Loxothylacus panopaei (Gissler, 1884)  Common name(s): Sacculinid parasitic barnacle
Synonyms:
Phylum Arthropoda  Subphylum Crustacea   Class Cirripedia    Order Rhizocephala     Suborder Kentrogonida      Family Sacculinidae
	Loxothylacus panopaei parasitizing the black-clawed crab Lophopanopeus bellus
	(Photo by: Dave Cowles July 2000)

How to Distinguish from Similar Species: Loxothylacus panopaei is the only sacculinid rhizocephalan (parasitic castrating barnacle that produces the large sack-like growth on the abdomen as above) listed for this area, and the only one listed as infecting Lophopanopeus bellus.

Geographical Range: Found at least in the Pacific Ocean and the Gulf of Mexico

Depth Range:_

Habitat: Locally, this species parasitizes the black-clawed crab Lophopanopeus bellus.  Elsewhere, it has been found parasitizing the crab Rhithropanopeus harrisii (not found in the Pacific Northwest)

Biology/Natural History: Rhizocephalan sacculinid barnacles such as Loxothylacus panopaei start life as a nauplius larva, then a cyprid as do many other crustaceans.  Instead of settling on rocks as other barnacles do, females settle at the base of the seta of a crab, usually recently molted.  They attach by the base of their first antennae.  Then they metamorphose, lose their eyes and legs, and inject the rest of their body into the host.  They begin to grow inside, sending projections through the crab body until most of the weight of the crab may actually be barnacle tissue.  When sexually mature the barnacle erupts in a sack-like or muchroom-like brood chamber from the abdomen, as seen above.  Male sacculinid barnacles (in the cyprid stage) settle within this sack.  The male extrudes cells which migrate into the female and form into a testis, which fertilizes the eggs the female lays.  The barnacle is called a parasitic castrator because they usually destroy the gonads of the host crab, causing them to display intermediate sexual characteristics.  They also inhibit the crab from molting.  Some crab hosts of this parasite may find refuge in low-salinity water.  Rhizocephalan barnacles have no appendages, digestive tract, segments, or plates; and the part which normally develops into a peduncle (as in goose barnacles) becomes a footlike absorptive process called "nutrient rhizoids" that ramify through the host.

References:

General References:
Barnes, 1980
Brusca and Brusca, 1990

Scientific Articles:
Glenner, H., 2001. Cypris metamorphosis, injection and earliest internal development of the kentrogonid rhizocephalan Loxothylacus panopaei Crustacea: Cirripedia: Rhizocephala: Sacculinidae. J. Morph. 249, 43–75.

Reisser, C.E., Forward Jr., R.B., 1991. Effect of salinity on osmoregulation and survival of a rhizocephalan parasite, Loxothylacus panopaei, and its crab host, Rhithropanopeus harrisii. Estuaries 14, 102– 106.

Walker, G., Clare, A.S., 1994. The effect of salinity on the development of Loxothylacus panopaei (Crustacea: Cirripedia: Rhizocephala). Estuaries 17, 276– 282.

This species does not seem to be common around Rosario, but I have seen it a few times over the years.  Perhaps I would see it more if I were to be specifically looking for it.