Ontogenetic niche specialization of the spider crab Libinia ferreirae associated with the medusa Lychnorhiza lucerna

Abstract

Symbiotic relationships in marine environments are not fixed and can change throughout the animal's life. This study investigated the ontogeny of symbiosis of the spider crab Libinia ferreirae with the host medusa Lychnorhiza lucerna. We described the type of relationship, the temporal correlation among species, and food habits. More than 50% of the sampled crabs were symbionts, most in early life stages. The highest number of crabs found in a single medusa was 11. Symbiosis was observed throughout most of the year but was more evident in warm periods. The crab has many benefits in this relationship with a medusa. One is the use of food resources captured by the medusa, primarily copepods. Because the crab steals the medusa's food, it is a kleptoparasitic relationship. There is a niche partition between symbiont and the free-living crabs as they occupy different habitats and use nonoverlapping food resources. Previous research reported that symbiosis first developed during the crab's last larval phase (megalopa) when crab and medusa are in the same habitat. Observation of the crab's behavior shows that symbiosis occurs when the crab can grab to the medusa when the host touches the sea bottom. The crab also took advantage of water currents, releasing itself from the substrate and then drifting toward the medusa. The symbiotic relationship that crabs have with the medusa provides then with a nursery, food resources, shelter, dispersion, and decreased competition with free-living adult crabs, all essential for the crab's survival.

Keywords: behavior; development; kleptoparasitism; megalopa; niche segregation.

Figure 1.

Development stage of the crab L. ferreirae. (A) Megalopa (zooplanktonic stage); (B) Juvenile individuals (N = 9, median 2.69 ± 0.13 mm CW) found associated with a single medusa specimen (167 mm UD) (notice the whitish coloration of the juvenile crab); (C) A benthic adult crab not associated with a medusa (notice the brownish coloration and the presence of epibionts on the carapace).

Figure 2.

Symbiotic relationship between symbiont crab and medusa host. (A) Abundance of L. lucerna medusa without symbionts and with L. ferreirae crab symbionts, in monthly collection from February 2013 to May 2014; (B) Relationship between the number of symbiont crabs (N = 311) in the host and the medusa’s UD (N = 128). X-axis values are the size class midpoint; (C) Number of symbiont crab by CW (mm) according to association abundance (1–11 crabs per medusa) in each medusa host.

Figure 3.

Crab density related to the size of CW of symbiont and free-living L. ferreirae crabs. The dotted line indicates the initial size transition to adult (around 35 mm CW), according to Gonçalves et al. (2017b).

Figure 4.

Symbiont crab and medusa host monthly size. (A) Monthly mean range of CW of L. ferreirae crabs associated with the medusae L. lucerna. (B) Monthly mean range of the UD of the medusae L. lucerna. (Median: percentile 25% and 75%; Min–Max, minimum–maximum values).

Figure 5.

Libinia ferreirae and L. lucerna correlation. (A) Relationship between UD and CW of associated crabs; (B) Relationship between the medusa’s mass and the crab’s mass.

Figure 6.

Percentage of food items found in the stomachs of L. ferreirae crabs in distinct life habitats. N= Number of individuals.

Figure 7.

Food habits of the crab L. ferreirae in distinct life habitats. These data were analyzed by NMDS. Circles—crabs associated with medusa, triangles—free-living crabs. Gray fill: adult crabs, white fill: juvenile crabs.

Figure 8.

Sequence of the association behavior between the crab L. ferreirae and the medusa host L. lucerna. This association occurs approximately in the first minute after contact with the medusa. Time presented in hh:mm format. (A) Introduction of medusa into the aquarium with 5 crabs. (B) Crabs notice the host. (C) First crabs are associated after approximation to medusa. (D) 2 crabs associated and another crab trying to associate—notice the behavior from the crab that are not in association by lifting the chelipeds in the attempt to attachment to the medusa. (E) Crab start drifting. (F) 1 crab during the drifting behavior in the attempt to associate with the medusa host. (G) 1 crab associated with the medusa. (H) >1 crab associated with a medusa. More details in the Supplementary Video 3.

Figure 9.

Crab agonistic behavior during association with the medusa host L. lucerna. (A) 2 L. ferreirae crabs in the medusa’s umbrella and tentacles during the agonistic behavior. (B) During agonistic behavior 1 crab opened the carapace of another, killing it in the process. (C) Dorsal view and (D) ventral view of the killed crab.