The significance of cephalopod beaks as a research tool: An update

Abstract

The use of cephalopod beaks in ecological and population dynamics studies has allowed major advances of our knowledge on the role of cephalopods in marine ecosystems in the last 60 years. Since the 1960’s, with the pioneering research by Malcolm Clarke and colleagues, cephalopod beaks (also named jaws or mandibles) have been described to species level and their measurements have been shown to be related to cephalopod body size and mass, which permitted important information to be obtained on numerous biological and ecological aspects of cephalopods in marine ecosystems. In the last decade, a range of new techniques has been applied to cephalopod beaks, permitting new kinds of insight into cephalopod biology and ecology. The workshop on cephalopod beaks of the Cephalopod International Advisory Council Conference (Sesimbra, Portugal) in 2022 aimed to review the most recent scientific developments in this field and to identify future challenges, particularly in relation to taxonomy, age, growth, chemical composition (i.e., DNA, proteomics, stable isotopes, trace elements) and physical (i.e., structural) analyses. In terms of taxonomy, new techniques (e.g., 3D geometric morphometrics) for identifying cephalopods from their beaks are being developed with promising results, although the need for experts and reference collections of cephalopod beaks will continue. The use of beak microstructure for age and growth studies has been validated. Stable isotope analyses on beaks have proven to be an excellent technique to get valuable information on the ecology of cephalopods (namely habitat and trophic position). Trace element analyses is also possible using beaks, where concentrations are significantly lower than in other tissues (e.g., muscle, digestive gland, gills). Extracting DNA from beaks was only possible in one study so far. Protein analyses can also be made using cephalopod beaks. Future challenges in research using cephalopod beaks are also discussed.

Life History of the Arctic Squid Gonatus fabricii (Cephalopoda: Oegopsida) Reconstructed by Analysis of Individual Ontogenetic Stable Isotopic Trajectories

“…Previous studies used eye lenses and gladii of cephalopods to analyse individual ontogenetic stable isotopic trajectories; however, they failed to identify the fast pace of changes in cephalopod ecological role early in ontogenesis [ 48 , 49 , 50 , 123 , 142 , 143 , 144 , 145 , 146 , 147 , 148 ]. Unlike beaks, neither eye lenses nor gladii were proven to have daily growth increment deposition [ 62 , 63 , 64 , 65 ]. Considering beak utility when necessary methodological considerations were performed for the anterior-most and posterior-most parts [ 51 , 61 , 62 , 63 , 69 ], beaks are recommended for ontogenetic SIA studies in the future.…”

“…Beaks are known to grow along their posterior border, where the most recent material is deposited, while the oldest material is deposited in the anterior tip [ 61 , 62 , 63 , 64 , 65 ]. The most anterior part of the beak, the tip of the rostrum, represents the earliest ontogenetic stages, and the most posterior, end of the hood and crest in the upper beak and the end of the hood and crest and wing in the lower beak, represents the latest [ 47 , 51 , 52 , 66 ].…”

“…Thus, values from the earliest ontogenetic stages are influenced by beak layers deposited during the later life periods [ 51 ]. Previous studies showed that new beak material is deposited every day in five species of squids and three species of octopods and in different areas of their beaks: see the recent review by Xavier et al [ 65 ]. However, new beak material deposition potentially depends on various factors, especially in deep-sea and cold-water species [ 65 , 67 , 68 ].…”

“…Previous studies showed that new beak material is deposited every day in five species of squids and three species of octopods and in different areas of their beaks: see the recent review by Xavier et al [ 65 ]. However, new beak material deposition potentially depends on various factors, especially in deep-sea and cold-water species [ 65 , 67 , 68 ]. Darkening of the beaks proceeds from the anterior part towards the posterior, as the beaks grow [ 51 , 56 , 58 , 61 ].…”

“…This stiffness is achieved by increasing the content of hydrophobic histidine-rich proteins, which causes dehydration and sclerotization of the tissues [ 70 , 71 ]. Thus, the tanned parts of the beaks are metabolically inert [ 47 , 51 , 52 , 61 , 65 , 66 , 69 , 70 , 71 ]. They are known to lose water, but not any other substances, during their progressive sclerotization [ 70 , 71 ].…”