranean Sea

Abstract

The present review critically assesses the records of cephalopods that have entered the Mediterranean Sea in the last few decades. It includes 13 species, namely Sepia dollfusi, Stoloteuthis leucoptera, Sepioteuthis lessoniana, Architeuthis dux, Cranchia scabra, Taonius pavo, Megalocranchia sp., Teuthowenia megalops, Cycloteuthis sirventi, Taningia danae, Octopus cyanea, Amphioctopus sp. and Tremoctopus gracilis. The presence of Sepia pharaonis needs to be confirmed, whereas that of Sepia gibba and Spirula spirula is excluded. The arrivals from the Atlantic Ocean through the Strait of Gibraltar are related to the entrance surface current, which either carried passively planktonic paralarvae or favoured in some other way the entrance of subadult and adult stray specimens. As a matter of fact, all Atlantic cephalopods are pelagic oegopsid squids, with the exception of the nekto-benthic sepiolid S. leucoptera; all of them have been found only in the western Mediterranean basin. None of them seemingly established a stable population there, apart from the latter species. On the contrary, the cephalopods entering the Mediterranean from the Red Sea through the Suez Canal (Lessepsian migrants) lead a benthic mode of life. At least two of them, namely S. lessoniana and Amphioctopus sp., set up stable populations in the eastern basin. Lastly the occurrences of the pelagic octopod T. gracilis are ascribed, in the literature, to human-mediated transfer.

First Integrative Morphological and Genetic Characterization of Tremoctopus violaceussensu stricto in the Mediterranean Sea

“…This study once again confirms that errors/misidentifications are common in public repositories [ 7 , 53 ], probably because there are not many and easily applicable taxonomic characters used in the field to distinguish them, especially separating T. violaceus and T. gracilis . These discrepancies between the morphological and molecular identification, suggest adopting a more precautionary approach when reassigning old records to a different species based solely on the colour patterns deducted from drawings/photos [ 8 , 9 , 10 ] or extending the geographical range of species (e.g., T. gracilis within the Mediterranean Sea), based solely on sporadic observations, described in conference papers almost impossible to find for consultation [ 19 ].…”

“…The difficulty in separating these taxa, as well as the paucity of molecular data, could cause several errors in identifications, especially involving T. violaceus and T. gracilis , as exemplified by the records of these two species outside their known geographical limits [ 7 ]. For instance, in the Mediterranean Sea, in the past, T. violaceus was the only member of the genus known to live in the area, while nowadays it is reported that two species of the genus Tremoctopus occur in the Mediterranean Sea, i.e., T. violaceus as the native species and T. gracilis as an alien species probably arriving in the Mediterranean due to human-mediated transfer or with Lessepsian migrations [ 8 , 10 , 11 ].…”

“…Most of the Mediterranean records of Tremoctopus sp., often classified as T. violaceus , refer to individuals stranded [ 12 , 13 , 14 , 15 ] or remain found in stomach contents of large pelagic predators such as swordfish Xiphias gladius Linnaeus, 1758 [ 16 ]. However, the occurrence of T. gracilis was reported in the Mediterranean Sea as early as in 1936 in the northern Adriatic Sea [ 17 ]; the individual, originally identified as T. violaceus , was later reclassified as T. gracilis based on the color pattern [ 10 ]. Recently, in August 2002, a large egg-carrying female of Tremoctopus sp.…”

“…In recent years, T. gracilis has been further recorded in Tunisian waters [ 19 , 20 ]. It cannot be excluded that other T. gracilis are misidentified with T. violaceus in the past, since T. violaceus is the only member of the genus known to live in the Mediterranean [ 10 ]. All these Mediterranean records of T. violaceus and T. gracilis refer to female specimens identified only by morphological characters that have been reported to be not always solid enough to avoid misidentifications [ 7 ].…”

Cephalopods, a gap in the European Marine Strategy Framework Directive and their future integration

“…Besides negative effects such as the reduction in native biodiversity and ecosystem shifts, there might also be benefits from NIS, such as the replacement of lost ecological functions and the strengthening of biogenic complexity (Kleitou et al 2021). A recent study describes 13 non-indigenous cephalopod species for the Mediterranean Sea, which entered the region over the last few decades, most likely by the following three different routes: the natural pathway Strait of Gibraltar, the human-constructed pathway Suez Canal or through other anthropogenic pathways like shipping (Bello et al 2020). Consequently, cephalopods should also be considered in the NIS assessment within the MSFD (D2).…”