In this study sperm morphology is studied in 15 Daphnia species and the morphological data are mapped on a Daphnia phylogeny. The authors found that despite the internal fertilization mode, Daphnia have among the smallest sperm recorded, as would be expected with external fertilization. The authors also conclude that increase in sperm length has evolved twice, that sperm encapsulation has been lost in a clade, and that this clade has very polymorphic sperm with long, and often numerous, filopodia.

Daphnia is an interesting model to study sperm morphology because the biology of sexual reproduction is often ignored in (cyclical) parthenogenetic species. Daphnia is part of the very diverse and successful group of cladocerans with cyclical parthenogenetic reproduction. The success of this reproduction mode is reflected in the known 620 species that radiated within this order, this is more than half of the known Branchiopod species diversity and the estimated number of cladoceran species is even two to four times higher (Forró et al. 2008). Looking at this particular model with a good phylogeny and some particularity in the mode of fertilization/reproduction, has thus a large value. Most Daphnia species are cyclical parthenogenetic and switch between sexual and asexual reproduction depending on the environmental conditions. Within the genus Daphnia, evolution to obligate asexuality has evolved in at least four independent occasions by three different mechanisms: (i) obligate parthenogenesis through hybridisation with or without polyploidy, (ii) asexuality has been acquired de novo in some populations and (iii) in certain lineages females reproduce by obligate parthenogenesis, whereas the clonally propagated males produce functional haploid sperm that allows them to breed with sexual females of normal cyclically parthenogenetic lineages (more on this in Decaestecker et al. 2009).

This study is made in the context of a body of research on the evolution of one of the most fundamental and taxonomically diverse cell types. There is surprisingly little known about the adaptive value underlying their morphology because it is very difficult to test this experimentally.  Studying sperm morphology across species is interesting to study evolution itself because it is a "simple trait". As the authors state: The understanding of the adaptive value of sperm morphology, such as length and shape, remains largely incomplete (Lüpold & Pitnick, 2018). Based on phylogenetic analyses across the animal kingdom, the general rule seems to be that fertilization mode (i.e. whether eggs are fertilized within or outside the female) is a key predictor of sperm length (Kahrl et al., 2021). There is a trade-off between sperm number and length (Immler et al., 2011). This study reports on one of the smallest sperm recorded despite the fertilization being internal. The brood pouch in Daphnia is an interesting particularity as fertilisation occurs internally, but it is not disconnected from the environment. It is also remarkable that there are two independent evolution lines of sperm size in this group. It suggests that those traits have an adaptive value. 

References

Decaestecker E, De Meester L, Mergeay J (2009) Cyclical Parthenogenesis in Daphnia: Sexual Versus Asexual Reproduction. In: Lost Sex: The Evolutionary Biology of Parthenogenesis (eds Schön I, Martens K, Dijk P), pp. 295–316. Springer Netherlands, Dordrecht. https://doi.org/10.1007/978-90-481-2770-2_15

Duneau David, Möst M, Ebert D (2022) Evolution of sperm morphology in a crustacean genus with fertilization inside an open brood pouch. bioRxiv, 2020.01.31.929414, ver. 4 peer-reviewed and recommended by Peer Community in Evolutionary Biology. https://doi.org/10.1101/2020.01.31.929414

Forró L, Korovchinsky NM, Kotov AA, Petrusek A (2008) Global diversity of cladocerans (Cladocera; Crustacea) in freshwater. Hydrobiologia, 595, 177–184. https://doi.org/10.1007/s10750-007-9013-5

Immler S, Pitnick S, Parker GA, Durrant KL, Lüpold S, Calhim S, Birkhead TR (2011) Resolving variation in the reproductive tradeoff between sperm size and number. Proceedings of the National Academy of Sciences, 108, 5325–5330. https://doi.org/10.1073/pnas.1009059108

Kahrl AF, Snook RR, Fitzpatrick JL (2021) Fertilization mode drives sperm length evolution across the animal tree of life. Nature Ecology & Evolution, 5, 1153–1164. https://doi.org/10.1038/s41559-021-01488-y

Lüpold S, Pitnick S (2018) Sperm form and function: what do we know about the role of sexual selection? Reproduction, 155, R229–R243. https://doi.org/10.1530/REP-17-0536