Key:  Superorders Rhizocephala and Acrothoracica and Infraclass Ascothoracida (parasitic barnacles)

Key written by Dave Cowles for the common species in the Pacific Northwest, based on references found at the end of the page.

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1a	Parasitic on anthozoans and echinoderms.  Have six thoracic and four abdominal segments.  May have piercing mouthparts or may live inside host tissue and absorb fluid directly.  In some species, the males may retain the ability to swim throughout life.  In our area, may parasitize Mediaster aequalis, Solaster stimpsoni, Dermasterias imbricata, or Crossaster papposus.	Infraclass Ascothoracida
1b	Parasitic on crustaceans or on seashells; not parasitic on anthozoans (but see choice 2a) and echinoderms.  Have no abdominal limbs.  (all in Infraclass Cirripedia)	2
2a	Tiny animals which burrow into shells of barnacles, or of molluscs or corals.  In our area, Trypetesa lateralis lives in gastropod shells inhabited by hermit crabs in central California.	Superorder Acrothoracica
2b	Parasitic on crustaceans such as crabs, shrimp, and hermit crabs (Superorder Rhizocephala)	3
3a	Externa has no receptacles, or they are not paired.  Do not form a kentrogon larva.  Mantle aperture of female externa develops late in development.  Males enter the mantle of the female externa directly through the integument instead of through an aperture.  Have no free nauplii--hatch as cyprids.  Males have no trichogon stage. (Order Akentrogonida)	4
3b	Externa has paired, cuticle-lined male receptacles.  Form a kentrogon larva.  Mantle aperture of female externa develops early in development of the externa.  Males enter the externa early in externa development through the mantle aperture.  Males have a trichogon larva.  Hatch as a nauplius(?) (Order Kentrogonida, Family Sacculinidae)	11
4a	Have many externa, all connected to the same internal body (interna).  Body not compressed and without lateral lobes.  Externa body is oval, pear-shaped, with a narrow stalk at on end.  Have no true mesentery.  Only a few spermatogenic islets exist in the mantle.  This is the only Rhizocephalan which retains its eyes in the cyprid larva.  Parasitize the hermit crab Discorsopagurus schmitti. (Family Thompsoniidae)	Thompsonia sp.
4b	Have only one externa	5
5a	Externa compressed, with many lateral lobes in two tiers (Family Mycetomorphidae) (Mycetomorpha vancouverensis has a mushroom-shaped body with numerous lobed processes.  Known to infest the shrimp Crangon communis, Metacrangon acclivis, and Metacrangon variabilis.  Infrequent.)	Mycetomorpha vancouverensis
5b	Mature externa broad, elongated, nearly cylindrical.  Stalk broad (1/5 externa length).  Immature externa disk-shaped, then globular, then elongated. Mantle aperture is apparent only at or after oviposition.  Cypris has no frontal horn glands, no frontal filaments, and no eyes. (Family Clistosaccidae)	6
6a	Host is a hermit crab	7
6b	Host is a lithodid crab or a shrimp	10
7a	Found in very deep water offshore of the Washington coast, on the hermit crab Parapagurus pilosimanus benedicti ("Pagurus armatus")	Angulosaccus tenuis
7b	Not restricted to very deep offshore water off Washington coast	8
8a	Found on hermit crabs Pagurus capillatus, Pagurus dalli, or Labdochirus splendescens in the Bering Sea, Alaska	Clistosaccus paguri
8b	Not found in the Bering Sea on the species named in 8a	9
9a	Found on an Elassochirus hermit crab	Peltogasterella gracilis
9b	Found on a Pagurus or Discorsopagurus schmitti hermit crab (P. paguri has only been found on Pagurus and P boschmae has only been found on Discorsopagurus schmitti, but P. gracilis has been found on both genera plus on Elassochirus)	Peltogaster boschmae, Peltogaster paguri, or Peltogasterella gracilis
10a	Host is a lithodid crab Lithodes aequispina, L. couesi, or Paralithodes camtschatica	Briarosaccus callosus
10b	Host is a shrimp from family Hippolytidae, Pandalidae, or Crangonidae	Sylon hippolytes
11a	Host is the Xanthid crab Lophopanopeus bellus	Family Kentrogonidae: Loxothylacus panopaei (Note: Probably a different species from this)
11b	Host is the deep sea pelagic shrimp Hymenodora glacialis [This parasite may not be a valid taxon, and may not occur off our coast so it is doubtful.  If it does exist, it may actually be closely related to Sylon.]	Trachylosaccus hymenodorae

    Sylon hippolytes

References used:
  Brusca and Brusca, 2003

  Butler, T.H., 1980.

  Hoeg, J. and A.V. Rybakov, 1992.  Revision of the Rhizocephala Akentrogonida (Cirripedia), with a list of all the species and a key to the identification of familes.  Journal of Crustacean Biology 12(4) 600-609

  Kozloff, 1996

See also:
  Grygier, M.J., 1982.  Dendrogaster (Crustacea: Ascothoracica) from California: sea-star parasites collected by the Albatross.  Proc. Calif. Acad. Sci. 42:  443-454

  Tomlinson, J.T., 1959.  A burrowing barnacle of the genus Trypetesa (Order Acrothoracica).  J. Wash. Acad. Sci. 43: 373-381

  Tomlinson, J.T., 1969.  The burrowing barnacles (Cirripedia: order Acrothoracica).  Bull. 296, U.S. Nat. Mus. v + 162 pp.