Discovery of novel eye features could lead to better farming
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;94:297-312), 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
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.