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First published online January 17, 2007
Journal of Experimental Biology 210, 432-437 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02673

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Dimorphic sperm and the unlikely route to fertilisation in the yellow seahorse

Katrien J. W. Van Look^1,*, Borys Dzyuba², Alex Cliffe³, Heather J. Koldewey³ and William V. Holt^1,*

¹ Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK
² Institute for Problems of Cryobiology and Cryomedicine of the National Academy of Sciences of the Ukraine, 23 Pereyaslavskaya Street, Kharkov 310015, Ukraine
³ Zoological Society of London, Regent's Park, London NW1 4RY, UK

View larger version (103K): [in this window] [in a new window]   Fig. 1. Yellow seahorse spermatozoa illustrating the two types. (A,B,D) Type 1 spermatozoa of different flagellar lengths; (C) Type 2 spermatozoon with the larger head. Scale bar, 13 µm.

View larger version (10K): [in this window] [in a new window]   Fig. 2. Head and flagellar measurements of Types 1 and 2 spermatozoa. (A) Principal components analysis (using sperm head length, width and length:width ratio) showed that the spermatozoa fell distinctly into the two groups (Types 1 and 2) with one outlier, and both sperm head length and width were significantly different (post-hoc ANOVA analyses: length, F1/42=135.4, P<0.001; width, F1/42=434.2, P<0.001, N=44). (B) Sperm heads of Type 1 spermatozoa measured 3.7 µm (median) in length (solid line), whereas sperm heads of Type 2 spermatozoa were significantly longer (dotted line), measuring 13.4 µm (median) in length. (C) Flagella of Type 1 spermatozoa (circles) varied in length between 6.3 and 69.3 µm (49.3 µm median), whereas flagellar lengths of Type 2 spermatozoa (triangles) ranged from 13.5 to 80.4 µm (25.1 µm median). The horizontal line in each group signifies the median value; *P=0.045 (N=44) indicates a significant difference by a Mann–Whitney test (two-tailed).

View larger version (12K): [in this window] [in a new window]   Fig. 3. Mating sequence of a yellow seahorse couple; hand drawn from video sequence. The time is shown above the images and demonstrates the duration when (A) the brood pouch remains opened (PO), (B) when gamete transfer occurs and (C) when the pouch is closed (PC).

View larger version (131K): [in this window] [in a new window]   Fig. 4. Image of a male yellow seahorse showing the exteriorly directed sperm duct, which is located 4.5 mm anterior to the opening of the brood pouch. SDO, sperm duct opening; AF, anal fin; SF, skin folds; PO, pouch opening. Scale bar, 2 mm.