Flatfish Made Easy ....and other Myths

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.

version 1.0; 2018

The Lineup

Right-HandersRock 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)

SouthpawsSpeckled sanddab (Citharichthys stigmaeus)Pacific sanddab (Citharichthys sordidus)California tonguefish (Symphurus atricaudus)

Switch Hitters Starry flounder (Platichthys stellatus)California halibut (Paralichthys californicus)

Slendersole(Lyopsetta exilis)

C-Osole(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. As the common name im-plies they are typically on sand bottoms, although the specimen at right was found looking very out of place on a rock bottom.

Flathead sole (Hippoglossoides elassodon)

Flathead sole have very large mouths, a sharp-ly angled jaw, and a pointed chin. The lateral line is quite straight and much more obvious underwater than it is in most other flatfish.

Flatheads live on open sand/mud bottoms. They can reach a length of 56 cm (22 in) but are so thin that they are of little commercial interest.

Pacific halibut (Hippoglossus stenolepis)

A large, somewhat streamlined, and very thick-bodied flatfish with a slightly forked tail. The eyes are relatively small and widely-spaced and the skin very smooth, due to it having tiny scales. The mottled dorsal coloration shown is typi-cal, while the underside is pure white. Maximum size 2.67 m (9 ft) and to at least 227 kg (500 lbs). The other flatfish in our area with slightly forked tails are arrowtooth flounder (not included) and California hali-but. Arrowtooths have a very large mouth that extends well past the eye with obvious, large teeth. California halibut have even smaller and more widely-spaced eyes and typically have a pattern of large “eyespots” on their upper side; the mouth also extends beyond the eye whereas it only reaches to mid-eye in Pacific halibut.

Full disclosure: while I took all the other photos in this book in situ, this one is from a dive in the dome at the Se-attle Aquarium. The only time I have encountered one underwater in the wild was in the Bering Sea, before I took up underwater photography. They are rarely shallow enough to be seen by divers in the Pacific Northwest, but I recall a large one taken by a spearfisherman many years ago at Sekiu, Washington.

Speckled sanddab (Citharichthys stigmaeus)

A small left-handed flatfish, only reaching 17 cm (6.8 in) in length and usually much smaller. The head is small and the mouth strongly curved. They can change color to match the bottom, and often blend in superbly.

This species will often follow divers about, search-ing for uncovered prey where the sand is disturbed by the diver’s fins. This is similar to their habit of following large rays in California to feed on the small invertebrates exposed as the rays excavate sed-iment in search of clams.

Speckled sanddabs are often confused with Pacific sandabs. Pacific sanddabs have a proportionately larger head and eyes, and grow much larger.

Pacific sanddab (Citharichthys sordidus)

Pacific sanddabs are a fairly thick-bodied, left-handed flatfish, reaching up to 41 cm (16 in) in length. Color is typi-cally a mottled brown. The head and eyes are proportionately much larger than those of the speckled sanddab, and the snout more rounded and projecting.

Although they are not of much commercial importance, Pacific sanddabs are particularly tasty and much sought after in some areas.

California tonguefish (Symphurus atricaudus)

These strange, left-handed flatfish are very bluntly rounded anteriorly with a tiny mouth and eyes, and have no discernable tail at the other end. They blend in well with the sand bottom, but often have dark bars at night that make them more obvious in a diver’s light. The maximum length is 21 cm (8 in).

This species only occasionally makes it up into the Pacific Northwest, during El Nino years.

Starry flounder (Platichthys stellatus)

Starry flounder are probably the most notable exception to the “don’t rely on color” flatfish rule. The black and yellow banded dorsal and anal fins are an unmistakeable character. Also note the distinct texture of the skin, with large, rough scales. This is one of our larger flatfish, reaching 91 cm (36 in) in length. Rather skittish and high strung, it is one of the more difficult flatfish to photograph, especially in the daytime.

Although they are classified with the right-handed flatfish, starries can go either way. They often occur in estuaries and readily enter fresh water, having been found as much as 121 km (75 miles) up the Columbia River. This species sometimes hybridizes with English sole.

The commercial fishermen I encountered in Alaska disliked this fish, complaining that it was difficult to filet and that its tough skin quickly dulled knives. I find them very tasty and definitely worth the trouble.

California halibut (Paralichthys californicus)

This is another flatfish that regularly switch hits, with about a 60/40 split of left/right eyed individuals. The eyes are small and widely spaced. The mouth is quite large and toothy, extending well past the lower eye; in Pacific halibut it only reaches the middle of the eye. The tail is slightly forked.

As you can see here they are masters of camouflage, and usually have some eyespot-like markings. This is our second-largest flatfish, reaching 1.52 m (5 ft) in length and highly sought after by both commercial and sport fishermen.

California halibut occasionally appear as far north as the outer coast of Washington State.