Belongs within: Aristioidea.
Hammer-clawed amphipods
Published 2 July 2025
For the longest time, amphipod taxonomy was, it must be said, a bit of a wasteland. A confusing distribution of characters and poor understanding of their evolution lead to bewilderment on the best way to arrange these crustaceans. As a result, there are a number of noteworthy groups whose recognition has only come fairly recently. Among these are the family Pakynidae.
The pakynids were first informally recognised as the ‘pachynid group’ by Lowry (1984). This group would later be formally established as the Pachynidae, a name that was later emended to Pakynidae when it was realised that the type genus Pachynus was preoccupied and renamed Pakynus (Lowry & Myers 2017). A dozen genera are currently included in the family. Pakynids have been recorded from oceans worldwide and are primarily recognised by the structure of the first gnathopod, the first more or less pincer-like appendage immediately behind the head. The propodus, the second-to-last segment of the gnathopod before the spine-like dactyl, is enlarged, and the preceding segment (the carpus) is narrowly compressed. Lowry (1984) recognised two major subgroups of the pakynids. In the ‘sculptured-tooth’ pakynids, the overall body form is more elongate and the teeth on the first maxilla are mostly sculptured. The ‘smooth-toothed’ pakynids have, naturally, smooth teeth on the first maxilla, and the body is stocky and comma-shaped.
Though numerous species of pakynid have been described to date, I have not found whether the significance of their distinctive morphology is understood. The robust gnathopods certainly suggest a focus on tougher foodstuffs than those preferred by other amphipods. Presumably, differences in habit also separate the sculptured- and smooth-toothed pakynids, with the latter being somehow less mobile. But the nature of their preferred prey, and how they obtain it, seemingly waits to be discovered.
Systematics of Pakynidae
Pakynidae [Pachynidae]LM17
|--Acheronia Lowry 1984LM17
|--Coriolisa Lowry & Stoddart 1994LM17
|--Drummondia Lowry 1984 nec Hook. 1828 (ICBN) nec Bat. & Maia 1963 (ICBN)LM17
|--Ekelofia Lowry 1984LM17
|--Figorella Barnard 1962LM17
| `--*F. tanidea Barnard 1962B69a
|--Pachychelium Stephensen 1925LM17
| |--*P. davidis Stephensen 1925B69a
| |--P. antarcticumN38
| `--P. oculatumN38
|--Pakynus Lowry & Myers 2017 [=Pachynus Bulycheva 1955 nec Rafinesque 1815 nec Reichenow 1881]LM17
| |--*P. chelatum (Bulycheva 1955) [=Pachynus chelatum]B69a
| `--‘Pachynus’ barnardi Hurley 1963B69b
|--Prachynella Barnard 1964LM17
| `--*P. lodo Barnard 1964B69a
|--Renella Lowry & Stoddart 2012LM17
|--Sheardella Lowry 1984LM17
|--Smaraldia Lowry & Stoddart 2012LM17
`--Ultimachelium Lowry & Stoddart 2012LM17
*Type species of generic name indicated
References
[B69a] Barnard, J. L. 1969a. The families and genera of marine gammaridean Amphipoda. United States National Museum Bulletin 271: i–vi, 1–535.
[B69b] Barnard, J. L. 1969b. A biological survey of Bahía de Los Angeles, Gulf of California, Mexico, IV. Benthic Amphipoda (Crustacea). Transactions of the San Diego Society of Natural History 15 (13): 175–228.
Lowry, J. K. 1984. Systematics of the pachynid group of lysianassoid Amphipoda (Crustacea). Records of the Australian Museum 36 (2): 51–105.
[LM17] Lowry, J. K., & A. A. Myers. 2017. A phylogeny and classification of the Amphipoda with the establishment of the new order Ingolfiellida (Crustacea: Peracarida). Zootaxa 4265 (1): 1–89.
[N38] Nicholls, G. E. 1938. Australasian Antarctic Expedition 1911–1914. Under the Leadership of Sir Douglas Mawson, O.B.E., B.E., D.Sc., F. R. S. Scientific Reports. Series C—Zoology and Botany vol. 2 pt 4. Amphipoda Gammaridea. David Harold Paisley, Government Printer: Sydney (Australia).