The existence of social monogamy (when a male and a female live together) has long puzzled researchers since both sexes usually gain more reproductive success by mating with multiple partners. This raises the question: under what conditions will monogamy provide reproductive advantages? The proposed research seeks to test three hypotheses that may explain the evolution of social monogamy in the mantis shrimp Pullosquilla litoralis: even spatial distribution of females, equal sex ratio between males and females, and the need for biparental care. It also tests the effects of even female spatial distribution and sex ratio on genetic monogamy (how many males sire each egg clutch) of P. litoralis to discover, how social mating systems and genetic mating systems interact to shape reproductive behaviors?. P. litoralis is a mantis shrimp that lives in pairs in burrows in sandy marine substrates where both males and females care for eggs by aerating them and removing fouled eggs. The proposed research will be conducted through field studies and experiments at Lizard Island Research Station, Australia. Field studies and experimentation in the marine lab will examine the effects of female spatial distribution and sex ratio on social monogamy in P. litoralis. Paternity testing will elucidate the effects of these factors on genetic monogamy. Laboratory experiments will also examine the effects of biparental care on the reproductive success of male and female P. litoralis. Finally, this research will contribute to management strategies for the conservation of marine crustaceans As charismatic invertebrates, mantis shrimps are excellent ambassadors for the conservation of coral reefs. Co-PI Wright will share her research with the public through K-12 classroom visits, a field blog that will be hosted by the UC Museum of Paleontology in coordination with the Understanding Science initiative, and undergraduate mentorship.

Project Report

Scientific Findings and Broader Impacts Although social monogamy and biparental care have been extensively studied in birds, mammals, and fish, few studies of these phenomena have been conducted on invertebrate species. Social monogamy is characteristic of several ecologically important marine crustaceans, while biparental care has only been characterized in a single genus of stomatopod crustacean (a.k.a. mantis shrimp), Pullosquilla. Pullosquilla species are small stomatopods (~1.5 cm) that live in heterosexual pairs in sandy burrows in shallow sand flats in the Indo-Pacific Ocean. Previous research indicates that male and female Pullosquilla provide equal amounts of care for their egg clutches by guarding them, oxygenating them, and removing fouled eggs. When food is abundant, pairs may double-clutch by producing a second clutch of eggs before the first clutch has hatched. In this case, males and females expend approximately twice as much time providing care to both clutches of eggs as they do for a single clutch. This project examined the effects of biparental care on the survival and development of stomatopod eggs to better understand the evolution and maintenance of biparental care. It also investigated the demography of stomatopods living in a sand flat to determine the prevalence of social monogamy in the population. All research was conducted at Lizard Island Research Station in the Great Barrier Reef, where Pullosquilla are abundant and easy to access in a shallow lagoon by SCUBA diving, between October 2011 and December 2011. An experiment was conducted in which Pullosquilla thomassini egg clutches were assigned to one of four parental care conditions: no parental care, paternal care only, maternal care only, or biparental care. We found that in the absence of any care, egg clutch survival and growth decreased. However, neither the number, nor the sex of the care providers had a significant effect on changes in egg clutch mass. Parental care conditions did not affect ovary size, the total number of eggs in a clutch, or egg size. Thus, while parental care increases reproduction of offspring in Pullosquilla, care by a single parent of either sex is sufficient to achieve this goal. These results suggest that biparental care is not evolutionarily maintained in P. thomassini by increasing the number of offspring hatching in an egg clutch. Instead, it is likely that biparental care is evolutionarily maintained by increasing the rate of egg clutch production and facilitating double-clutching. Future studies will investigate the role of biparental care in facilitating double-clutching to determine if this may be an alternative mode of evolutionary maintenance of biparental care. Preliminary research funded by this grant also suggests that Pullosquilla may be genetically monogamous (egg clutches are sired by a single male). Further research will investigate whether this pattern holds for other Pullosquilla populations. The findings of this project emphasize the importance of studying a diversity of taxa and environments when trying to understand the evolution of important behavioral traits. For example, it is often assumed that when biparental care is widespread in a species, it increases the viability of the current brood of offspring. However, in P. thomassini there is no evidence that biparental care increases either the survival or development of embryos. Additionally, the evolution of social monogamy in many animals is attributed to a need for biparental care. Stomatopods appear to provide a counter-example in which social monogamy likely facilitated the evolution of biparental care and other forms of paternal effort. Educational Broader Impacts During this project, co-PI Wright shared her research with students at Carl B. Munck Elementary School in the Oakland Unified School District, a school where 58% of students are eligible for a reduced-fee or free lunch. Co-PI Wright involved them in her field season with a series of six in-person lessons using SCUBA diving examples to teach basic principles of physics and marine biology. She created three video blogs during her field season at Lizard Island Research Station and communicated with students via email, answering their questions. By doing this, co-PI Wright endeavored to increased student enthusiasm for science and marine conservation. As a young female scientist, she also acted as a role-model to demonstrate that scientists are diverse and approachable and that science may be a realistic career goal for students. This project also provided training opportunities for several undergraduates and a recent post-graduate. Steve Doo (post-grad from UC Davis) and Leslie Hillman (undergraduate from Duke) were volunteer field assistants with co-PI Wright at Lizard Island Research Station and learned marine survey techniques and experimental design methodology. Co-PI Wright mentored Ms. Hillman as she conducted an independent research project on fish predation on stomatopod crustaceans. UC Berkeley undergraduate Doriane Weiler assisted co-PI Wright in conducted paternity testing on specimens collected in the field, learning molecular biology techniques that will aid her in pursuing a career in medicine.

National Science Foundation (NSF)
Division of Integrative Organismal Systems (IOS)
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Michelle M. Elekonich
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University of California Berkeley
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