Biologically rich rock-pools (RPs) are prominent component of rocky intertidal habitat and this study elucidates the factors affecting the phytoplankton distribution from high-tide (HT), mid-tide (MT) and low-tide (LT) RPs (Anjuna rocky shores, Goa, India), during the summer season. Results revealed that the RP location determines the nature of phytoplankton assemblage. MT-RPs and LT-RPs supported rich microalgal diversity (species number/morphological shapes) than HT-RPs. However, the cell abundance was higher in latter than former RPs. In frequently flushed MT-RPs and LT-RPs, pennate diatoms (Pseudonitzschia, Nitzschia, Thalasionema, Navicula, and Licmophora) representing elongated shapes dominated, whereas in stagnant HT-RPs spherical/combined shaped dinoflagellates (Amphidinium carterae sensu stricto and Bysmatrum gregarium) dominated. Interestingly, even though the taxonomic assemblages changed temporally the shape dominance remained consistent suggesting shape as a robust morphological trait. Additionally, the study also revealed the existence of species-specific allelopathy on other microalgae by Amphidinium (potential harmful algae) indicating a type of adaptive strategy, in addition to eurytolerant capabilities, for its predominance in HT-RPs, which experiences varying environmental (light, temperature, and salinity) conditions.

$\bf{Highlights}$

$\bullet$ Investigated factors influencing distribution of phytoplankton (species and shape-wise) in intertidal rock pools (RPs) of Anjuna rocky shores, Goa.

$\bullet$ Location of RPs determined nature of phytoplankton community, i.e., less diversity and high abundance in high-tide RPs than mid/low-tide RPs.

$\bullet$ Spherical-shaped dinoflagellates and elongated shape diatoms dominated phytoplankton community of high and mid/low tide RPs, respectively.

$\bullet$ Even though the species composition changed temporally the shape dominance remained consistent suggesting the latter as a robust morphological trait.

$\bullet$ Dominant dinoflagellate, Amphidinium caterae sensu stricto, also showed species-specific allelopathy on other phytoplankton.