CONSEIL INTERNATIONAL POUR L’EXPLORATION DE LA MER Fiches d’identification PLEURONECTIDAE des oeufs et lames de (BY J. H. NICHOLS) poissons 1971 Fiches Nos 4-6 https://doi.org/10.17895/ices.pub.5170 2 Single-oil .globule s- A F Many oil globules B m Terminal mouth G Notocord Tentocle Morgtnol fins kc H \ \Ypigrnent bands Pectoral fins with pigment J YLoterol spines D Precaudal or abdominal vertibrne Preanal are0 I Postonal area E K length 3 Introduction This sheet deals only with the Pleuronectidae occurring within the areas listed in Table 1. Initial separation of flatfish larvae into the three families of Pleuronectidae, Bothidae and Soleidae must be made with the aid of the first larval key, before the specific Pleuronectidae key can be used. There are eight species occurring in the area covered by this sheet, and they can with some experience be easily separated. Some are unlikely to occur in the same area at the same time; therefore reference should be made to their distribution and spawning times (Tables 1 and 2). In addition to the larval key, each species is discussed separately and points of likely confusion between species are clarified. Descriptions of the eggs of the eight species are included separately, but for identification reference should be made to a complete egg key (EHRENBAUM1911; HOEK1911), or a size chart (SIMPSON1956). SIMPSON'Schart is summarized in Table 3. Material and methods Whenever possible my own specimens have been used, either from vertical plankton net samples from the wild, or from hatchery-reared stock. These specimens have all been fixed and preserved in 2 o/o hexamine buffered formalin, which may have caused up to 10 o/o shrinkage in some of them. All the figures have been drawn direct from a stereomicroscope with the Zeiss camera lucida attachment. For specimens not available in'iny own collection I have relied upon the best drawings from the many descriptions of these larvae in earlier works. These illustrations have all been redrawn, using the same technique as for my own specimens. Characters used in identifying pleuronectid larvae The black pigment is usually the only colouring remaining after fixation in formalin and even this disappears slowly after long preservation. The distribution of black pigment and especially the patterns formed on the postanal portion of the body are important diagnostic characters, particularly in the early larval stages. The pigment is most easily viewed with incident light, this often being the only method for viewing the delicate black spots, such as those which occur on the pectoral fins of Limnda limanda. The number of fin rays is in many cases specific and they can be counted after the onset of asymmetry, preferably using transmitted light. Vertebral counts should only be made as a last resort, since it is usually necessary to stain with Alizarin red (GURR1963) or at least use a clearing technique before the vertebrae of fixed specimens can be seen. The key and figures have all been compiled from specimens in excellent condition. However, with the increasing use of high-speed plankton samplers, the possibility of extensive damage, e.g. loss of dermal pigment, torn marginal fins and the destruction of the yolk-sac and gut, must be borne in mind. The possibility of reversed specimens of Pleuronectidae, though very small, should be considered when separating metamorphosing specimens of the three families. Reversed specimens of any of the species may occur, but are common only in PlaticthysJesus. A guide to the nomenclature used in this sheet will be found in Figures A-K. Identification A. Eggs Order Heterosomata - Family Pleuronectidae The eggs are planktonic, round, containing no oil globules. The yolk is homogeneously distributed. 1. Large egg, diameter 1.38-3.50 mm, with a large perivitelline space; spawning time 11-VI ............ ..... Hippoglossoides platessoides 2. Large or very large egg with a small perivitelline space. 2.1. Diameter of egg 3.00-4.25 mm, perhaps even larger, imperfectly known; spawning time 111-VIII. .......... Hippoglossus hippoglossus 2.2. Diameter of egg 4.00-4.50 mm, imperfectly known; spawning time 111-VI ............................. Reinhardtiur hippoglossoides 2.3. Diameter of egg 1.66-2.20 mm; spawning time XII-VI ................................................... Pleuroncctes platessa 2.4. Diameter of egg 1.13-1.45 mm; spawning time IV-X, but in the Channel 11-111. Black pigment develops on the yolk. Microstomus kiff 3. Medium or small egg with a small perivitelline space. 3.1. Diameter of egg 1.07-1.25 mm; spawning time 111-VIII .............................................. Glyptocephalrcs cynoglossus 3.2. Diameter of egg 0.82-1.13 mm; spawning time I-VII. ........................................................ PlaticthysJIesus 3.3. Diameter of egg 0.66-0.98 mm; spawning time I-VIII ...................................................... Limanda limanda B. Larvae Order Heterosomata The anus is almost at right angles to the larval axis and extends to the edge of the marginal fin. The body is laterally flattened. The pelvic fins are small and barely evident during the early larval stages; the dorsal and anal fins are single and continuous. There is asymmetry of the head at metamorphosis, with the subsequent adaption for lying on the right or left side. Three families are present in this order and they can be separated by the following characteristics. 4 1. Earb larval stages before the development of the hypural rudiment. Single oil globule in the yolk-sac, persisting through the early larval stages (Figure A). ........................... Bothidae Many oil globules in the yolk-sac; yolk segmented; vesicular expansion of the embryonic dorsal fin above the mid-brain; mid-brain protruding (Figure B) .................................................................... Soleidae No oil globule in the yolk-sac .................................................................. Pleuronectidae 2. Mid-larval stages prior to the onset of asymmetry A tentacle present on the head (Figure C) .............................................................. Bothidae Lateral (otocystic) spines on the head (Figure D) ......................................... ... ............. Bothidae Preanal area of larva about one half the total area, mid-brain not protruding .................................. Bothidae Preanal area of larva about one half the total area, mid-brain protruding (Figure E) ............................ Soleidae Preanal area of larva much less than half the total area and closer to one third .... Pleuronectidae 3. Metamorphosing larvae Right eye migrating; mouth terminal ................................................................ Bothidae Head distinctly rounded with a lateral mouth. Size at metamorphosis small, less than 9 mm; left eye migrating (Figure F) Soleidae Mouth terminal. Size at metamorphosis greater than 9 mm; left eye migrating (Figure G) ..................... Pleuronectidae Family Pleuronectidae Key to the pelagic stages prior to the onset of asymmetry. Pigment in the form of three to six bands or patches in the marginal fins and/or on the body ............. 1 Pigment not in the form of bands or patches. ....................................................... 2 1. Distance from the tip of the jaw to the anus less than one third of the total length of the larvae. .................... 1.1 Distance from the tip of the jaw to the anus greater than one third the total length of the larvae ....................... 1.2 1.1. Six postanal bands or patches of pigment on the body, three of which are distinct and present in the early larval stages in the marginal fins (Figure 5a-g) .......................................................... GlyPtocephalus cynoglossus 1.2. Less than six postanal bands or patches of pigment on the body. 1.2.1. Three or four indistinct bands of pigment on the postanal portion of the body. The marginal fin pigment correspond- ing to these bands is indistinct and variable (Figure 4a-b) ................................ Hippoglossoides platessoides 1.2.2. Four distinct postanal bands of pigment on the body with distinct patches corresponding to these bands at the edges of the marginal fins, giving the larva a distinctly striated appearance. Patches of pigment on the body, gut and dorsal marginal fin directly above the anus. The marginal fins are broad (Figure 6a-d). ............. Microstomus kitt 2. Larva large, greater than 7 mm at hatching and reaching at least 30 mm before metamorphosis; very long straight-lined lower jaw ..................................................................................... 2.1 Larva small, less than 7 mm on hatching, metamorphosing at 10-15 mm ......................................... 2.2 2.1. 2.1.1. Larva exceedingly nmrow and elongated; body and marginal fin pigment very light (Figure 8a-c) . Rekhardtius hippoglossoides 2.1.2. Larva broad with a stout head and a strong depression in front of the eyes; “pug nose” (Figure 7a-c) Hippoglossus hippoglossus 2.2. Double row of pigment spots along the whole length of the ventral postanal portion of the body, i.e. a single row on either side at the base of the anal fin. ................................................................ 2.2.1 This double row of pigment spots absent .............................................................. 2.2.2 2.2.1. 2.2.1.1. Pigment present in the form of distinct black pigment spots on the edges of the pectoral fins. Larva small at hatching, about 3 mm (Figure 2a-f)