Evidence for use of both capital and income breeding strategies in the mangrove tree crab, Aratus pisonii

Abstract

Two common strategies organisms use to finance reproduction are capital breeding (using energy stored prior to reproduction) and income breeding (using energy gathered during the reproductive period). Understanding which of these two strategies a species uses can help in predicting its population dynamics and how it will respond to environmental change. Brachyuran crabs have historically been considered capital breeders as a group, but recent evidence has challenged this assumption. Here, we focus on the mangrove tree crab, Aratus pisonii, and examine its breeding strategy on the Atlantic Florida coast. We collected crabs during and after their breeding season (March-October) and dissected them to discern how energy was stored and utilized for reproduction. We found patterns of reproduction and energy storage that are consistent with both the use of stored energy (capital) and energy acquired (income) during the breeding season. We also found that energy acquisition and storage patterns that supported reproduction were influenced by unequal tidal patterns associated with the syzygy tide inequality cycle. Contrary to previous assumptions for crabs, we suggest that species of crab that produce multiple clutches of eggs during long breeding seasons (many tropical and subtropical species) may commonly use income breeding strategies.

Figure 1

Proportion of Aratus pisonii that were vitellogenic (A) and ovigerous (B) on each Julian sampling date, showing a unimodal distribution consistent with a defined reproductive period.

Figure 2

Residual hepatopancreas mass of Aratus pisonii (after accounting for body mass) as a function of Julian sampling date. Symbols indicate sampling site, where circles are Round Island, triangles are Oslo, plus sign is North Causeway, and the multiplication sign is Pepper park. Gray shaded sampling dates indicate periods of new moon, while white indicates full moon.

Figure 3

Residual mass of individual Aratus pisonii eggs (after accounting for body size) as a function of Julian collection date. The heavy lines of each box plot shows the median, the box encompasses the first and third quartiles of the data, whiskers encompass 95% of the data, and circles indicate outliers that fall outside of this range. The dashed line is drawn to highlight decreasing egg mass through time.

Figure 4

Residual number of eggs per clutch in Aratus pisonii (after accounting for body size) as a function of Julian sampling date. Symbols indicate sampling site, where circles are Round Island, triangles are Oslo, plus sign is North Causeway, and the multiplication sign is Pepper park. Gray shaded sampling dates indicate periods of new moon, while white indicates full moon.

Figure 5

Residual gut mass (A) and gut width (B) in Aratus pisonii (after accounting for body size) as a function of Julian sampling date, proxies for the amount and quality of food consumed, respectively. Symbols indicate sampling site, where circles are Round Island, triangles are Oslo, plus sign is North Causeway, and the multiplication sign is Pepper park. Gray shaded sampling dates indicate periods of new moon, while white indicates full moon.