Population Structure and Relative Growth of the Marbled Crab (Pachygrapsus marmoratus) in the Aegean Sea (Turkey) Fikret Öndes 1 & Onur Karadal 1 Received: 5 March 2020 /Revised: 18 June 2020 /Accepted: 22 June 2020 # Springer Nature Switzerland AG 2020 Abstract Although crab Pachygrapsus marmoratus, is one of the common species in the intertidal zones of the Aegean coasts of Turkey, its’ population structure has not been investigated to date. This study aimed to provide information on population characteristics of Pachygrapsus marmoratus, including the sex ratio, size distribution, spawning period, relative growth and catch per unit effort (CPUE). The samples were collected by hand monthly between January 2018 and December 2018, from the rocky shores of Izmir, Turkey. The results indicated that males were dominant except for the months of July, August and September. The CW (carapace width) of female and male crabs ranged from 5.5 to 36.0 mm and 5.5 to 40.0 mm, respectively. The berried (egg- carrying) females were found in May, June and July and their CW varied from 13.0 to 27.0 mm. Only 0.8% of the P. marmoratus population was shown to be infected with the parasite Sacculina sp. The CPUE of P. marmoratus significantly changed monthly and there was a significant correlation between the CPUE and seawater temperature. Concerning the relative growth, the carapace width-cheliped length relationship in males and carapace width-abdomen width relationship in females showed positive allom- etry. Males were significantly heavier in comparison to females of the same size. Future survey designs would need to investigate whether spatial differences in the population characteristics and size at maturity occur in the Aegean coasts of Turkey. Keywords Pachygrapsus marmoratus . Ecology . Catch per unit effort . Allometry . Eastern Mediterranean Introduction Rocky shores support ecological functions including nursery provision, spawning area, migration, and reproductive areas (Seitz et al. 2014). In addition to their ecological importance, rocky shores provide many opportunities to humans including leisure, relaxation, and their fauna and flora members are used for food, bait and ornamental aims. However, as in other parts of the world, the intertidal ecosystems of the Mediterranean have been threatened for the last several decades by increased human activities, such as poaching, habitat destruction, pollu- tion, and global warming (Milanese et al. 2011; Marchini et al. 2019). Thus, it is important to protect and monitor the fauna and flora of these ecosystems. One of the important inhabi- tants of intertidal areas are grapsid crabs, which generally exhibit high abundance and intense activity. They also influ- ence the sessile community owing to the foraging activities and contribute to the energetic flow, because they are also consumed by some vertebrate species such as gulls (Flores and Paula 2002a). The main prey of grapsid crabs are algae, mussels, limpets and barnacles (Cannicci et al. 2002; Puccio et al. 2006). The main prey of grapsid crabs can show tempo- ral differences based on seasonal changes in density of algae and invertebrate species in rocky shores (Cannicci et al. 2007). Due to ecological importance, the population biology of many grapsids have been investigated over the past 80 years, whilst some of them such as marbled crab have been rarely studied in some areas, particularly around the Eastern Mediterranean (Kocataş 1971). The marbled crab, Pachygrapsus marmoratus (Fabricius 1787), is distributed in the Mediterranean, the North-Eastern Atlantic coasts of Southern Europe and the Black Sea (Karadal 2018). P. marmoratus mainly lives in the intertidal belt of rocky shores and the depth distribution can reach up to 6 m (Sümer et al. 2016). In addition to the rocky shores, this species was also found in sandy, muddy habitats in estuaries (Galil et al. 2011). P. marmoratus can reach a maximum size of 50.9 mm (CW) (Sümer et al. 2016). The size at onset of maturity (SOM) was estimated as 9.9 mm CW for males and 10.6 mm CW for females in Portugal (Flores and Paula * Fikret Öndes fikret.ondes@ikcu.edu.tr 1 Faculty of Fisheries, Izmir Katip Çelebi University, 35620 Izmir, Çiğli, Turkey https://doi.org/10.1007/s41208-020-00234-8 / Published online: 3 July 2020 Thalassas: An International Journal of Marine Sciences (2020) 36:657–665