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. 2008 May 20:6:21.
doi: 10.1186/1741-7007-6-21.

Induced metamorphosis in crustacean y-larvae: towards a solution to a 100-year-old riddle

Henrik Glenner  1 , Jens T Høeg, Mark J Grygier, Yoshihisa Fujita
Affiliations

Induced metamorphosis in crustacean y-larvae: towards a solution to a 100-year-old riddle

Henrik Glenner et al. BMC Biol. .

Abstract

Background: The y-larva, a crustacean larval type first identified more than 100 years ago, has been found in marine plankton samples collected in the arctic, temperate and tropical regions of all oceans. The great species diversity found among y-larvae (we have identified more than 40 species at our study site alone) indicates that the adult organism may play a significant ecological role. However, despite intense efforts, the adult y-organism has never been identified, and nothing is therefore known about its biology.

Results: We have successfully and repeatedly induced metamorphosis of y-larvae into a novel, highly reduced juvenile stage by applying the crustacean molting hormone 20-HE. The new stage is slug-like, unsegmented and lacks both limbs and almost all other traits normally characterizing arthropods, but it is capable of vigorous peristaltic motions.

Conclusion: From our observations on live and preserved material we conclude that adult Facetotecta are endoparasitic in still to be identified marine hosts and with a juvenile stage that represents a remarkable convergence to that seen in parasitic barnacles (Crustacea Cirripedia Rhizocephala). From the distribution and abundance of facetotectan y-larvae in the world's oceans we furthermore suggest that these parasites are widespread and could play an important role in the marine environment.

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Figures

Figure 1
Figure 1
Metamorphosis in y-larvae (Crustacea Facetotecta). (A) Light microscopy. Lateral view of a juvenile facetotectan (ypsigon stage) after 28 hours exposure to 20-hydroxyecdysone (20-HE), leaving the molted cuticle of the preceding y-cypris stage. The pigment of the degenerating compound eyes is seen anteriorly in the ypsigon. The shield and swimming limbs of the cyprid are left behind. (B) Lateral view of a free ypsigon after escape from its y-cypris 26 hours after exposure to 20-HE. Note the absence of segmentation and limbs. The elongated brownish body may represent the vestigial gut. (C) Dorsal view of free ypsigon disconnected from the empty y-cypris cuticle.
Figure 2
Figure 2
Morphology of the ypsigon stage (Crustacea Facetotecta). (A) Horizontal transmission electron microscopy section through the ypsigon shown in Figure 1C. Collage of multiple frames. (B) Two high-magnification frames from A. The epidermis is clearly overlain by an ultrathin cuticle. (C) Light microscopy. Lateral view of a molting ypsigon, 48 hours after escape from the y-cyprid. (D) Light microscopy. Lateral view of the specimen in C, but 4 hours later. The first ypsigon cuticle, laid down while still within the y-cyprid, is now being shed and replaced by the new one. The interior structure of the ypsigon does not change noticeably during the molt.
Figure 3
Figure 3
Ypsigon within the y-cyprid. A fully formed ypsigon located within the y-cyprid and just starting to escape through a hole near the anterior end. All y-cyprid tissues are included in the new ypsigon stage, leaving behind only the empty cuticle and all appendages of the y-cyprid. Note how the thorax (with natatory appendages) and abdomen of the y-cyprid shows distinct segmentation, while the slug-shaped ypsigon is entirely unsegmented.
Figure 4
Figure 4
'Plump' type ypsigon escaping from the y-cyprid. Ypsigon escaping from the y-cyprid after 28 hours exposure to 20-hydroxyecdysone. This 'plump' type of ypsigon differs from the more elongated forms shown in Figures 1–3 and Additional files 1, 2, 3, 4, 5. It illustrates that the different 'species' of y-larvae recognized from molted naupliar skins (exuviae) is at least to some extent matched by variations in the shape of the juvenile ypsigon stage.
Figure 5
Figure 5
'Plump' type ypsigon. A 'plump' type of ypsigon (as in Figure 4), just after escape from its y-cyprid.
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References

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