**5. Conclusions**

Collagen was successfully isolated from lizardfish by-product scales by using acid and pepsin extraction methods with yields of 4.2% and 4.7% (based on the dry weight). The analysis of SDS-PAGE and UV indicated that both ASC and PSC were type I collagen. The FTIR and CD spectra of ASC and PSC were similar; the collagen maintained the triple-helical structures well, indicating that the triple-helix structure of collagen was not disrupted by pepsin digestion. The two types of collagen exhibited multilayer overlapping and porous sheet-like microstructure under SEM. The analysis of the amino acid structure showed that the ASC and PSC had high amino acid contents at 237 residues/1000 residues and 236 residues/1000 residues, respectively. Solubility tests showed that ASC and PSC exhibited high solubility in the acidic pH ranges (pH 1–4) and low NaCl levels (1–6%, *w*/*v*). Moreover, the ASC from lizardfish scales exhibited higher Tmax (43.2 ◦C) compared to rat tail collagen (39.4 ◦C) and calf skin collagen (35 ◦C), indicating its potential as an alternative to collagen of terrestrial source. A dynamic rheological examination indicated that the preparation method may affect the viscoelasticity of the collagen, and that PSC exhibited better viscoelasticity than ASC. Both ASC and PSC were not toxic to the HaCaT and MC3T3- E1 cells, and the relative cell viability of ASC was higher than that of PSC during the 48 h of cell culture. Overall, the results sugges<sup>t</sup> that lizardfish scales ASC may be considered a potential alternative to terrestrial collagen for further use in the biomedical area.

**Author Contributions:** J.C. conceptualization, validation, resources, writing—original draft preparation, writing—review and editing, supervision, project administration, funding acquisition; Y.L. conceptualization, methodology, formal analysis, data curation, writing—original draft preparation; G.W. conceptualization, methodology, software, formal analysis, data curation. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 42076120, 41676129, 41106149; Scientific Research Foundation of the Third Institute of Oceanography, SOA, gran<sup>t</sup> number 2019010; the Marine Economic Innovation & Development Project of Beihai, gran<sup>t</sup> number Bhsfs008. The APC was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 42076120.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data supporting the conclusions of this article are included in this article.

**Acknowledgments:** The authors would like to thank Jianlin He from the Third Institute of Oceanography, Ministry of Natural Resources, for his technical help for cell compatibility and cell morphology analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
