Fig 2: Peltogaster (Peltogastridae) whole externa and root system attached to a part of the main abdominal root trunk From Bresciani & Hoeg 2001.
Female cyprids settle on the exoskeleton of a host crab at the base of a plumose seta and metamorphose into a special stage known as the kentrogon. The kentrogon penetrates the exoskeleton of the crab with a hollow stylet and injects the primordial parasite into the hemocoelic fluid. After an internal phase of a few months to 3 years, the parasite produces an external virginal reproductive body (externa) situated under the abdomen of the host. The externa attracts male cyprids, which implant as dwarf males and remain with the female externa for the duration of the latter’s lifetime and fertilize all its broods. Externa failing to receive at least one male cannot mature and eventually perish, leaving a scar on the host exoskeleton.
Fig 4: First photo shows the cyprid larva, antennae are marked with arrows, from Pasternak et al 2005. Second photo shows the kentrogon stage injecting cells in to the host body, from Hoeg 1987.
Sacculina carcini is a rhizocephalan parasite that attacks the crab C. maenas. S. carcini can have on their hosts severe and lasting effects on these include growth, morphology, physiology, and behavior of the host crab. The parasite can arrest the moult cycle of its host crab (which therefore suffers increased fouling). As a consequence of rootlet growth this parasite castrates both male and female hosts and phenotypically feminizes the males. Both sexes suffer alterations in behavior such as response to the externa and Sacculina eggs as their own brood. For example a common behavior of fecunded female crabs is to climb to some high rock and groom its abdomen to release the fertilized eggs from the brood pouch. Moving her claw the mother crab stir the water generating a flow. In the same way male parasitized by Sacculina will display a similiar behavior when the parasite offpring is ready to hatch, he will groom the Externa and release the next generation of his foster sons. “Sacculinized” host became an integrated developmental system that include the castrated crab, the breeding externa and the dwarf males. These creatures had been the focus of interesting cromosome and molecular studies that we can discuss in a next episode.
Fig 5: Rhizocephalan Extena on infected host species showing rootlets distribution in different parasite-host systems: A: Peltogaster paguri (Peltogastridae) on Pagurus bernhardus. B: Sacculina carcini (Sacculinidae) on Carcinus maenas. C: Sylon hippolytes (Clistosaccidae) on Spirontocaris lilljeborgi, from Bresciani & Hoeg 2001.
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