Identifying Species Using Ear Stone Shapes

Jenn Hoskins
21st June, 2025

Identifying Species Using Ear Stone Shapes

While visually similar, the otoliths of the black scabbardfish (Aphanopus carbo) (a–c) and the intermediate scabbardfish (A. intermedius) (d–f) possess subtle shape differences that this study's wavelet analysis uses to accurately identify each cryptic species.

Image adapted from: Vasconcelos et al. / CC BY (Source)

Key Findings

  • Researchers in the northeastern Atlantic developed a highly accurate method using fish ear stones and genetic data to tell apart two very similar scabbardfish species
  • They found that A. carbo typically dominates catches (55-60%) with stable proportions over decades, though it declined in 2010, and A. intermedius prefers offshore areas
  • These insights, influenced by environmental factors and fishing expansion, enable more precise and sustainable management of these economically important fish
Accurate identification of fish species is a cornerstone of effective fisheries management. Without knowing exactly which species are being caught, it becomes impossible to properly assess their populations, set sustainable catch limits, or protect vulnerable stocks. This challenge is particularly acute when dealing with what are known as "cryptic species"—two or more distinct species that look so similar they are often mistakenly classified as one. The scabbardfish, specifically Aphanopus carbo and Aphanopus intermedius, represent such a conundrum in the northeastern Atlantic, where they are economically important but difficult to tell apart by traditional means. A recent study by researchers at the Universidad de Las Palmas de Gran Canaria[1] tackled this problem by exploring the presence and distribution of these two scabbardfish species over four decades. The core issue is that while these species might appear identical to the naked eye, they could have different population sizes, growth rates, or habitat preferences, meaning that managing them as a single unit could lead to overfishing one species while the other remains abundant. This is a long-standing issue in biology; the challenge of cryptic species has been recognized for nearly 300 years, and while modern tools like DNA sequencing have provided new ways to differentiate them, many questions about their prevalence and distribution remain open[2]. The study aimed to provide a more precise way to distinguish between A. carbo and A. intermedius and understand their population dynamics. To do this, the researchers analyzed otoliths, which are small, calcium carbonate structures found in the inner ear of bony fish. Often called "ear stones," otoliths grow throughout a fish's life, adding layers much like tree rings, and their shape can be unique to a species. Samples of otoliths were collected from scabbardfish caught off Madeira and the African coast between 1990 and 2021. The method employed for analyzing these otoliths was highly sophisticated: a wavelet-based contour analysis. Wavelets are mathematical tools that can break down complex shapes, like the outline of an otolith, into different components, allowing for the capture of very fine details. This technique has proven useful in other fish groups; for instance, a similar approach using wavelet analysis of otolith contours was successfully used to identify different species of gobies in the Adriatic and Mediterranean seas, demonstrating its efficiency in distinguishing species based on these subtle anatomical differences[3]. By applying this advanced method to scabbardfish otoliths, the current study aimed to improve species discrimination beyond what traditional morphological examination allows. Crucially, the researchers enhanced the accuracy of their otolith analysis by using reference specimens that had been definitively identified through genetic methods. This integration of genetic information, as highlighted in earlier research, is vital because genetics can reveal hidden information like species misidentification or even hybridization, which might otherwise go unnoticed[4]. The findings of the study provided crucial insights. It revealed that the proportions of A. carbo and A. intermedius in catches remained relatively stable over time, with A. carbo generally being more dominant, accounting for about 55-60% of the catch. However, a notable decline in A. carbo was observed in 2010. The research also indicated that A. intermedius tended to be more prevalent in offshore areas, a pattern that might be linked to higher salinity levels near Madeira Island during the species' spawning season from October to December. Environmental factors, such as changes in ocean temperature and salinity at depths greater than 800 meters, along with the expanding reach of the longline fishery into new fishing grounds, likely influenced these observed distribution patterns. These results have significant implications for fisheries management. The ability to accurately distinguish between these two cryptic scabbardfish species, even when they look identical, means that management strategies can be tailored to the specific needs of each population. This directly addresses the problem of misidentification that can plague fisheries assessments. For example, in the case of anglerfish, it was discovered that a significant fraction of specimens identified morphologically as one species were genetically another, and even that natural hybridization occurred, leading to a population of hybrids. Such findings underscore the "potentially negative consequences of ignoring genetic data for assessing fisheries resources"[4]. By combining the detailed otolith analysis with genetic validation, the scabbardfish study provides a robust framework for assessing these valuable fish stocks. It allows for a more precise understanding of their distribution, helping to avoid the pitfalls of managing multiple distinct biological units as if they were a single, uniform stock, thereby enhancing the long-term sustainability of the scabbardfish fishery in the northeastern Atlantic.

Animal ScienceMarine Biology

References

Main Study

1) Clarifying species identity in Aphanopus using wavelet-based otolith shape analysis

Published 18th June, 2025

https://doi.org/10.1371/journal.pone.0326199

Related Studies

2) Cryptic species as a window on diversity and conservation.

Journal: Trends in ecology & evolution, Issue: Vol 22, Issue 3, Mar 2007

3) Identifying sagittal otoliths of Mediterranean Sea gobies: variability among phylogenetic lineages.

https://doi.org/10.1111/jfb.13615

4) Evidence of stock connectivity, hybridization, and misidentification in white anglerfish supports the need of a genetics-informed fisheries management framework.

https://doi.org/10.1111/eva.13278


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