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First published online December 1, 2006
Journal of Experimental Biology 209, 4974-4983 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02586
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Males also have their time of the month! Cyclic disposal of old spermatophores, timed by the molt cycle, in a marine shrimp

Shmuel Parnes*, Shaul Raviv*, Asaf Shechter and Amir Sagi{dagger}

Department of Life Sciences and National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva 84105, Israel


Figure 1
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  		Fig. 1. Scheme showing the discovery of periodicity in the disappearance of spermatophores in Litopenaeus vannamei. (Left) Long-term observations on three different types of shrimp population. (Middle) The percentage of males found without spermatophores. (Right) A flow chart summarizing the logical flow of thought that led to our hypothesis. (A) Field observations twice a month on maturing populations of shrimp in aquaculture ponds showed the females to be arrested in a pre-pubertal state. The percentage of juvenile males found without spermatophores - initially 100% - decreased steadily to an average of 9.7±8.6% during a 5-month period as the animals matured to become sub-adults. The remainder of the males were found to carry mature spermatophores containing fully developed spermatozoa. (B) In an experimental adult population, males were found with mature spermatophores most of the time during which the females were not receptive, and the average percentage of males found without spermatophores during this time was 5.6±3.2% (Bi). When females were going through vitellogenic cycles and were receptive (Bii, red font), the average percentage of males found without spermatophores was 12.4±11.3. (C) In an experimental all-male population, 5.0±1.4% of the males were found without spermatophores.

 

Figure 2
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  		Fig. 2. Progression of melanization in a L. vannamei male from the mixed adult male/adult female population over a 9-week period. The pictures show the ventral side in the posterior region of the cephalothorax. First week: the spermatophores are white; third week: slight melanization (arrow) is visible in the most distal part of the left spermatophore, immediately next to the gonopore area; fifth week: melanin accumulation is clearly seen in the left spermatophore (arrows); ninth week: melanin has spread over the left spermatophore (arrows). Broken arrow indicates slight melanization in the right spermatophore.

 

Figure 3
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  		Fig. 3. Progress of spermatophore melanization in 7* representative L. vannamei males that were followed throughout 32 weeks, including a male-only period of 9 weeks (weeks 24-32). The light-gray rectangle that covers the histogram in the area between weeks 5-15 represents the period of time in which the females were vitellogenically active. In week 5, one of the seven males was found slightly melanized and then `white' again on week 6. Then, three males developed melanization between weeks 7-10 but all of them recovered and were found `white' between weeks 12-14. All the recoveries occurred strictly during the period in which the females were vitellogenically active (weeks 5-15). *Only 6 males remained at week 26 and only 5 at week 29.

 

Figure 4
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  		Fig. 4. Partially melanized L. vannamei male, and dissected components of the distal part of its reproductive system. (A) Ventral view of the intact male. Symmetrical dark brown melanin spots are clearly visible through the cuticle near the genital papillae. (B) The dissected ampoules with intact spermatophores. Here, the muscular ampoules with their intact spermatophores are stretched and turgid. (C) The same ampoules (top) after the spermatophores (bottom) had been dislodged from them by applying a gentle pressure on the edge of the ampoules opposite to the gonopore, mimicking ejaculation. The empty ampoules then lost their turgidity and became flaccid and slack. Note the melanized flake that separated out from the right spermatophore (arrowhead). Scale bars, 5 mm (A,B), 3 mm (C).

 

Figure 5
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  		Fig. 5. Spermatophore dynamics in the experimental adult male/adult female and all-male shrimp populations in relation to the period during which the females matured and were receptive to males (pink rectangle, weeks 5-15). Time is shown on the x axis in weeks. The y axis gives the percentage of males that were found with empty ampoules at a certain week. Black circles indicate the average daily percentage of molting males out of the total number of males in the population. Green bars represent males with empty ampoules on the night of ecdysis, before they molted (`molt-related'). Red bars represent males with empty ampoules in the middle of the molt cycle (`sex-related'). It is clear that the latter were found only during the period that the females were sexually receptive. On the other hand, males with empty ampoules that were about to molt were found throughout the observation period, irrespective of the presence of females, since such males were also observed in the all-male population.

 

Figure 6
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  		Fig. 6. Periodic disappearance of spermatophores in L. vannamei males related to the molt cycle. (A-C) The spermatophore disappearance phenomenon in a representative male shrimp. Above the photographs is a colour-coded diagram; the white circles indicate the presence of spermatophores in the ampoules; the green circle, empty ampoules without spermatophores; and the yellow triangle, the molt event (ecdysis). In all three pictures, the ampoules are marked with broken arrows. (A) Intermolt with intact spermatophores. Solid arrows indicate the genital papillae that house the sexual openings. (B) ~12 h before ecdysis in latepremolt; the ampoules are empty but are visible as two lumps underneath the cuticle. (C) ~36 h post molt; a new pair of spermatophores can be seen through the cuticle. The lower part of the figure shows three representative individual male time tracks, which are color coded as described above. Each of these time tracks describes five observed molt events that cover four complete molt cycles in which the spermatophore disappearance phenomenon was observed. Two missed observations, for which we did not manage to observe the ampoules before the animal molted, are also indicated. Nevertheless, in both cases the animal molted on the same night.

 

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© The Company of Biologists Ltd 2006