The masquerade game: marine mimicry adaptation between egg-cowries and octocorals

Background. Background matching, as a camouflage strategy, is one of the most outstanding examples of adaptation, where little error or mismatch means high vulnerability to predation. It is assumed that the interplay of natural selection and adaptation are the main evolutionary forces shaping the great diversity of phenotypes observed in mimicry; however, there may be other significant processes that intervene in the development of mimicry such as phenotypic plasticity. Based on observations of background mismatching during reproduction events of egg-cowries, sea snails of the family Ovulidae that mimic the octocoral where they inhabit, we wondered if they match the host species diversity. Using observations in the field and molecular systematics, we set out to establish whether the different egg-cowrie color/shape polymorphisms correspond to distinct lineages restricted to specific octocoral species. Methods. Collection and observations of egg-cowries and their octocoral hosts were done using SCUBA diving between 2009 and 2012 at two localities in the Tropical Eastern Pacific (TEP), Malpelo Island and Cabo Corrientes (Colombia). Detailed host preference observations were done bi-annually at Malpelo Island. We analyzed the DNA sequence of the mitochondrial genes COIand 16S rDNA, extensively used in phylogenetic and DNA barcoding studies, to assess the evolutionary relationship among different egg-cowrie colorations and morphologies. Results. No genetic divergence among egg-cowries associated to different species of the same octocoral genus was observed based on the two mitochondrial genes analyzed. For instance, all egg-cowrie individuals from the two sampled localities observed on 8 different Pacifigorgia-Eugorgia species showed negligible mitochondrial divergence yet large morphologic divergence, which suggests that morphologies belonging to at least two sea snail species, Simnia avena(=S. aequalis) and Simnialena rufa, can cross-fertilize. Discussion. Our study system comprised background-matching mimicry, of the masquerade type, between egg-cowries (Simnia/Simnialena) and octocorals (Pacifigorgia/Eugorgia/Leptogorgia). We observed mimicry mismatches related to fitness trade-offs, such as reproductive aggregations vs. vulnerability against predators. Despite the general assumption that coevolution of mimicry involves speciation, egg-cowries with different hosts and colorations comprise the same lineages. Consequently, we infer that there would be significant tradeoffs between mimicry and the pursuit of reproductive aggregations in egg-cowries. The findings of this study not only contribute to the understanding of the evolution of mimicry in egg-cowries, a poorly studied group of marine gastropods, but also to the development of a new biologically meaningful board game that could be implemented as a learning tool.

presented here summarizes the valuable input received from authors and colleagues who played it multiple times. In brief, this game envisions a biologically relevant driving process that promotes its understanding via the player's exposure to natural processes in ecology and evolution.
The chosen name was 'The Masquerade Game' due to the type of mimicry observed in egg-cowries and its potential ludic meaning concordant with our results. In addition to the cards, we use a 6x6 square board (equally divided into two territories, colonized by two sea fan types), two dice of different color, numerous cowry-like pieces of two different colors (e.g., beans), and two different toy-like pieces to represent the two kinds of predators. Players choose a side (a sea fan population of a specific color) and are given the same amount of randomly chosen colored "egg-cowries" (six) to start with. The game begins with each player placing the cowries on the board and ends when either all sea fan squares (on one side) have been colonized by at least one cowry of their matching color or when only cowries of one color remain.
Since the goal was to develop a game that faithfully represents the natural system, we included several natural events in the dynamics of the game: group migration of the adult egg-cowries for reproduction, hatching of the resulting eggs, mismatched cowries being more susceptible to predators, as well as external factors such as the effects of sea fan mortality (disease and invasive species). Each turn cards assigned to natural events are drawn, and dice specify the location of the events, if required (each die for the x or y position on the board), except for predators, which have a physical presence on the board. The goal of the game is for each player to fill each and all of the squares of their color (Color 1 or color 2) with at least one cowry of the same color. There can S2 be no more than six cowries in one particular square, following the idea that sea fans are a limited resource and cannot sustain overpopulation.
The element of chance is an important factor, but to make the game more interesting we allowed players some control over certain outcomes, so that strategy could be employed. We did this in various ways. First, we allowed each player to choose which egg-cowry to move on each turn and the direction of its movement. Also, when a reproduction card is drawn, the player can choose which adults move for reproduction if two or more are equidistant from the reproduction site. We also allowed players to choose the predator´s path on the board during their turn. Finally, when overcrowding occurs due to hatching of the egg-cowries, players can choose which new cowries to remove (which results in the removal of mismatched recruits) and where to place offspring within a given region. A kit to play the game is available (figures S1A and table S1). However, it is important to us that the game be easily accessible; for that reason it can be played with six sided dice and a standard 52-card deck and a printed board (Table S2) or even a chess board ( Figure S1B) (using part of the board). A summary of the rules is presented in the Table S3 and Card kit at the end.

Remarks.
We developed a board game whose main scientific objective was to support the idea that there are conditions for which our hypothesis results in a viable process. Events with high probability were chosen to be those that occur in a regular and almost predictable manner, and are the main drivers of population dynamics including cowry movement, reproductive aggregation, recruitment onto a coral host of the larvae as well as predation. Less probable events are of the disturbance type, which are less frequent and may have catastrophic results on the population and include disease, invasive species as well as a super-predator.
We were actually surprised that playing the game was actually entertaining and included several moments of excitement for people we invited to play. The game lasted an average of a half hour, and the standard distribution of that time was relatively low, with some outliers, especially in the early versions of the game. As expected, the game evolved towards an equilibrium in which either population of egg-cowries colonized their respective sea fan with a few mismatched cowries. This manual simulation based on a game supports the idea that a small number of mismatches due to reproductive aggregations can be maintained over time despite the existence of predators. Two implementations of the game in action can be seen in figures S2 A and B.
Creating a board game based on biologically meaningful rules was not the only motivation for developing the masquerade game. We also wanted to create a game that could go beyond its original inspiration, i.e., it can be played with non-biological subjects and yet be biologically meaningful. We aimed to provide implicit and emergent cognition on natural history and selection as well as the element of chance in nature.
Given the difficulty of teaching mainstream biology in some schools (Yates & Marek, 2014), this kind of game S3 can be an aid to the biology curriculum in the same fashion that computer games and other ludic activities are (Sadler et al., 2013). Our goal is not evaluating its role in formal education, instead, in this context, the board game is similar to a very basic simulation of population dynamics on the one hand, and on the other hand it is an opportunity to intertwine outreach activities in the scientific process, as opposed to it being an afterthought.
Inclusion of educational material with a scientific article may contribute to bringing young students closer to the scientific process first hand and is consistent with the current trend in many journals to include sections such as an "Author Summary" for non-specialists.
We believe that an integration of the outreach activities in the scientific process can result in a better integration of the community in the scientific process, an improved understanding of science by the community, and potentially better science since the community gets an opportunity to contribute ideas as well as knowledge.
We firmly believe that natural history provides very good examples to promote the outreach of ecology and evolution, particularly when these examples are drawn from the student´s surroundings. Furthermore, a game whose outcomes are organically discovered by students is a form of active learning that could be used as a platform for teaching the scientific method and concepts in evolution, ecology, and conservation, which may support the teaching of mainstream biology in some regions where it competes with other interpretations (Long, 2012).
A game may be as useful as simulations to explore different outcomes of an evolutionary process.
Simulations are important tools in that they can explore the parametric space of interest in a short amount of time, but have fundamental limitations: first, since the rules are set from the beginning, a computer simulator cannot question them or come up with new strategies. This intentionality in human players-that so far is difficult to program-can lead to players asking questions about the assumptions, the rules, think of alternative processes and other key aspects of the process under study that a computer would not be able to come up with without an actual understanding of the phenomenon. While designing this game we had to go through multiple iterations of the rules, and this process forced us to think about many possible natural events that can affect the game-and the natural history-in a way that makes it playable and relevant. Gaming can then be thought as a complementary tool to predict and model a natural system; it can be used to explore the large conceptual and parameter space of the natural world and focus on a part of it. These are natural history events that occur in a regular and almost predictive manner, which are the main drivers of population dynamics.
These events alter the number or position of cowries on the board, so helping to win the game but also to break down the uniformity that the players seek to win the game. Some cards require the player to throw two dice of different colors. One will represent the position on the x-axis (x-color dice) and yaxis (y-color dive) where the event will happen.
Reproductive aggregation 9 This is the major finding of this study, all types of egg-cowries color gather for reproduction and ovoposition into encapsulated eggs on the sea fan surface. As in other ovulids, both males and females exhibit gregarious behavior an gather on a single sea fan colony after copulating with multiple males briefly earlier (Nowlis, 1993).
Throw the dice to determine the position; the closest cowries will congregate in that square.
The number of cowries to participate is shown on the card.
Afterwards, they settle on coral hosts.
If the player has a reproduction card, (they can play this to ensure all offspring are their color) the player will put cowries on that square and/or any adjacent square.
If the number of cowries in any square is more than 6, any additional cowries die; but the order in which the cowries are placed on the squares is chosen by the player playing the card. The player moves the predator (from it´s previous position).
The number of squares are shown on the card. The predator cannot move diagonally. In each square that it visits during the movement it will eat any cowries whose color is not matched to the background coral´s color.

Movement 16
Here, it is assumed that eventually cowries can move for no particular reason and look for another sea fan host.
Players move one cowry at every turn, and here they can move any of the cowries of their own color, which increases the chances for winning the game; move freely (one square, forward, back sideways or diagonally).

S6
Less probable events

Total of 9
These are events of the disturbance type, which are less frequent and have catastrophic results on the population.
These movements, as in an intermediate disturbance scenario, will shake the fate of game drastically. Moreover, these cards can delay the player that was close to win and it introduces evenness between players. The player who gets this card must throw two dices of different color to determine the position as above.
Super predator 2 The longnose hawkfish (Oxycirrhites typus) is a predator that also has a masquerade type background-matching mimicry for the same kind of sea fan (Pacifigorgia). It is closely associated to gorgonians and black corals are given the same amount of "cowries" (6) to start with. The game begins with each player drawing cowries S11 randomly (with no control over their color) and ends when only one color remains (the winner) or when one seafan has been completely colonized by the cowries of matching color. We used the cards to assign these events as well as dice to specify the location if required (each dice for x or y axis of the board), except in the case of the predators, which had a physical presence on the board (the predator cannot move diagonally). The goal of the game is for each player to fill each and all of the squares of their color (Color 1 or color2) with at least one cowry of the color of their squares. There can be no more than six cowries in one particular square. -Spades are used as predator cards: The number on the card shows how many steps the predator can take, eating all mismatched cowries in it´s path.
-Hearts are used as reproduction cards: The number on the card representing the number of cowries to take part (and number over 6 will result in just six cowries congregating).
The dice are rolled to show position, and the closest cowries congregate there.
If all cowries are the same color the player with that color keeps the card.
-Clubs are used as recruitment cards: The number on the card representing the number of cowries added, the dice are rolled to show position, and cowries are placed there or in adjacent squares, if more than six cowries are in one square the last ones added will die (be removed)was it the last ones added or did the player have control over which ones to remove?
If the player has a reproduction card they may play it and add cowries of their own colour, if not cowries are drawn randomly.
-Diamonds are used for the less probable events: 9 and 10 can be used for the super predator.
The dice are rolled to determine the position of attack and then the player chooses a corner of the square and the predator attacks all cowries adjacent to this (up to 4 squares)any cowries in these square are removed from the game board.
6,7 and 8 can be used as invasive species cards.
The dice are rolled to show a position, all cowries must move to adjacent squares, if there are more than 6 cowries in one square the most recent arrivals die (are removed).
2,3,4 and 5 can be used as Sea Fan disease cards The dice are rolled and all cowries in that square are removed.

Table S2
Summarized playing rules of the Masquerade game Each player should place six randomly chosen cowries on each their side of the board, A predator on the center of the board and a super predator (outside the board waiting) Each player then throws a die, and the player with the highest number gets to start; the die can be thrown again to decide the color that each player chooses, in case of disagreement.
In each turn a player: Moves one of his/her cowries by one space Draws a card (from the "events" cards) and follows the instructions of the card. These include:

Spawning
Movement of the predator Catastrophic (also favorable) events such as pollution, disease, etc… Additional movement The objective of the game is for each player to fill each and all of the squares of their color (Color 1 or color2) with at least cowry of the color of their squares.
There can be no more than six cowries in one particular square.
Make sure you read the paper before you play! Enjoy!