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Biology

Variety in the personal lives of triggerfish

 

The Paper: Ziadi-Künzli, F., Tachihara, K., 2016. Female defence polygyny and plasticity in the mating system of the demersal triggerfish Rhinecanthus aculeatus (Pisces: Balistidae) from Okinawa Island. Mar. Biol. 163, 27. doi:10.1007/s00227-015-2780-z

 

Lagoon triggerfish in Frankfurt's zoo. rhinecanthus aculeatus

Lagoon triggerfish in Frankfurt’s zoo.
rhinecanthus aculeatus. Credit: Joachim S. Müller

 

Introduction

Love can take many forms- triggerfish know what I’m talking about. Any living organism is (evolutionarily speaking) just trying to reproduce and preserve their genes in the world. Animals employ all sorts of strategies to attract mates and keep them happy enough to create successful offspring. Monogamy, polygamy, or promiscuity all have their benefits and drawbacks.

Fish vary in mating strategies, finding what works based on the traits of that species. Methods of mating differs as well as the amount of parental care needed. Sometimes males show off to attract mates or fight to earn a partner. Some species stick to one strategy while others are more apt to go for one or another based on the environment. The availability of a mate, food, or shelter can determine what mating strategy is best at that time.

A variety of reproductive strategies have been recorded in members of the triggerfish family (Balistidae). In some cases mothers do all the parental care, sometimes it is shared. Males can be territorial year round or only during breeding season. Some males are polygnyous (multiple females in a harem) while others opt for monogamy. This study investigated what ecological or population factors determine if a fish will be polygamous or monogamous.

 

Methods

The study took place Sesoko Island, Japan and followed one species of triggerfish in the area, Rhinecanthus aculeatus. Triggerfish on the nearby reef were monitored from October 2012 to November 2013. The researchers observed each fish in the area, identifying its sex, size and territory size. The location and nature of aggressive interactions between fish were noted and categorized as chasing, pursuing, or biting.

Mating success was observed in terms of female spawning events and later parental egg care. Male mating success was measured by the number of spawning events by the females they had primary or sole access to at the time. Mating status was described as polygynous, monogamous, or solitary.

Triggerfish. Gulf of Mexico, McGrail Bank. Credit: NURC/UNCW and NOAA/FGBNMS.

Another triggerfish species swimming through its reef habitat. Gulf of Mexico, McGrail Bank.
Credit: NURC/UNCW and NOAA/FGBNMS.

Results and Significance

Females had territories in the nearshore areas and were very near each other without overlapping. Each territory contained coral that could be used as shelter. Females were mostly either part of a harem or in a monogamous pair but several females maintained a solitary territory without a male living on it. The solitary females still laid and cared for eggs, but the reproductive partner was not clear. In all but one cases females were smaller than the male they were partnered with. Larger females had larger territories and more matings. Females were the more aggressive sex and fought with other females and males that were not in their reproductive group. New females were claimed by males within three weeks or established a solitary territory. Females in any of the three mating strategies did not have different numbers of matings.

The males defended territories that contained their corresponding females, up to four total. Smaller males were monogamous while larger ones had harems. Males with harems had more matings. Monogamous pairs occupied 30% of the territories while haremic groups occupied about 50%, the rest were made up of solitary fish.

Females appear to maintain their own territories and reproduce regardless of whether they have males on the territory or not or how many other females the male may be connected to. Males benefit by having a harem but are only able to do so as they become large enough to defend a larger territory encompassing multiple females. This study shows that mating strategies can differ based on the demographics of the population. Bigger females and bigger males appear to be most desirable and also most able to defend their territories or compete for mates. Long term observations like the ones used here help us understand how animal behaviors can change through time.

Human relationships are complex and looks like fish ones can be too!

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