Pelagia benovici Piraino, Aglieri, Scorrano & Boero 2014, sp. nov.

Pelagia benovici Piraino, Aglieri, Scorrano & Boero sp. nov.

Holotype: male specimen (adult), collected from Gulf of Venice (Chioggia), November 2013, 46 mm bell diameter. Deposited in the Collection of the Museum of Adriatic Zoology Giuseppe Olivi (Palazzo Grassi, Chioggia, University of Padova). Accession number: CN54 CH.

Paratype I: female specimen (adult), Gulf of Venice (Chioggia), November 2013, 50 mm bell diameter. Deposited in the Collection of the Museum of Adriatic Zoology Giuseppe Olivi (Palazzo Grassi, Chioggia, University of Padova). Accession number: CN55 CH. Other material: 10 specimens. Gulf of Venice (Chioggia), November 2013 32–45 mm (range of bell diameter). Deposited in the Collection of Marine Invertebrates at the Laboratory of Zoology and Marine Biology of the University of Salento (Lecce). Accession numbers: UNIS _SCY_001–10. Five specimens photographed in the field, but not collected, by Mr. Fabrizio Marcuzzo at Punta Sottile, Muggia (Trieste) on December 5 th, 2013

Description (based on holotype and paratype). Preserved medusa almost flat, with thin transparent mesogleal jelly. Live specimens hemispherical during active swimming strokes (Fig. 2A). Exumbrella yellow-ochre in colour, homogeneously covered by prominent cnidocyst warts of various shapes, from rounded to oval to pointed, with whitish refringent tip of wart. Sixteen marginal lappets, rectangular, with rounded corners (Figs. 2B–D, 3A). Eight adradial, hollow, white and transparent tentacles, up to three times the diameter of umbrella in length. Large tentacle bases laterally compressed to ovoid, with medio-peripheral main axis, distally tapering into cylindrical shape (Figs. 2A–D, 3A). Absence of longitudinal muscular foldings in tentacle mesoglea (Fig. 3B). Eight marginal sensory organs (Fig. 2D), lacking ocelli, each located in a shallow pit formed by ectodermal outgrowth of the umbrellar margin and by overlapping sides of marginal lappets (Fig. 4A). Well-developed coronal muscle on subumbrellar surface. Simple radial septa terminating between sense organs and tentacles, dividing the gastrovascular sinus into 8 tentacular and 8 rhopalial separate pouches, tentacular pouches slightly larger than rhopalial ones (Fig. 4B). Stomach without gastric septa, with bundles of gastric filaments arranged in interradial groups, originating at the transition between stomach and gastrovascular sinus (Fig. 4C). Four interradial, elongated milky white, ribbon-like gonochoric gonads (holotype: male; paratype: female), horse-shoe shaped, convex; ribbon protrudes out of subumbrellar surface at periphery of gastric pouches; each ribbon spans two tentacular and one intermediate rhopaliar pouch (Figs. 2D, 3A, 4B–D). Manubrium whitish, transparent, ≤1.5 times the diameter of umbrella in length, with very short oral tube and long delicate oral arms with frilled edges (Figs. 2A,B,D), covered by colourless cnidocyst warts (Fig. 4C). Perradial subumbrellar surfaces between gonads covered by brownish cnidocyst warts, smaller than exumbrellar warts (Fig. 4B, C), also scattered over the gonad foldings (Fig. 4D, 5A,B).

Cnidome (Fig. 5C–F). At least 3 cnidocyst types: holotrichous O-isorhizas (spherical, length 7–11 µm; width 7–11µm), microbasic euryteles (ovoid, length 9–11µm; width 4–5 µm), and a third larger type, provisionally identified by light microscopy as heterotrichous microbasic birhopaloid II type (ovoid, length 16–24 µm; width 14–17 µm).

Etymology. The species is named after the late Prof. Adam Benovic, who dedicated his life to the study of gelatinous plankton, especially in the Adriatic Sea.

Type locality. Gulf of Venice, Adriatic Sea, Mediterranean Sea.

Molecular analysis. Seven COI sequences from P. benovici specimens were identical to each other (mean within-species pairwise K2P distance = 0.001, S.E. = 0.001), and significantly dissimilar from the 14 P. noctiluca sequences from Mediterranean Sea and South Atlantic Ocean specimens (mean between-species pairwise K2P distance = 0.362, S.E. = 0.040) and from 8 P. cf. panopyra sequences (mean between-species pairwise K2P distance = 0.342, S.E. = 0.038). (Table 1). Unrooted NJ and ML trees had the same topology (data not shown) including consistently distinct P. noctiluca and P. benovici clades. The COI Bayesian tree (Fig. 6A) also shows consistent separation of P. benovici sp. nov. from any of its morphologically closest relatives, P. noctiluca and P. cf. panopyra.

Bayesian phylogenetic analysis of 28S (Fig. 6B) consistently showed the same topology as the NJ and ML analyses of 28S (not shown), including monophyly of the Pelagiidae, with all representatives of Pelagia, Chrysaora and Sanderia that were considered. However, this tree does not show reciprocal monophyly of Pelagia benovici sp. nov. and P. noctiluca; 28S sequences from medusae that were distinguished morphologically as different species occur together in two distinct clades.

* Sequences downloaded from GenBank; n/a: not available.

Discussion and systematic remarks. Within the order Semaestomeae, four families are widely recognized (see Kramp 1961; Russell 1970; Bayha & Dawson 2010)— Pelagiidae, Cyaneidae, Ulmariidae and Drymonematidae —and a fifth family, Phacellophoridae, has been proposed by Straehler-Pohl et al. (2011). The Pelagiidae are easily recognized by emergence of the tentacles at the umbrella margin, the absence of branched pouches of the gastrovascular sinus, and the absence of a ring canal.

Pelagiidae contains three genera (Gershwin & Collins 2002)— Chrysaora, Pelagia, and Sanderia —of which one, Chrysaora, was recently revised by Morandini and Marques (2010). Pelagiid jellyfish bearing eight marginal tentacles and eight rhopalia, as the new jellyfish described here, belong either to Chrysaora or to the monospecific Pelagia. These two genera are easily distinguished by a number of morphological characters. Namely, Pelagia have conspicuous exumbrellar cnidocyst warts, shallow rhopaliar pits, 16 marginal lappets, and radial septa terminating between rhopalia and tentacles. By contrast, Chrysaora jellyfish have radial septa terminating proximate to the base of the tentacle and a higher number of marginal lappets (32–48 [but see C. colorata below]); additional anatomical features are distinctive characters species within Chrysaora, such as the occurrence of quadralinga and a heavy manubrium in C. colorata and C. achlyos (Gershwin & Collins 2002). So far, C. colorata is the only known Chrysaora species with 8 marginal tentacles, but it is distinguished by its outer morphology (large size and star-shaped exumbrellar marks) from the three nominal Pelagia species, the well-known P. noctiluca (Forskål 1775), P. panopyra Péron & Lesueur 1809, and P. flaveola Eschscholtz 1829 (Gershwin and Collins 2002; Cornelius 2013). A striking difference also resides in life history, in the direct metamorphosis planula-ephyra in Pelagia, whereas Chrysaora species have a polypoid stage.

A thorough analysis of the older literature (Maas 1903; Mayer 1910; Krasinska 1914; Menon 1930; Kramp 1961; Russell 1964, 1970) revealed the morphological features therefore clearly indicate that the new species must be ascribed to the genus Pelagia, and the molecular analysis confirmed its distinctiveness from P. noctiluca, with the complication that phylogenetic analyses of 28S suggest incomplete lineage sorting of this more slowly evolving nuclear marker, amplification of paralogues, or hybridization between P. benovici and P. noctiluca. Each of these three processes could be responsible for the conflict between gene and species trees. So far, besides P. noctiluca, all other nominal species of this genus are considered doubtful (Kramp 1961; Gul & Morandini 2013; Cornelius 2013). Indeed, no nominal species of Pelagia can be referred to the presently described material (Tab. 2), and Pelagia benovici sp. nov. is described here as a new species by a combination of morphological features, notably:

• densely distributed and irregularly shaped (rounded to arrow-pointed) exumbrellar cnidocyst warts (Fig. 7);

• milky white (same colour both in female and male specimens) horse-shoe shaped, outwardly convex gonads (Fig. 7), protruding out of the subumbrellar surface;

• club-shaped cnidocyst warts on gonadal ribbons (Fig. 4D);

• white transparent colour of tentacles, manubrium and oral arms (Fig. 8), tentacles without longitudinal muscular folds (Fig. 3B).

The findings of sexually mature specimens at sites several hundreds of kilometers apart indicate the possibility of an established population spreading in the North Adriatic. Gershwin & Collins (2002) remarked that pelagiid systematics was oddly neglected, in spite of the conspicuous sizes, distinctive morphologies, and painful stings of these jellyfish. The same authors predicted that new pelagiids likely would be discovered, and P. benovici seems a point in case.

Native or introduced? The first alien jellyfish in the Mediterranean Sea, Cassiopea andromeda, was found near Cyprus at the beginning of the 20th century (Maas 1903). Since then, three non-native jellyfish species have been recorded in the Mediterranean Sea: Phyllorhiza punctata and Rhopilema nomadica (Galil 1990), and Marivagia stellata Galil and Gershwin 2010 (Galil et al. 2010). R. nomadica and M. stellata also were new to science.

When M. stellata was described as a new species from the Israeli Mediterranean coast, Galil et al. (2010) cautiously considered it as a non-native species and a probable Lessepsian immigrant, assuming that a native, coastal conspicuous jellyfish, markedly different from all known scyphozoans of the Mediterranean Sea, would hardly have escaped attention until the 21 st century. Recently, the finding of M. stellata off the coast of Kerala, India (Galil et al. 2013) corroborated this hypothesis and established this species as the fourth Erythrean alien jellyfish species introduced in the Mediterranean Sea through the Suez Canal. Similarly, given the number of marine biological stations in the Adriatic Sea, the long history of investigations on gelatinous zooplankton in the area, and the increasing attention on jellyfish blooms in recent years, it is highly unlikely that P. benovici remained unnoticed until 2013, when a large population of mature jellyfish suddenly appeared and then persisted in a restricted area. The North Adriatic Sea, particularly the Gulf of Venice, is a major hotspot for introduction of alien species by shipping- and aquaculture-mediated in Europe (Occhipinti et al. 2011; Galil 2012) and an increase in shipping related invasions has been noted recently (Galil 2009). Re-discovery of rare jellyfish (e.g. Rhizostoma luteum in the Gibraltar Strait or the native Drymonema dalmatinum in the Adriatic Sea) after almost a century-long absence (Bayha & Dawson 2010 for a review; Prieto et al. 2013) suggests some scyphozoans might remain undetected for a very long time and re-appear suddenly, perhaps linked to the presence of an asexually reproducing polyp stage in the life cycle (Boero et al. 2008). However, this is unlikely to be the case for P. benovici. The genus Pelagia is thought to lack a polyp stage, although this needs to be confirmed for P. benovici. By its currently restricted distribution in the Gulf of Venice (Fig. 1), its conspicuous bloom, and each medusa’s unignorable size, P. benovici seems most likely to be another alien species introduced by human activities into the Mediterranean Sea.

Most probably, P. benovici was transported as viable jellyfish in the ballast waters of ships coming from the native area of this species, where it remains still undetected. This is the third case of a new species discovered in the Mediterranean but native to different seas, after the Erythrean immigrants Alpheus migrans (Decapoda) Lewinson and Holthuis 1978, and the rhizostome jellyfish Marivagia stellata. The life cycle of P. benovici still remains unknown, but this new taxon looks like an invasive species with the potential to form large blooms. It may have potential to spread across the Adriatic and neighbouring seas, raising the need for research efforts on mechanisms driving bioinvasions and on the impact of outbreak-forming species, as Boero (2013) recently advocated.