Physocephalata

Belongs within: Amphipoda.

The Physocephalata are a group of planktonic marine amphipods. The name means ‘swollen head’ and refers to the conspicuously enlarged head of many (but not all) members of the group. Members of the family Phronimidae are known for their habit of hollowing out the salps or siphonophores on which they feed to form a barrel-shaped residence.

Another case of mistaken identity

Published 29 January 2009

Just the other day, Adam Yates showed us a couple of photos of a fossil that had been identified as dinosaurian, but actually belonged to a fish. Identifying isolated pieces of things can be a hazardous activity and a mistaken identification can become something of a self-fulfilling prophecy—once the idea of a certain identity for your specimen has developed, you will tend to find “characters” that support your identification. Palaeontology, of course, presents researchers with no shortage of fragmentary remains, and it is not entirely surprising that a few snafus have occured. Adam referred to the case of Aachenosaurus multidens, a “hadrosaur” described in 1888 that was soon reidentified as a piece of petrified wood. A similar fate befell the “sauropod jaw” Succinodon putzeri (making the first four letters of the species name even more apropos). But while the most famous (and most dramatic) examples of such misidentifications involve fossils, studies of recent organisms have not been entirely free of impostors.

The figure above from Huys (2001) shows two views of the paratype of Megallecto thirioti, described by Gotto in 1986. The two specimens originally assigned to this species came from a plankton haul off the coast of Mauretania. Gotto identified them as parasitic copepods belonging to the family Splanchnotrophidae, and suggested that their hosts might be pteropods from the same haul.

Parasitic copepods can certainly be very strange creatures. While free-living males (and larvae of both sexes) may look like fairly ordinary copepods, the parasitic females may have highly derived morphologies that barely resemble crustaceans, let alone copepods. Consider the female of another splanchnotrophid, Arthurius elysiae (also from Huys 2001):

When Huys (2001) revised the Splanchnotrophidae, however, he discovered that Gotto’s Megallecto was (A) not a splanchnotrophid, and (B) not even a copepod. In fact:

‘Megallecto‘ was nothing but a large chunk of the detached head of Phrosina semilunata, a pelagic amphipod. Phrosina belongs to a group of amphipods known as Hyperiidea. Most hyperiids feed on gelatinous plankton such as jellyfish or salps. They may or may not feed on pteropods.

Systematics of Physocephalata

Characters (from Bowman & Gruner 1973): Head large, longer than pereonite 1. Eyes large (small to moderately large in Vibilia), occupying most of head surface (excluding rostrum in Oxycephalidae). Maxilla 1 without inner lobe. Maxilliped with inner lobes completely fused. Pereopods 1 and 2 usually chelate or subchelate, less commonly simple.

<==Physocephalata
    |--Lycaeopsidae [Lycaeopsoidea]MD01
    |--VibilioideaMD01
    |    |--CystisomatidaeMD01
    |    |--ParaphronimidaeMD01
    |    `--VibiliaE12 [VibiliidaeMD01]
    |         |--V. borealis Bate & Westwood 1869 [incl. V. kroyeri Bovallius 1887]N00
    |         `--V. jeangerardii Lucas 1846E12
    |--PlatysceloideaMD01
    |    |--AnapronoidaeMD01
    |    |--LycaeidaeMD01
    |    |--OxycephalusB26 [OxycephalidaeMD01]
    |    |--ParascelidaeMD01
    |    |--PlatyscelidaeMD01
    |    `--PronoidaeMD01
    `--PhronimoideaMD01
         |--DairellidaeMD01
         |--Phronima Latreille 1802L02 [PhronimidaeMD01]
         |    |--*P. sedentaria [=Cancer sedentarius]L02
         |    `--P. custosR26
         |--Phrosina Risso 1822H01 [incl. Megallecto Gotto 1986H01; PhrosinidaeMD01]
         |    |--*P. semilunata Risso 1822 [incl. *Megallecto thirioti Gotto 1986]H01
         |    `--P. macrophthalma Risso 1826R26
         `--HyperiidaeMD01
              |--HyperiaBD95
              |    |--H. galba [incl. Lestrigonus exulans, L. kinahani, Hyperia latreillei, H. spinigera Bovallius 1887]N00
              |    |--H. latissima Bovallius 1889BD95
              |    `--H. medusarum (see below for synonymy)N00
              |--HyperocheN00
              |    |--H. martineziiN00
              |    |--H. prehensilis (Bate & Westwood 1869) [=Hyperia prehensilis]N00
              |    `--H. tauriformis (Bate & Westwood 1869) (see below for synonymy)N00
              |--ParathemistoN00
              |    |--P. gracilipes (Norman 1869) [=Hyperia gracilipes; incl. P. longipes Bovallius 1887]N00
              |    |--P. guadichaudiiW81
              |    `--P. oblivia (Kröyer 1838) [=Hyperia oblivia]N00
              `--EuthemistoN00
                   |--E. bispinosa (Boeck 1870) [=Themisto bispinosa]N00
                   |--E. compressa (Goës 1865) (see below for synonymy)N00
                   |--E. crassicornis (Kröyer 1838)S00, N00 [=Themisto crassicornisN00]
                   `--E. libellula (Mandt 1822) (see below for synonymy)N00

Euthemisto compressa (Goës 1865) [=Themisto compressa; incl. Lestrigonus spinidorsalis Spence Bate 1878, Hyperia spinidorsalis]^N00

Euthemisto libellula (Mandt 1822) [=Gammarus libellula; incl. Themisto arctica Kröyer 1838 N00, Euthemisto nordenskioeldi Bovallius 1887]^N00

Hyperia medusarum [=Cancer medusarum, Metoecus medusarum, Oniscus medusarum, Tauria medusarum; incl. Talitrus cyaneae, Hyperia cyaneae, H. spinipes]^N00

Hyperoche tauriformis (Bate & Westwood 1869) [=Hyperia tauriformis; incl. Tauria abyssorum Boeck 1872, Hyperoche abyssorum, Parathemisto abyssorum, Hyperoche kroeyeri Bovallius 1889, Hyperoche luetkeni Bovallius 1889]^N00

*Type species of generic name indicated

References

[BD95] Bakalem, A., & J.-C. Dauvin. 1995. Inventaire des crustacés amphipodes (Gammaridea, Caprellidea, Hyperiidea) des costes d’Algérie: essai de synthèse. Mésogée 54: 49–62.

[B26] Bigelow, H. B. 1926. Plankton of the offshore waters of the Gulf of Maine. Bulletin of the Bureau of Fisheries 40 (2): 1–509.

Bowman, T. E., & H.-E. Gruner. 1973. The families and genera of Hyperiidea (Crustacea: Amphipoda). Smithsonian Contributions to Zoology 146: 1–64.

[E12] Evenhuis, N. L. 2012. Publication and dating of the Exploration Scientifique de l’Algérie: Histoire Naturelle des Animaux Articulés (1846–1849) by Pierre Hippolyte Lucas. Zootaxa 3448: 1–61.

[H01] Huys, R. 2001. Splanchnotrophid systematics: a case of polyphyly and taxonomic myopia. Journal of Crustacean Biology 21 (1): 106–156.

[L02] Latreille, P. A. 1802. Histoire Naturelle, générale et particulière des crustacés et des insectes vol. 3. Familles naturelles des genres. F. Dufart: Paris.

[MD01] Martin, J. W., & G. E. Davis. 2001. An updated classification of the Recent Crustacea. Natural History Museum Los Angeles County, Science Series 39: 1–124.

[N00] Norman, A. M. 1900. British Amphipoda of the tribe Hyperiidea and the families Orchestiidae and some Lysianassidae. Annals and Magazine of Natural History, series 7, 5: 126–144.

[R26] Risso, A. 1826. Histoire naturelle des principales productions de l’Europe méridionale et particulièrement de celles des environs de Nice et des Alpes maritimes vol. 5. F.-G. Levrault: Paris.

[S00] Stebbing, T. R. R. 1900. Arctic Crustacea: Bruce collection. Annals and Magazine of Natural History, series 7, 5: 1–16.

[W81] Watson, L. 1981. Sea Guide to Whales of the World. Hutchinson: London.