Quick identification guide for siphonophores and hydromedusae (Cnidaria: Hydrozoa) reported as harmful for finfish aquaculture in the Northeast Atlantic.

Luis Martell1, Joan J Soto-Angel1, Erlend Hallaren1, Sanna Majaneva2, Aino Hosia1 QUICK IDENTIFICATION GUIDE SIPHONOPHORES AND HYDROMEDUSAE (CNIDARIA: HYDROZOA) REPORTED AS HARMFUL FOR FINFISH AQUACULTURE IN THE NORTHEAST ATLANTIC 1University Museum of Bergen, Department of Natural History, University of Bergen, Norway 2Akvaplan-niva, Norway Contact: [email protected] Body with typical jellyfish shape (hydromedusae) Table of contents IMPORTANT: This guide covers all pelagic hydrozoans reported as harmful to aquaculture in Northeast Atlantic waters. However, as >100 other non-harmful species occur in the region, definitive identification of specimens may require expert advice and broader ID resources. 2 Lizzia blondina page 3 Obelia spp. page 6 Phialella quadrata page 5 Solmaris sp. page 7 Velella velella page 8 Apolemia sp. page 10 Physalia physalis page 9 Muggiaea atlantica page 12 Dipleurosoma typicum page 4 Body with shape different from typical jellyfish (siphonophores and floating hydroid colonies) Lizzia blondina Similar species How does it look? four club-shaped tentacles above mouth often producing smaller jellyfish on the region around the mouth eight tentacle bulbs on body margin, four of which may carry more than one tentacle 2 mm 3 Rathkea octopunctata is also small and has jellyfish buds, but differs from L. blondina in the structure of the mouth The mouth of R. octopunctata has branched lips instead of the unbranched club-shaped tentacles of L. blondina Where in NE Atlantic waters? Mediterranean Sea to mid-Norway When? summer to autumn Where does it live? common and sometimes abundant in coastal waters Life cycle: jellyfish produced by polyps living on the bottom Where does it live? coastal waters, although little information is available for this species Life cycle: jellyfish produced by polyps living on the bottom Similar species How does it look? body with 5 to18 irregularly distributed canals issuing from the centre, often carrying beanor sausage-shaped gonads black dots (ocelli) at the base of tentacles 5 mm 4 Laodicea also have black dots at the base of tentacles, but unlike D. typicum only have 4 radial canals Halopsis ocellata is larger (up to 150 mm) and does not have ocelli. It has long, wavy gonads along the regularly distributed radial canals Dipleurosoma typicum gonads on the inner half of the radial canals, close to centre Where does it live? coastal waters, although little information is available for this species Life cycle: jellyfish produced by polyps living on the bottom Where in NE Atlantic waters? British Isles, Faroe-Shetland Channel, Norway When? spring to autumn, most common June to September Similar species How does it look? canals forming a cross. On these canals, 4 short gonads with a groove in the middle. 16 to 32 tentacles of the same type reliable identification of young P. quadrata from Clytia species is only possible through DNA analysis 6 mm 5 Several Clytia species look very similar, so accurate identification should be confirmed by an expert The number of statocysts (small, bubble-like organs on the body edge) differs between Clytia species (many) and P. quadrata (eight) Phialella quadrata Where in NE Atlantic waters? widespread in coastal Atlantic waters When? recorded all year, but more abundant from April to September Where does it live? coastal species typically found in nearshore environments Life cycle: jellyfish produced by polyps living on the bottom Similar species How does it look? flat body with numerous short, tightly packed tentacles of uniform length round gonads along 4 straight canals forming a cross 2 mm 6 Young Clytia jellyfish may look similar, but they are not flat and have fewer and different-looking tentacles Solmaris sp. has a vaguely similar shape and size but have no visible structures on the body Obelia spp. the jellyfish of the different species of Obelia cannot be distinguished from one another Where in NE Atlantic waters? widely distributed in all NE Atlantic When? all year, with peaks in spring through autumn. Where does it live? very common in coastal waters, occasionally offshore Life cycle: meroplanktonic, produced by polyps living on the bottom Similar species How does it look? body simple: flat and without visible canals or gonads several statocysts (tiny, orange bubble-like organs) on the body margin 3 mm 7 Obelia jellyfish are also flat and small, but they have canals forming a cross across their body The presence of a distinct mouth and gonads also distinguishes Obelia from Solmaris Solmaris sp. Where in NE Atlantic waters? UK, Faroe Islands, midto northern Norway When? insufficient data, coastally more common from July to September Where does it live? primarily in oceanic surface waters; occasionally coastal Life cycle: always in water column How does it look? floating colonies with round shape and characteristic upright sail intense deep-blue colour and floating on the surface make it easy to detect 7 cm 8 Similar species Portuguese man o' war (Physalia physalis)are larger and have a gas-filled float and longer tentacles Blue button (Porpita porpita), found in the Mediterranean but not in temperate NE Atlantic waters, lacks a sail Velella velella By-the-wind sailor Where in NE Atlantic waters? west UK waters, very rarely up to 62° N in Norway When? offshore year-round; coastal sightings peak in spring Where does it live? floating on the surface of the sea, often blown ashore by winds Life cycle: colony living at air-water interface, floating on surface How does it look? colonies with a large, gas-filled float that stays above the water surface massive aggregations often follow strong onshore winds or shifting currents 15 cm 9 Similar species Velella velella has smaller colonies with a sail instead of a float Physalia physalis is unmistakable in the NE Atlantic, but other, related species occur elsewhere in the world Physalia physalis Portuguese man o’ war long tentacles with stinging cells hanging under the float Where in NE Atlantic waters? tropical to temperate waters, but not in Norway When? offshore year-round; coastal sightings in spring and early summer Where does it live? floating on the surface of the sea, often blown ashore by winds Life cycle: colony living at air-water interface, floating on surface