Lump­fish vi­sion

Dis­cov­ery of novel eye fea­tures could lead to bet­ter farm­ing

Fish Farmer - - Contents - Editor’s Welcome - BY ROBERT L. GENDRON, RAAHYMA AH­MAD AND HELENE PARADIS*

OCEAN teleost Cy­clopterus lum­pus (C Lum­pus, Lump­fish) is a cleaner fish species which in re­cent years has been listed as threat­ened (1-4). Lump­fish are be­com­ing im­por­tant in aqua­cul­ture for their nat­u­ral abil­ity to de­louse salmon, which of­fers an at­trac­tive and sus­tain­able al­ter­na­tive to chem­i­cally based de­lous­ing agents.

Since lump­fish are preda­tory, the health of the visual sys­tem of these fish is im­por­tant for both their sur­vival in the wild, for liv­ing in cul­ture and for salmon lice re­moval in aqua­cul­ture.

A bet­ter un­der­stand­ing of the bi­o­log­i­cal struc­tures and func­tions spe­cific to lump­fish, par­tic­u­larly their or­gans of sense, could ul­ti­mately un­der­lie strate­gies to pro­tect these fish both in cul­ture and in the wild.

There was pre­vi­ously no sci­en­tific lit­er­a­ture on the char­ac­ter­is­tics of the lump­fish eye. Hav­ing ex­ten­sive ex­pe­ri­ence in mouse and hu­man eye, our lab­o­ra­tory, with the re­sources of the Dr Joe Brown Aquatic Re­search Build­ing at Me­mo­rial Univer­sity (Fig­ure 1), has re­cently stud­ied the cul­tured lump­fish eye dur­ing post hatch de­vel­op­ment (5).

We have ap­plied both clas­si­cal and state-of-the art meth­ods such as his

tol­ogy, im­muno­his­to­chem­istry, fun­dus imag­ing and spec­tral do­main op­ti­cal co­her­ence to­mog­ra­phy (SD-OCT) reti­nal imag­ing in or­der to sys­tem­at­i­cally de­fine fea­tures of the lump­fish eye (Fig­ure 2).

We found in the study, pub­lished in Jour­nal of Fish Bi­ol­ogy (J Fish Biol. 2019;94[2]:297-312), that cul­tured lump­fish har­bour sev­eral fea­tures in their eyes that set them apart from other teleosts, and from mam­mals.

Lump­fish have a very promi­nent re­trac­tor lentis, a mus­cle-like struc­ture which serves to move the lens around their eye globes to al­low them to fo­cus on visual tar­gets.

This struc­ture is phys­i­cally as­so­ci­ated with an­other ro­bust vas­cu­lar struc­ture called the rete mirabile in the pos­te­rior (rear) por­tions of the eye which nour­ishes not only the retina but, be­cause of its ro­bust­ness, pos­si­bly other more an­te­rior (front) por­tions of the eye.

Quite sur­pris­ingly, we found that lump­fish eyes, un­like mam­mals, pos­sess two par­ti­tioned sep­a­rate lobes of reti­nal tis­sue con­tain­ing both light sens­ing pho­tore­cep­tors and other pos­si­bly re­gen­er­a­tive cells.

The smaller reti­nal lobe po­si­tioned at the base of the eye is rem­i­nis­cent of an ac­ces­sory retina pre­vi­ously de­scribed only in deep sea fish (6).We think that lump­fish might be us­ing these ex­tra reti­nal lobes to op­ti­mise their ca­pa­bil­i­ties to detect light rays ar­riv­ing at the eye from above.

Fur­ther work will be needed to con­firm this in­ter­est­ing no­tion. Con­sid­er­ing that lump­fish are now known to move through sig­nif­i­cant ranges and depths of the wa­ter col­umn, the ex­tra reti­nal tis­sues might be use­ful to them in darker and deeper re­gions of the wa­ter col­umn.

We also found that, like deep sea fish, lump­fish eyes har­bour ex­ten­sive tape­tum ma­te­rial, a mir­ror-like layer that serves to re­flect light ei­ther back into the light sens­ing cells or to bounce light rays around in­side the eye max­imis­ing the light sig­nal.

Again, fur­ther work will be needed to ex­plore these no­tions.At tank side, cul­tured lump­fish

are very cu­ri­ous and of­ten swim with eyes en­tirely out of the wa­ter, as if they are look­ing at some­thing (see Fig­ure).This be­hav­iour also raises in­ter­est­ing ques­tions about how lump­fish might see their world.

In the con­text of aqua­cul­ture and fish farm­ing, we pro­pose that a bet­ter un­der­stand­ing of the lump­fish eye, as well as its other or­gans of sense, can be used to bol­ster the health and well­be­ing of these rather amaz­ing an­i­mals.

Some of the imag­ing tech­niques we used in our study are ac­ces­si­ble and can be em­ployed by aqua­cul­ture staff.

Our work also in­di­cates that live, non-in­va­sive method­olo­gies such as op­ti­cal co­her­ence to­mog­ra­phy can be used to quickly and com­pre­hen­sively as­sess lump­fish eye health, much like is done for hu­mans in an op­tometrist’s of­fice.

Aqua­cul­ture fa­cil­i­ties could per­haps one day be de­signed with some con­sid­er­a­tion for bet­ter health mon­i­tor­ing of lump­fish and how the lump­fish might sense its world.

Al­ter­na­tively, other cul­ture con­di­tions, such as feed and phys­i­cal at­tributes of the wa­ter, might also be ad­justed based on a bet­ter un­der­stand­ing of lump­fish vi­sion and other senses.

“The ex­tra reti­nal tis­sues might be use­ful to them in darker and deeper re­gions of the wa­ter col­umn”

*Di­vi­sion of Biomed­i­cal Sci­ences, Fac­ulty of Medicine, Me­mo­rial Univer­sity, St John’s, NL, Canada. With thanks to Danny Boyce and the staff of the Dr Joe Brown Aquatic Re­search Build­ing, Depart­ment of Ocean Science, Me­mo­rial Univer­sity.

1. Jonassen et al., 2017, Jour­nal of Fish Dis­eases. 40(12):1903-1914. 2. Im­s­land et al., 2014 Aqua­cul­ture. s424–425. 18–23. 3. Lo­rance et al., 2015. Cy­clopterus lum­pus.The IUCN Red List of

Threat­ened Species 2015 e.T18237406A­45078284. 4. Com­mit­tee on the Sta­tus of En­dan­gered Wildlife in Canada

[COSEWIC], Gov­ern­ment of Canada) 5.Ah­mad et al., 2019. J Fish Biol. 94(2):297-312. 6. Par­tridge et al., 2014. Pro­ceed­ings of the Royal So­ci­ety Bi­o­log­i­cal

Sci­ences. 281(1782):20133223.

Be­low: Fig­ure 2- novel eye struc­tures of the lump­fish. Op­po­site: Fig­ure 1ju­ve­nile lump­fish in the Dr Joe Brown Aquatic Re­search Build­ing at Me­mo­rial Univer­sity. Mid­dle and right pan­els show fish at sur­face of wa­ter or with eyes en­tirely out of wa­ter.

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