Genetics

Anglerfish, Frogfish, Batfish (and more), Oh My!

Maile, A.J., Smith, W.L. & M. P. Davis (2025). A total-evidence phylogenetic approach to understanding the evolution, depth transitions, and body-shape changes in the anglerfishes and allies (Acanthuriformes: Lophioidei). PLoS One, 20(5): e0322369. journal.pone.0322369.

What Is a Ray to a Fish?

Actinopterygii, or ray-finned fishes such as salmon and tuna, are fish with fins supported by thin, bony-spines, somewhat like a handheld folding fan. Ray-finned fishes make up over 50% of living vertebrates (animals with a backbone), and about 99% of all living fishes, or over 34,000 species. Since there are so many, they are also one of the most diverse groups of fishes, with a variety of eating habits and lifestyles.

One group of ray-finned fishes is the Lophiiformes, or the anglerfish, named aptly for their approach to hunting using a modified dorsal (back) ray as a lure to their prey. Anglerfish and their relatives were discovered first by Aristotle, and have been fascinating scientists and capturing the imagination of fiction authors ever since. However, since their discovery, the relationships between anglerfish and its relatives, including monkfishes, coffinfishes, batfishes, and frogfishes, has eluded scientists. Beyond knowing that each of the aforementioned groups used a lure, how they relate to one another and how they are related to other fish has been under investigation, with several varying perspectives.

However, in a first comprehensive study of both the genetics (or DNA) and body morphology (body-shape and organization) of anglerfishes and their relatives, Maile and his team have determined a new evolutionary pattern of relationship between these groups. Both genetics and morphology play a crucial role in understanding the evolutionary relationships of animals. Although genetics are important, certain traits of animals are preserved morphologically between relatives, even if the genetics don’t necessarily reflect the relationship. So, Maile thought it would be best to review both!

Figure 1. A European Anglerfish, Lophius piscatorius, photographed on the seafloor off the coast of France, captured by Donald Davesne, via Wikimedia Commons.

(Angler)fishing for a New Perspective 

Maile’s team used both DNA and morphology to understand how monkfishes, frogfishes, batfishes, coffinfishes, and deep-sea anglerfishes are related to each other. They ran a series of genetic tests, and compared how all of the bodies were shaped, as well as where certain key organs and limbs were located. They then compared that information to where each group of anglerfishes were found in the ocean, and what type of habitat they usually live in.

Each group had specific depths and habitat types, which the scientists were investigating to understand the relationship with the DNA and body types. The anglerfish live in benthic (seafloor), demersal (near, but just above, the seafloor), and pelagic (swimming) habitats; depending on where they live, they will have different body shapes for the best hunting and living strategies. And they typically inhabited shallow to mid depths (<200 m) and deep-sea depths (>200 m).

To generally summarize the groups that Maile’s team reviewed (Fig. 2):

  • Monkfishes are benthic and demersal shallow fishes
  • Frogfishes are benthic and demersal shallow fishes
  • Batfishes are benthic and demersal, and inhabit shallow to deep-sea environments
  • Coffinfishes are benthic and demersal deep-sea fishes
  • Deep-sea anglerfish are pelagic deep-sea fishes
Figure 2. Images with representatives of each group. A, Monkfish, B-C, Frogfishes, D-E, Batfishes, F, Coffinfish, G-J, Deep-sea anglerfish. Adapted from Figure 1, Maile et al., 2025.

A 200 Meter Dividing Line 

Maile and his team determined that body shape alone was no longer a good diagnostic tool to understand the relationships within the anglerfishes, because within each group there was body variation, and depth had little relationship with body shape either. Instead, the scientists determined that there is a possible relationship with prey items, or environment variance, rather than just depth.

In fact, Maile’s team thinks there is an ancestor that inhabited around 200 m depth from which both shallow and deep groups deviated from. The general body-shape divide, though it wasn’t 100% of the time the case, was more monkfish, batfish, and coffinfish in one group, and frogfish and deep-sea anglerfish in another. But, depth was probably less a deciding factor in body shape than previously thought, and Maile’s study indicated that there was a strong correlation with genetics. That conclusion does agree with recent genetic studies, too.

The genetic analysis that Maile did helps other scientists better understand genetics in other animals, not just fish. Maile’s team thinks that anglerfish are one large group, which has not been the case for understanding anglerfish so far. And the relationship between body-variation and depth gives other scientists a new perspective in understanding body variance in other animals.

So, Maile’s team has made an update to the evolutionary relationship of the anglerfish. This is not only a positive step in understanding fish and fish evolution, but the rest of the evolutionary tree. Understanding one group helps scientists make stronger inferences and understanding of others, both genetically and morphologically! And, this further helps scientists around the world help preserve biodiversity in the future, in our changing planet.

 

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