Evolution & diversity in the deepest-living Fishes

One of our research team’s main goals is to understand how fishes have evolved and adapted into deep-sea environments. In our new paper in Marine Biology, we explore what it takes for fishes to live at the ocean’s greatest depths.

Revision of the depth record of bony fishes with notes on hadal snailfishes (Liparidae, Scorpaeniformes) and cusk eels (Ophidiidae, Ophidiiformes)

Mackenzie Gerringer, Thomas Linley, & Jørgen G. Nielsen 

Bony fishes are extremely successful in the marine environment, having evolved into nearly every ocean habitat. However, bony fishes do not seem to inhabit the ocean’s deepest depths, likely due to constraints of pressure adaptation. How deep do bony fishes live?

Few studies have examined the deepest living vertebrates, because sampling in hadal environments, depths 6000–11,000 m, is technologically challenging. In this study, we review the literature on records of the deepest living bony fishes.

Current depth records are held by the hadal snailfish Pseudoliparis swirei (family Liparidae) in the Mariana Trench, collection depth 7966 m, filmed to 8178 m, and the cusk eel Abyssobrotula galatheae (family Ophidiidae) in the Puerto Rico Trench, collection depth 7965 m.

Observations of abyssal and hadal fish communities suggest that hadal snailfishes are endemic to trenches but occasionally cross into abyssal areas. On the other hand, cusk eels dwell on the abyssal plains, but can extend their ranges into the trenches. These habitat differences allow both snailfishes and cusk eels to occupy distinct niches in the greatest ocean depths.

We then comment on the ecological and physiological significance of these two major hadal families and present recommendations for future research.

This paper has been featured as a Highlight Article, with editorial comment from Dr. Scott Hamilton available here.

The Mariana Snailfish, Pseudoliparis swirei Gerringer & Linley 2017. The deepest-living fish collected from the ocean floor.

Do deep-sea fishes have low-density bones?

Habitat influences skeletal morphology and density in the snailfishes (family Liparidae)

The snailfish was sampled by the ROV Global Explorer during a benthic dive. Image courtesy of Microcosm Film, The Hidden Ocean 2016: Chukchi Borderlands.

We tested the hypothesis that deep-sea fishes have poorly mineralized bone relative to shallower-dwelling species using data from a single family that spans a large depth range. The family Liparidae (snailfishes, Cottiformes) has representatives across the entire habitable depth range for bony fishes (0 m–> 8000 m), making them an ideal model for studying depth-related trends in a confined phylogeny.

We used micro-computed tomography (micro-CT) scanning to test three aspects of skeletal reduction in snailfishes (50 species) across a full range of habitat depths: 1) reduction of structural dimensions, 2) loss of skeletal elements, and 3) reduction in bone density.

Liparis florae, a shallow-living snailfish, showing bones analyzed in this study.

Using depth data from the literature, we found that with increasing depth, the length of the dentary, neurocranium, and suborbital bones decreases. The ventral suction disk decreases width with increasing maximum habitat depth and is lost entirely in some deeper-living taxa, though not all.

Although visual declines in bone density in deeper-living taxa were evident across full skeletons, individual densities of the lower jaw, vertebra, suction disk, hypural plate, and otoliths did not significantly decline with any depth metric.

However, pelagic and polar taxa tended to show lower density bones compared to other species in the family.

We propose that skeletal reductions allow snailfishes to maintain neutral buoyancy at great depths in the water column, while supporting efficient feeding and locomotion strategies. These findings suggest that changes in skeletal structure are non-linear and are driven not only by hydrostatic pressure, but by other environmental factors and by evolutionary ancestry, calling the existing paradigm into question.

Read our full open access paper here!

The life and times of hadal snailfishes

IOB_snailfish

The deepest-living fishes in the ocean are small, pink, and look delicate, but they are thriving in an environment named for the proverbial hell.

How are they surviving under such high pressures? What do they eat? How long do they live?  Why do snailfishes do so well in deep-sea trenches?

This new review article, published in Integrative Organismal Biology, synthesizes what we know about this amazing group of animals.

A bucket-list fish…

Cusk eel, an aphyonine.
The ghostly cusk eel, an aphyonine fish seen by NOAA OER.

The remotely-operated vehicle, Deep Discoverer, explores the deep sea live, while anyone with an internet connection can tune in. The sense of excitement and discovery is palpable. As we watch NOAA’s Okeanos Explorer live feed, we sometimes discuss: what would be the top things you would want to see. This fish made the top of our lists and last year, we saw it! It’s an aphyonine cusk eel, in the family Ophidiidae, and this is the first time one has been seen alive!

Continue reading “A bucket-list fish…”

Gelatinous Tissues and Robot Snailfish

Robot snailfish
Robotic model of a hadal snailfish to investigate swimming performance.

Some deep-sea fish are full of a gelatinous goo—a watery tissue layer. These tissues show up in several different types of fishes, but why are they there? Our new open-access paper in Royal Society Open Science tackles this question. We describe which fishes have gelatinous tissue, show the chemistry of what gelatinous tissues are made of, and test some of the functions. Gelatinous tissues likely help deep-sea fishes maintain buoyancy. They may also act as faring, changing the shape of the fish to reduce drag. And, of course, we needed to build a robot hadal snailfish! Check out the full paper and coverage by Science News!

Expanding Your Horizons

Mackenzie Gerringer shows deep-sea fishes at EYH event in Hawaii.
Mackenzie Gerringer shows deep-sea fishes at EYH event in Hawaii.

A wonderful weekend at the University of Hawaii – Expanding Your Horizons Event! Exciting to encourage girls to stay interested in STEM fields with our Journey to the Deep workshop.