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Sea urchin aggregation is a common phenomenon in coastlines. However, it remains controversial whether sea urchins form resource aggregations or behavioral aggregations in a non-spawning season. To clarify, we studied the aggregative responses to food and predators in the sea urchin Mesocentrotus nudus when high fitness areas (HFAs) were scarce versus sufficient. By taking the occupied area of each sea urchin (test diameter + spines = 4.5 cm) as a square (4.5 cm × 4.5 cm), we set scarce HFAs for the sea urchins in Experiment 1 (the squares of HFAs: the area occupied by experimental sea urchins = 1:1) and sufficient HFAs for the sea urchins in Experiment 2 (the squares of HFAs: the area occupied by experimental sea urchins = 2:1). If M. nudus form resource aggregations, they would aggregate passively under the scarce HFAs conditions, but not in the sufficient HFAs conditions. Conversely, if M. nudus form behavioral aggregation, aggregation would occur in both scarce and sufficient HFAs. The present results showed that in the scarce HFAs, M. nudus in the food and predator groups were significantly closer to the food and further from predators, and had significantly more aggregated numbers in HFAs than those in the control group. Sea urchins did not aggregate in response to food or predators under the sufficient HFAs, although significantly more sea urchins of the experimental group was found in HFAs than that of the control group. Sea urchins (at least M. nudus) form resource aggregations that are driven by the scarce HFAs. This provides valuable information into the mechanisms of the aggregation of sea urchins.

Understanding the ecological role of shelters is greatly hampered by the scarcity of long-term laboratory experiments on the trade-off between fitness benefits and costs. This lack probably leads to an underestimation of the negative and/or positive effects on behaviors and growth of marine invertebrates in benthic ecosystems. Although our previous study revealed a significant effect on fitness-related traits of Glyptocidaris crenularis after 31 months, the present study extended it and investigated fitness benefits and/or costs of long-term sheltering on sea urchins to over 7 years. The present long-term study suggests that the previously reported reduction in feeding rate probably resulted from a reduction in reflexive feeding motions (Aristotle’s lantern reflex) rather than changes in foraging behavior. Actively seeking sheltering behavior was negatively impacted in individuals with continuous access to shelters. However, covering and righting behaviors did not differ in sheltered sea urchins, indicating that these behaviors are maintained to escape from adverse environments regardless of shelter. Body size of sea urchins in the group with shelters was significantly lower than those without shelters after 7 years. Weights of gonads and gut were not significantly different after 7 years despite previous observations of differences after ~2.5 years. The present study provides valuable information on the trade-off between fitness benefits and costs to sea urchins residing in shelters. However, the present study is only a laboratory investigation for one urchin species (G. crenularis) which does not consider the complexity of natural environments. Field studies should be carried out with G. crenularis and other sea urchin species, before a more universal conclusion can be drawn.

Small sea urchins Strongylocentrotus intermedius (1–2 cm of test diameter) are exposed to different environments of light intensities after being reseeded to the sea bottom. With little information available about the behavioral responses of S. intermedius to different light intensities in the environment, we carried out an investigation on how S. intermedius is affected by three light intensity environments in terms of phototaxis, foraging and righting behaviors. They were no light (zero lx), low light intensity (24–209 lx) and high light intensity (252–2,280 lx). Light intensity had obvious different effects on phototaxis. In low light intensity, sea urchins moved more and spent significantly more time at the higher intensity (69–209 lx) (P = 0.046). S. intermedius in high light intensity, in contrast, spent significantly more time at lower intensity (252–690 lx) (P = 0.005). Unexpectedly, no significant difference of movement (average velocity and total distance covered) was found among the three light intensities (P > 0.05). Foraging behavior of S. intermedius was significantly different among the light intensities. In the no light environment, only three of ten S. intermedius found food within 7 min. In low light intensity, nine of 10 sea urchins showed successful foraging behavior to the food placed at 209 lx, which was significantly higher than the ratio of the number (two of 10) when food was placed at 24 lx (P = 0.005). In the high light intensity, in contrast, significantly less sea urchins (three of 10) found food placed at the higher light intensity (2,280 lx) compared with the lower light intensity (252 lx) (10/10, P = 0.003). Furthermore, S. intermedius showed significantly longer righting response time in the high light intensity compared with both no light (P = 0.001) and low light intensity (P = 0.031). No significant difference was found in righting behavior between no light and low light intensity (P = 0.892). The present study indicates that light intensity significantly affects phototaxis, foraging and righting behaviors of S. intermedius and that ~200 lx might be the appropriate light intensity for reseeding small S. intermedius.

Transgenerational effects are important for phenotypic plasticity and adaptation of marine invertebrates in the changing ocean. Ultraviolet-B (UV-B) radiation is an increasing threat to marine invertebrates. For the first time, we reported positive and negative transgenerational effects of UV-B radiation on egg size, fertilization, hatchability and larval size of a marine invertebrate. Strongylocentrotus intermedius exposed to UV-B radiation showed positive transgenerational effects and adaptation on egg size, hatching rate and post-oral arm length of larvae. Negative transgenerational effects were found in body length, stomach length and stomach width of larvae whose parents were exposed to UV-B radiation. Sires probably play important roles in transgenerational effects of UV-B. The present study provides valuable information into transgenerational effects of UV-B radiation on fitness related traits of sea urchins (at least Strongylocentrotus intermedius).