The Bizarre Ways Worms Have Sex – A Glimpse into the Amazing Diversity of Sexual Reproduction in the Protostomia

Introduction

The characteristics of sexual reproduction are incredibly diverse, not just across Kingdom Animalia, but also in all species capable of sexual reproduction. Just within the Protostomia, a clade of animals in which the blastophore develops into the mouth instead of the anus during embryonic development, sexual reproduction mechanisms already differ greatly. This may be because the Protostomia themselves are very diverse, which consist of two major branches. These are the Lophotrochozoa, which have trochophore larvae, and the Ecdysozoa, which undergo ecdysis, or moulting.

This essay aims to explore the diversity of sexual reproduction in the Protostomia, by comparing the similarities and differences in sexual reproduction in one representative phylum from each major branch of the Protostomia. In this essay, sexual reproduction is defined as the mating system by which offspring are formed from the union of a male gamete (ie. the sperm) and a female gamete (ie. the egg) (Mackiel & Dillon, 2021).?The representative phyla I have chosen are the Annelida (ie. “true” segmented worms) and the Onychophora (ie. velvet worms), which are lophotrochozoans and ecdysozoans respectively. I chose to compare these phyla due to their similar appearance, as both are segmented and are commonly called “worms”. Thus, comparing these two seemingly similar representative phyla may illustrate the vast diversity of sexual reproduction mechanisms in the Protostomia, which may not be readily apparent. In this essay, I will compare the Annelida and the Onychophora in terms of their anatomy, sex, the types of fertilisation they undergo, methods of sperm delivery, and modes of reproduction.

Fundamental Anatomical and Sex Differences

Firstly, it is important to address the fundamental differences in anatomy between the Annelida and the Onychophora, as anatomical differences lead to differences in sexual reproduction. Among the Annelida, the Clitellata, which comprise the Oligochaeta and Hirudinea (ie. leeches), are hermaphroditic, and while most polychaetes are diecious, some can also be hermaphroditic (Verdonschot, 2015).??In contrast, the vast majority of onychophorans are dioecious (Wright, 2014). Thus, reciprocal cross-fertilisation (Govedich & Moser, 2015), where two copulating hermaphrodites exchange sperm and fertilise each other’s eggs (Verdonschot, 2015), are not possible in onychophorans, where only the sperms of the male onychophoran can fertilise the eggs of the female onychophoran. Obviously, onychophorans are also unable to engage in self-fertilisation, which is possible in clitellates, although this is not as common as reciprocal cross-fertilisation?(Verdonschot, 2015).

Type of Fertilisation

Since the basic differences in anatomy and sex between the Annelida and the Onychophora have been addressed, the type of fertilisation they undergo can now be discussed. Here, the Onychophora are more straightforward – they all undergo internal fertilisation (Velvet Worm, 2020).?In contrast, different classes of annelids undergo different types of fertilisation. While polychaetes and oligochaetes undergo external fertilisation, albeit using different mechanisms?(Verdonschot, 2015), leeches undergo internal fertilisation, just like onychophorans (Govedich & Moser, 2015).?In fact, the mechanism of internal fertilisation in onychophorans and leeches is surprisingly similar. After the onychophorans and leeches have received sperm from their copulating partner, the sperm migrate through their body to the ovaries in the female onychophoran (Velvet Worm, 2020), or the ovisac in the receiving leech, to fertilise the eggs (Govedich & Moser, 2015).

Method of Sperm Delivery

Not only is the type of fertilisation similar between onychophorans and leeches, some onychophorans also share similar methods of sperm delivery with leeches and even some oligochaete species. During copulation, male onychophorans and these clitellates deposit spermatophores (ie. external sperm packets) onto the body of the female onychophoran or the receiving clitellate partner respectively (Govedich & Moser, 2015; Timm & Martin, 2015; Velvet Worm, 2020).?The sperm then cause the skin or body wall of both the onychophoran and clitellate partner to rupture so that they can migrate through the body of the receiving partner to fertilise their eggs (Govedich & Moser, 2015; Velvet Worm, 2020).

Presence of Spermathecae

Some onychophorans and most oligochaetes?(Timm & Martin, 2015) also share similar anatomical organs. These onychophorans and oligochaetes possess spermathecae?(Verdonschot, 2015), also known as sperm receptacles?(Velvet Worm, 2020), which are “responsible for receiving, maintaining, and releasing sperm to fertilise eggs”?(Pascini & Martins, 2017). Spermathecae are present near the ovaries in female onychophorans or egg sac in clitellates (Figure 1), and both the onychophorans and oligochaetes may store the received sperms in their spermatheca after copulation (Velvet Worm, 2020; Verdonschot, 2015).

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Figure 1: The sexual organs and clitellum of the Tubificidae, a family of Oligochaeta?(Verdonschot, 2015). The Hirudinea also have similar organs.

It is important to note that the onychophorans described in the previous two paragraphs may possess both spermathecae and the ability to produce spermatophores?simultaneously (Velvet Worm, 2020). On the other hand, clitellates can either have spermathecae, which are present in most oligochaetes?(Timm & Martin, 2015), or they can produce spermatophores, which leeches (Govedich & Moser, 2015) and some oligochaetes?do (Timm & Martin, 2015). They cannot possess both characteristics at the same time, unlike the onychophorans.

Mode of Reproduction

Finally, although both onychophorans and leeches undergo internal fertilisation and produce spermatophores, their modes of reproduction are very different. The Hirudinea deposit their fertilised eggs into a cocoon produced by their clitellum via the female genital pore (Figure 1), sliding the cocoon off their body at the anterior end (Govedich & Moser, 2015; Verdonschot, 2015).?Thus, they are oviparous, as most embryonic development occurs outside the body of the leech. In contrast, onychophorans can be oviparous, ovoviviparous (ie. fertilised eggs develop inside the female with simultaneous hatching and birth) or viviparous (ie. live young develop inside the female), depending on the species (Eriksson & Tait, 2012).?On the other hand, polychaetes and oligochaetes have completely different reproductive strategies, as they undergo external fertilisation (Verdonschot, 2015).?

Conclusion

In conclusion, while there are some surprising similarities between sexual reproduction in the Onychophora and the Annelida, especially in the Hirudinea, sexual reproduction in these two phyla is still very different overall. Merely comparing sexual reproduction in only one representative phyla from the Lophotrochozoa and Ecdysozoa can offer an insightful glimpse into the incredible diversity of sexual reproduction in the Protostomia.

Bibliography

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Govedich, F. R., & Moser, W. E. (2015). Clitellata: Hirudinida and Acanthobdellida. Thorp and Covich’s Freshwater Invertebrates: Ecology and General Biology: Fourth Edition, 1, 565–588. https://doi.org/10.1016/B978-0-12-385026-3.00023-1

Mackiel, A., & Dillon, H. (2021). Sexual Reproduction. Encyclopedia of Evolutionary Psychological Science, 7456–7462. https://doi.org/10.1007/978-3-319-19650-3_437

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Timm, T., & Martin, P. J. (2015). Clitellata: Oligochaeta. Thorp and Covich’s Freshwater Invertebrates: Ecology and General Biology: Fourth Edition, 1, 529–549. https://doi.org/10.1016/B978-0-12-385026-3.00021-8

Velvet worm. (2020, December 14). The Australian Museum. https://australian.museum/learn/animals/worms/velvet-worm/

Verdonschot, P. F. M. (2015). Introduction to Annelida and the Class Polychaeta. Thorp and Covich’s Freshwater Invertebrates: Ecology and General Biology: Fourth Edition, 1, 509–528. https://doi.org/10.1016/B978-0-12-385026-3.00020-6

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