Gunnel, Gunnel, Gone!

By The Marine Detective on April 19, 2021

Meet the Penpoint Gunnel, another fabulous fish face and master of camouflage.

The colour of Penpoint Gunnels varies as much as the colour of seaweed . . . from olive green, to golden brown to red. In fact, the colour of Penpoint Gunnels is generally such a perfect match to their seaweed habitat, that they sometimes seem to disappear into it. Presto – gone!

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There’s a great paper from 1966 by Don Wilkie on the colour of Penpoint Gunnels. An interpretation of the paper by FISHBIO includes: “The coloration of adult penpoint gunnels typically matches the dominant algal community of their habitat. Green individuals are found in the upper intertidal zone where green algae (and eelgrass) is most common, brown specimens most frequently occur in the mid-to-lower intertidal zone where brown algae mainly occurs, and in deeper water where red algae become increasingly prevalent, penpoint gunnels tend to be red as well.”

Here’s a mystery. Penpoint Gunnels CAN’T change their colour to match their surroundings. So how can they have the variety of colour and be so well-matched to their habitat, as is evident in my photos below? Read on!

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How can they be such a match to the algae when they cannot change their colour? Is colour determined through the genetics of their parents? Research suggests not! Is it determined by their diet when they are adults? Also no!

It appears that the colour is determined by (1) the vegetation upon which the transparent / colourless larvae settle and/or (2) by which amphipods the larvae eat.

From Wilkie: “Field and laboratory studies were undertaken to examine the ecological role of colouration in the penpoint gunnel Apodichthys flavidus . . . A. flavidus was found to prefer cover under rocks to that within vegetation, but when provided with vegetation alone chose that which it matched. The colour phases observed in A. flavidus were found to be determined directly by the pigments they contained not by differences in stages of chromatophore expansion . . . . Colour change experiments showed that A. flavidus cannot undergo complete changes of colour phase in response to environment alone. Diet has an influence on colour, but complete colour changes were not produced experimentally.

Larvae were reared from the eggs of green and brown individuals. All developed colouration more similar to that of the Artemia [brine shrimp] upon which they were fed than to their parental type . . . It is suggested that the colouration of A. flavidus has a cryptic function which is of importance primarily during food seeking. It is hypothesized that the vegetation upon which A. flavidus larvae settle in conjunction with early diet primarily determines the colouration of individuals.“

So, while Penpoint Gunnels cannot change colour, they appear to be able to recognize and select the vegetation for which their colour is a good match. What this also suggests is that the depth where an individual started of his/her life as a larva, will be the depth where they would/should live out their life.

As described above, because of the limits of how deep wavelengths of light can travel, there are zones of seaweeds / algae. Green seaweeds are in the shallows, then there are brown seaweeds, and then red seaweeds are the deepest (their pigment can best absorb the blue-green light that can penetrate to greater depths). If a Penpoint Gunnel started off as a larvae feeding on amphipods that are well-matched to green seaweed, the research supports this is what would make them green coloured. If that individual moved deeper into the brown or red zone, they would not have the appropriately coloured seaweed to match their colour.

ID Challenges

It can be wonderfully difficult to discern Penpoint Gunnels from the other gunnel species off the coast of British Columbia (6 species total).

If you get a really good look at the back end of a Penpoint Gunnel, that really helps in IDIng the species. The “penpoint” refers to the first spine of the anal fin. It’s large and grooved like a fountain pen point. Yes, I know that most often that ID tip is not really going to help with a live individual. 🙂

I find it the most difficult to discern Penpoint Gunnels from Crescent Gunnels (Pholis laeta to 25 cm) and Saddleback Gunnels (Pholis ornata and 30 cm). Those species also have a wide variety in colour and have the black bar by their eye and, Penpoint Gunnels also can have markings along their backs. I don’t believe it is known how their colour of Crescent and Saddleback Gunnels is determined.

Then there are also Rockweed Gunnels, Longfin Gunnels, and Red Gunnels off the coast of British Columbia. Oh, and there are other elongate fish found in similar habitats, like species of prickleback and cockscomb!

The next six photos are included to maybe help with IDing gunnels. They are all NOT Penpoint Gunnels.

Then, at the end of the blog, there’s a fun fishing finding venture for you.

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Find the Fish!

Many of you may know that every Friday I do a “Find the Fish Friday’ challenge and have two children’s books by the same name. These are the “Where’s Waldo” of the fish world with the intent being that, when people search for the fish in my images, they are also absorbing what the life looks like in the dark, rich Northeast Pacific Ocean.

Below are three such challenges where there is one fish to be found in each photo and it is a Penpoint Gunnel. At the very end of the blog I reveal the location of the fishes. Enjoy!

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Summary for Penpoint Gunnels

Species information from “Certainly More Than You Want to Know About the Pacific Coast” by Dr. Milton Love includes:

Maximum length to 46 cm. The Ranges: Kodiak Island to Santa Barbara and Gaviota (Southern California). They are abundant from at least Prince William Sound (northern Gulf of Alaska) to Central California. Intertidal to 8, including tide pools. Most fish live int he intertidal or barely subtidal . . .

Penpints are long, thin, and eel-like, distinguished by a deeply grooved spine on the front of the anal fin (hence the name “penpoint”), a line extending downward through the eyes, and no pelvic fins. The body colour is highly variable: orange, red, and magenta, bright green, olive, or bronze. While usually a solid colour, the body can be highly mottled, with a row of dark or light spots along the midline . . .”

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Answers to the three Find the Fish challenges above

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Sources

• FISHBIO, Colour Match, 2014
• Forest, Lisa & Busby, Morgan. (2006). Development of larval and early juvenile penpoint gunnel (Apodichthys flavidus) (family: Pholidae). Fishery Bulletin- National Oceanic and Atmospheric Administration. 104.
• Lamb, A., & Edgell, P. (2010). Coastal fishes of the Pacific Northwest. Madeira Park, B.C: Harbour Pub.
• Love, M. S. (2011). Certainly more than you want to know about the fishes of the Pacific Coast: A postmodern experience. Santa Barbara, Calif: Really Big Press.
• Wilkie, D. W. (1966). Colour pigments in the penpoint gunnel Apodichthys flavidus and their ecological significance. Vancouver.