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This is only the second report of the bopyrid isopodParabopyrella thomasi(Nierstrasz & Brender à Brandis, 1929) since it was described. The species infests the arrow shrimpTozeuma carolinenseKingsley, 1878, and this report of new material extends its known geographic distribution to the Caribbean coasts of Mexico. We provide data on some morphological variations in both sexes of this species as well as on the fecundity of this ectoparasite.

Morphometric data are presented for developmental stages ofBopyrinellathorii(Richardson, 1904), obtained from 380 individuals ofThorfloridanusKingsley, 1878 infested by this parasite and collected by trawl net in Bahía de la Ascensión, Quintana Roo, Mexico. The close relationships between the sizes of host and parasite () and between sizes of the two sexes of the parasite () suggest the simultaneous growth of host and parasite and the permanence of the pairing of a male and female parasite throughout their lives. Cryptoniscus larvae ofB. thoriiwere sometimes attached intramuscularly to other body parts than the host’s gill chambers. Mature females unpaired with a male had ovarian activity visible through their exoskeleton, which reveals thatB. thoriicould produce oocytes even in the absence of a male. Bilateral double infestation, present in 4.2% of theT. floridanusspecimens analysed, was more frequent in smaller size classes of the host.

Abstract This study describes changes in the secondary sexual characters of Thor floridanus Kingsley, 1878 males, caused by the isopod Bopyrinella thorii (Richardson, 1904). Of 380 specimens of T. floridanus examined, the females were more often infested (n = 313) than the males (n = 41), and had a wider size range (0.7-2.85 mm POCL) compared to the males (0.9-2.32 mm POCL); no ovigerous females of T. floridanus were found. In 25 males the appendix masculina was reduced in size, the endopod of the second pleopod was barely ornamented, and the flexor margin of the propod of the third pereiopod was similar to that of the female of the species; all those males were classified as “feminized”. Those individuals were infested mainly by pairs of mature parasites, and 75% of the “primary” males carried cryptoniscus larvae, suggesting that the “feminization” of the males is caused by the parasite, as a result of the sexual development of hosts and parasites.

The association of Bopyrinella thorii (Richardson, 1904) with Thor floridanus Kingsley, 1878 in seagrass beds of the Mexican Caribbean has been previously reported upon, although without mention of other data relative to the parasitic species' biology. The purpose of this study was to provide information on reproductive aspects of B. thorii from Bahia de la Ascension, Caribbean coast of Mexico. We estimated that B. thorii females initiate reproduction at 1.75 mm total length (TL) and that the size at sexual maturity is 2.1 mm ± 0.95 TL. The fecundity of B. thorii varied from 83 eggs in smaller females to 1,036 eggs in larger ones. The relationship between the number of eggs and the total length of the females was significant (r = 0.769). This relationship, however, was not progressive, as females of one same size presented a variable number of eggs, particularly in the larger specimens. The size of B. thorii eggs varied from 0.113 to 0.175 mm in the egg to embryo II phases, and the size of the epicaridium larvae varied from 0.176 to 0.213 mm. No differences were observed in the size and developmental phase of the eggs within a single brood. La asociacion de Bopyrinella thorii (Richardson, 1904) con Thor floridanus Kingsley, 1878 en praderas de pastos marinos del Caribe mexicano, se ha reportado previamente sin aportar mayor informacion sobre la biologia del parasito involucrado. El presente trabajo tiene como objetivo proporcionar informacion sobre aspectos reproductivos de B. thorii en la Bahia de la Ascension, costa del Caribe Mexicano. Se estimo que las hembras de B. thorii inician su reproduccion cuando alcanzan 1,75 mm de longitud total (LT) y que la talla de madurez sexual de la especie es de 2,1 mm ± 0,95 (LT). La fecundidad de B. thorii vario de 83 huevos en las hembras mas pequenas a 1036 huevos en las de mayor tamano. La relacion entre el numero de huevos y la longitud total de la hembra es significativa (r = 0,769) aunque no progresiva, pues se observo variacion en el numero de huevos entre hembras de la misma talla, la cual fue mas marcada en las hembras mas grandes. El tamano de los huevos de B. thorii fue de 0,113 a 0,175 mm en las fases huevo a embrion II; y el tamano de las larvas epicaridium vario de 0,176 a 0,213 mm. No se observo variacion en el tamano y fase de desarrollo de los huevos de una misma masa ovigera.