The functional morphology and role of the thelycum in insemination,and its relation to the mating system in the seagrass shrimp Ambidexter symmetricus (Decapoda: Processidae)

There is considerable variation in structures known to function in the transfer and storage of sperm in female decapod crustaceans. The thelycum is a secondary sexual character that forms from the posterior thoracic sterna of female shrimps (especially penaeoids and sergestoids). Females in the caridean shrimp family Processidae have a thelycum‐like structure which rarely occurs in other caridean females. We tested the hypothesis that the processid thelycum serves as a spermatheca for either short‐term attachment or long‐term sperm storage. When inseminated females of the processid Ambidexter symmetricus were isolated after mating, newly spawned and then incubated embryos hatched, but in the continued absence of a male, females were unable to fertilize a subsequent spawn. Our observations on A. symmetricus show that sperm were not retained after female spawning, and thus the thelycum is not used for long‐term sperm storage as in many penaeoids. In A. symmetricus, the thelycum may serve as an external median spermatheca (seminal receptacle) for temporary attachment and storage of a sperm mass during the 2–3 h interval between mating and spawning. Observations on mating behavior support the hypothesis of a pure‐search (promiscuous) mating system in A. symmetricus, with males showing little interest before, and copulating with females only after, the female parturial molt. Mating encounters were short (<2 min). This mating system is like that of other caridean shrimps with populations structured similarly to those in A. symmetricus: a relatively high density of mobile individuals and sexual dimorphism in body size (reproductive females larger than males) but not in cheliped weaponry (similar in males and females).     

Molecular modification of Penaeus monodon sperm in female thelycum and its consequent responses

Using Penaeus monodon as the model, we demonstrated the molecular changes and the mechanism of thelycal-dependent sperm modification resulting in an enhanced acrosome reaction (AR) response. Attention was paid to the modification of the sperm plasma membrane which was mediated through an adsorption or removal of sperm peripheral and integral membrane proteins as indicated by the different profiles of these proteins in spermatophore (S) and thelycal (T) sperm. In vitro adsorption of Alexa-488 conjugated T proteins onto the entire S-sperm surface confirmed protein transfer in a time-dependent manner. Specific anchoring of 83 and 140 kDa proteins to sperm peripheral proteins as well as 53/55 and 60 kDa proteins to sperm lipids was demonstrated. Apart from membrane modification, a substantial increase in protein tyrosine phosphorylation was shown to be closely associated with T-dependent sperm modification event. The physiological significance of this sperm modification in enhancing sperm AR response, which required at least 3 days of T residence in order for the sperm to gain a complete AR response, was also elucidated.     

Larval development of Euphausia superba Dana, 1852 and a phylogenetic analysis of the Euphausiacea

Literature data and new investigations by SEM of selected ontogenetic stages of the Antarctic Krill, Euphausia superba Dana, 1852 revealed morphological characters that are either missing in, or significantly changed towards, the adult. Besides adult features, such ontogenetic characters enabled us to propose a hypothesis of the phylogenetic relationships of and within the Euphausiacea on the basis of a computer aided cladistic analysis. These are of the form (Bentheuphausia amblyops + Euphausiidae = ('Thysanopoda' + Nematobrachion + Euphausiinae = (Meganyctiphanes norvegica + Euphausiini, new name + Nematoscelini, new name = (Nyctiphanes + Nematoscelina, new name)))). From this analysis the taxon names 'Euphausiina', 'Nematoscelini', and 'Nematoscelina' are introduced for in-groups of the taxon Euphausiacea as representing monophyletic units. The position of a set of ontogenetic characters remains relatively uncertain due to the still unknown larval development of Bentheuphausia amblyops (G. O. Sars, 1883).