Sex with Flexible Partners #2

During the first installment in this series on reproductive strategy (available here) the topic of hermaphroditism was introduced and a quick summary of simultaneous hermaphroditism was provided. Moving forward from that discussion, this short essay will provide a synopsis of a category of hermaphroditism referred to as “sequential hermaphroditism.”

Recall from the first essay that simultaneous hermaphroditism refers to organisms that can function as both a female and a male during a single mating session. These individuals have the ability to produce both ova and sperm and, with social dynamics temporarily laid aside, can essentially mate with any adult member of its population. Using this abbreviated definition of simultaneous hermaphroditism, notice that the word “function” has a specific meaning in this context; not only does it imply the presence of reproductive organs, but it also requires the action of using those organs during mating - this is a minor, but important point.

For example, a “functional female” is an organism that undertakes the female role within the context of its social hierarchy and produces only ova during a single mating session; it exhibits the outward coloration and morphology of a female member of its species. If that functional female organism should then be taken into a laboratory, dissected by a biologist and found to have male gonad tissue as an addition to its female anatomy, it would still be considered a functional female, even though male tissues were found to be present. The same applies for a “functional male;” it would still be considered a functional male even if female reproductive tissues were later discovered. This is a significant delineation when discussing hermaphroditic organisms because frequently sexual characteristics are very difficult to distinguish. For example, on close examination of fishes belonging to the genus Lythrypnus they are often found to possess reproductive tissues of both females and males; this may erroneously lead some to conclude that the fish are simultaneously hermaphroditic, but in fact individuals function entirely as one of the two sexes when in the wild; it’s either a male or a female, not both. If at some point Lythrypnus changes its function from one sex to the other, it still wouldn’t be classified as a simultaneous hermaphrodite – because it still wouldn’t function as two sexes simultaneously – rather, it would be said to be “Sequentially Hermaphroditic.”

[Lythrypnus]

Sequential hermaphroditism can occur in one of two ways; either a functional male can change to a functional female, or vice versa, a female can change to a male. Organisms which are born male and later become female are said to exhibit “protandry.” Here, the prefix “proto-” is derived from Latin and translates as “first;” the root word “andro” refers to androgen, a steroid hormone responsible for masculine characteristics. Protandry therefore means “first a male.” Correspondingly, “protogyny,” means “first a female” and is used when describing the trait of a female undergoing sex change to a male. Both protandry and protogyny are used as reproductive strategies in a variety of hermaphroditic organisms. Sometimes sex changes occurs at a predetermined age in conjunction with the normal growth and maturation process, in other cases this change can occur as a response to environmental or social triggers. With the polychaete worm Ophryotrocha puerilis, sex change occurs as a strategy for dealing with the expense of creating ova, the same expense that lead to the compromise of egg trading in the Hamlet fish (Hypoplectius) mentioned in the first essay.

Recall from the Hamlet fish discussion that because of anisogamy - a size difference between male and female gametes - the cost of producing ova is greater than that associated with making sperm; eggs are expensive, sperm are cheap. Bearing this in mind, it follows that the females of a species with greatest access to resources will tend to grow larger and, in turn, be able to produce more eggs than females with more-limited access to resources. The larger the female, the greater fecundity she exhibits. Monogamous polychaete worms take this lesson to heart, within the male-female pair bond the female is the larger of the two annelids. The male polychaete typically develops more slowly and is normally smaller in size; therefore the assigning of burdensome egg development duties to the larger member of the duet is a better reproductive strategy. At least until the male hits a growth spurt!

[Ophryotrocha puerilis]

Although the male polychaete is a slow starter in regards to growth, at a certain point in adulthood he really hits his stride and rapidly outsizes the female. This creates a reproductive opportunity for the pair-bond. With increased size, the male is now better positioned to handle egg development. There’s only one problem – he’s a male! Not wasting any time, the male changes to a female and takes up the task of producing female gametes. In concert with this change, the old female, now the smaller of the two, changes to a male and begins to produce sperm. This double sex change improves the sexual efficacy of the pair and results in more offspring produced. However, there is one additional complication with this highly plastic stratagem…

Now that the original female has changed to male and is no longer responsible for egg production, her primed-up metabolism, which up to this point had to supply energy to both her and her eggs, causes the fish to really “pack on the pounds.” In time, the original female, which is now male, is once again larger than the egg-producing female of the pair-bond...

No worries though, the pair just change back to their original sexes – that’s what being a flexible partner is all about.

In talking about sequential hermaphroditism strategy in relation to body size, the polychaete worm exhibited a relationship known as the “size-advantage hypothesis.” The size-advantage hypothesis explains that when an organism reproduces best as one sex whilst either “young and small” or while “old and big,” sex changes can occur when transitioning between these two categories. The male worm benefited from being young and small, but as it got older and larger this benefit was reduced; in order to benefit from being older and bigger it had to change sexes – to a female.

It should be mentioned that like the sequential variety, some simultaneous hermaphrodites utilize this size-advantage relationship as well. The freshwater snail Helisoma trivolvis makes use of it every time it encounters a potential mate. Unlike the Hamlet fish, Helisoma doesn’t share in the costs of egg production, when two snails meet whichever one happens to be the larger takes on the role of the female for that particular mating period.

[Helisoma trivolvis]

During the next Sex with Flexible Partners installment, the size-advantage hypothesis will be tested in situations for which the “eggs are expensive, sperm are cheap” rule doesn’t necessarily apply and the topic of hermaphroditism as a reproductive strategy will be drawn to a close.

P MUNDAY, P BUSTON, R WARNER (2006). Diversity and flexibility of sex-change strategies in animals Trends in Ecology & Evolution, 21 (2), 89-95 DOI: 10.1016/j.tree.2005.10.020

Kohei Ohta, Mayumi Hirano, Takayuki Mine, Hiroshi Mizutani, Akihiko Yamaguchi, Michiya Matsuyama (2007). Body color change and serum steroid hormone levels throughout the process of sex change in the adult wrasse, Pseudolabrus sieboldi Marine Biology, 153 (5), 843-852 DOI: 10.1007/s00227-007-0856-0

C. G. Norton, A. F. Johnson, R. L. Mueller (2008). Relative size influences gender role in the freshwater hermaphroditic snail, Helisoma trivolvis Behavioral Ecology, 19 (6), 1122-1127 DOI: 10.1093/beheco/arn099