Flatfish Made Easy ....and other Myths version 1.0; 2018 Gregory C. Jensen All images ©Gregory C. Jensen In my ongoing effort to alleviate some of the misery of identifying marine life in the Pacific Northwest, I’ve put togeth- er this little guide to help divers identify the various flatfish they encounter. Many of the problems with identifying flatfish stem from the fact that the characters typically used to distinguish them are ones that are only practical when examining the fish out of water. Features like the shape and branching of the lateral line, how far a fin reaches when folded forward, etc. may work just fine on the deck of a boat, but are of little help underwater. The first thing to determine when trying to identify a flatfish is whether it is right-or left-eyed (also referred to as right-or left-handed). As you probably know, flatfish start out as normal-looking planktonic fish larva with an eye on each side of the head. As it gets ready to settle to the bottom, one eye migrates to the other side; the naming refers to the eye that didn’t migrate, so a right-eyed fish is one in which the left eye moved to the right side of the head. Other than the eye the basic design of the fish is like any ‘normal’ fish, with a dorsal fin on the top, pelvic and anal fins on the bottom, and an upturned mouth. The easiest way (for me at least) to quickly determine the ‘handedness’ of the fish is seeing which way it faces when viewed with the mouth upturned- if it points left, it’s left-handed or left-eyed. There are a few ‘ambidextrous’ species that can go either way. In the case of starry flounder, those in California are about equally divided in their handedness while those in Alaska are predominantly left-handed; they are almost exclu- sively lefties in the western Pacific. Other useful features are the relative size of the eyes and mouth, and whether it has an angular jawline or not. Color is often not particularly helpful as many species can change color or pattern to better match their surroundings, but is useful in a few cases. The Lineup Right-Handers Rock sole (Lepidopsetta bilineata) Slender sole (Lyopsetta exilis) Dover sole (Microstomus pacificus) English sole (Parophrys vetulus) Butter sole (Isopsetta isolepis) C-O sole (Pleuronichthys coenosus) Curlfin sole (Pleuronichthys decurrens) Diamond turbot (Pleuronichthys guttulatus) Sand sole (Psettichthys melanostictus) Flathead sole (Hippoglossoides elassodon) Pacific halibut (Hippoglossus stenolepis) Southpaws Speckled sanddab (Citharichthys stigmaeus) Pacific sanddab (Citharichthys sordidus) California tonguefish (Symphurus atricaudus) Switch Hitters Starry flounder (Platichthys stellatus) California halibut (Paralichthys californicus) Slender sole (Lyopsetta exilis) C-O sole (Pleuronichthys coenosus) Rock Sole (Lepidopsetta bilineata / L. polyxystra) Rock sole were fairly recently divided into two species, the southern rock sole (L. bilineata) and northern (L. polyxys- tra). Unfortunately the differences involve gill raker, pore, and scale counts that are impossible to perform underwater, so they are treated together here. They have large, bulbous eyes and a fairly small mouth with very thick lips. The scales are large and obvious. They sometimes rest in a position that I haven’t observed in other flatfish, with the body being elevated above the bottom due to the dorsal and anal fins being positioned vertically. While rock sole normally have a ‘popeye’ appearance, they sometimes flatten them down into their orbits and this can alter their appearance considerably. Our other soles with bulging eyes (e.g., C-O, curlfin) don’t have these large pits associated with their eyes. Maximum size is 69 cm (27 in). Slender sole (Lyopsetta exilis) Slender sole are typically encountered on soft bottoms during night dives. They have a prominent, angular jawline, large and obvious scales, and a large mouth. In addition to having a rather narrow body, these fish are very thin and often appear to be draped like a rag on the bottom. There is often a large unpigmented area in the middle of the body, as seen in the above photo. This species reaches about 40 cm (16 in) in length, but is so thin that it has never generated any interest from com- mercial fishermen. Closeup of the head showing the large mouth and the angular jaw. The reddish color is due to the gills showing through the very thin bones of the operculum. Dover sole (Microstomus pacificus) Dover sole are also known as ‘slime sole’ due to the copious amounts of mucus they exude when hauled up in a net, making them very difficult to pick up barehanded. Despite this difficulty in handling, they are a very important com- mercial species and filets and stuffed Dover sole grace the menus of many high-end seafood restaurants. Dovers have small heads with tiny mouths and large, bulbous eyes. They are usually brownish with some rings or round spots. The maximum size is 76 cm (30 in). In addition to making them uber-slippery, the slime may have additional defensive properties. I had a captive ju- venile for a couple of years, and it was a fascinating and very personable fish. One day when I was putting food in the tank a sculpin inadvertantly bit the Dover during the feeding frenzy; it immediately released it, shook its head, and seemed unable to close its mouth for some time after. The reaction was reminiscent of the reaction of predators to the Moses sole, a flatfish found in the Red Sea that exudes a potent shark repellent. English sole (Parophrys vetulus) This is a rather slender, streamlined flatfish with a no- ticeably pointed head. The left (i.e., migrating) eye ex- tends out beyond the margin of the head, hence one of the ‘out of water’ characters used for identification is checking to see if the eye is visible when looking at the fish from below. English sole have very small scales. Maximum size is about 61 cm (24 in). English sole once supported a substantial trawl fishery in Puget Sound, but increases in tumors and unsightly parasitic nematode worms have made them unmarketable. They sometimes hybridize with starry flounders. Butter sole (Isopsetta isolepis) Though common in trawl catches, butter sole seem to be rarely encountered (or at least recognized) by divers, perhaps because they are largely buried and tend to be in less frequented muddy areas. They have unusually small heads and typically have yellow spots and yellow trim along the edge of their fins. Butters get up to 55 cm (22 in), but are generally so small and thin that they have no commercial importance. C-O sole (Pleuronichthys coenosus) These very low-key flatfish are named for the “C-O” pat- tern on their tail, which sometimes takes a bit of imagi- nation to see. They also usually have a conspicuous dark spot in the middle of the body. Color varies widely; those in areas with coralline algae can even have patches of matching pink on them. Juveniles are often nearly solid white. The eyes are extremely large and bulbous and the mouth very small. Maximum size is 36 cm (14 in). The first rays of the dorsal fin start below the front of the left eye; compare to the similar curlfin sole where the rays start near the mouth. The caudal penduncle is exception- ally wide in adult C-O sole and the eyes are proportion- ately smaller than those of curlfins. Curlfin sole (Pleuronichthys decurrens) Very similar to the C-O sole, with extremely large eyes and a tiny mouth. The anteriormost part of the dorsal fin is held folded underneath the head and body, and it lacks the C-O pattern on the tail and the middorsal spot characteristic of that species. The color pattern tends to have a marbled appearance. Dorsal fin rays begin far under the head, by the mouth. Compare to the C-O sole where the first rays begin under the front of the left eye. Proportionately speaking, curlfins have even larger eyes than C-O soles and much narrower caudal pe- duncles. The maximum size of this species is about 36 cm (14 in). Diamond turbot (Pleuronichthys guttulatus) As the common name implies, this species has a broad, almost perfect diamond shape. The head and mouth are very small and the tail broadly rounded. They typically have a rather uniform pattern of blue spots on the upper side. The largest measure about 46 cm (18 in). Diamond turbot are found on mud and sand bottoms, often in coastal estuaries. They are not commercially fished but are taken by sport anglers. This species has not yet been recorded in the Pacific Northwest. Since its range extends into northern California (and considering the many recent sightings of other Californian species in Washington), it is included here in anticipation of its arrival. Sand sole (Psettichthys melanostic- tus) This rather distinctive flatfish has a large mouth and angular jaw, and the first rays of the dorsal fin are very long and largely without webbing. The feature that al- ways stands out for me are the unusually tiny eyes that are widely-spaced and pointing in different directions, giving it a Marty Feldman kind of look. The only other west coast flatfish with similar eyes are the California and Pacific halibuts, which both lack the angular jaw and the long, filamentous dorsal rays; they also have slightly forked tails while sand sole have rounded ones. Sand sole are fairly large, reaching 63 cm (25 in) in length and are one of the more desireable flatfish from a commercial standpoint.