Regarding the presence of the Caribbean M. aff. magnirhaphidifera in the east Pacific Ocean
From the ten species found in Mexican Pacific waters, only one, M. (C.) aff. magnirhaphidifera is known in the West Atlantic.
The possible amphiamericanism of M. (C.) magnirhaphidifera is difficult to explain. Currently, the Isthmus of Panama represents a barrier of fresh water across what was once a large neotropical marine environment approximately three million years ago (Coates and Obando, 1996; Craig et al., 2004). However, the two freshwater lakes that the boats have to cross have proved insufficient as a fresh-water barrier (Jones and Dawson, 1973). Several species of marine fishes have survived during the crossing through the canal (McCosker and Dawson, 2004), and several invertebrates were found in the locks when they were drained for cleaning, which show that they have been transported through the canal by associating with fouling material on the underside of ships (Hildebrand, 1939). In fact, morphologically similar invertebrate species from the Atlantic and the Pacific are presently found on both sides of the Isthmus of Panama (Weinberg and Starczak, 1989; Knowlton and Mills, 1992; Knowlton et al., 1993).
If we consider the presence of M. (C.) magnirhaphidifera in the East Pacific Ocean as an invasion, this means that this species survived in fresh water during a period required for a ship to cross of the canal (close to 8 hours (McCosker and Dawson, 1975). The influence of the salinity, in the particular case of marine sponges, has been documented only in a few cases, which show that some marine sponges, particularly clionaids, can live in a salinity of 1.5-2.0, and they can also recover from several days of exposure to salinities as low as 1 (Hopkins, 1956; Hartman, 1958). The presence of some Caribbean Clionids species in the East Pacific Ocean such as C. amplicavata or C. flavifodina could have this explanation (Carballo et al., 2004a), but the confirmation that some sponges can survive during the crossing is supported by the presence of the sponge Haliclona permollis in the locks when they were drained (Hildebrand, 1939). Currently, the true identity of this species is uncertain because H. permollis is a junior synonym of Haliclona (Reniera) cinerea, which is not yet reported reliably from the NW Atlantic (Porifera DataBase), but whatever species it may be its presence shows that it has been transported through the canal by associating with fouling material on the underside of ships. The mechanism by which sponges can endure low salinity has not been studied enough, but it is known that the species Microciona prolifera occurs naturally in salinities ranging from 7 to 38 (De Laubenfels, 1947) and uses free amino acids for osmotic compensation (Knight et al., 1992). Even though there are species that can support several hours in emersion (Carballo et al., 2004b).
However, despite the possibility that M. aff. magnirhaphidifera has survived the crossing of the Panama Canal, it is not possible to decide whether Pacific M. magnirhaphidifera has recently been introduced into the Pacific, or on the contrary, it forms part of a sister-species pair which was separated together with the closure of the natural water way between the Caribbean and the eastern Pacific. De Laubenfels (1936b) found a large group of similar species that lived in both coasts of Panama, which suggested that they may have been introduced from one to the other side through the canal. Independently of the true identity of the De Laubenfels’ records, when putative transisthmian populations of the sponge Spirastrella cf. mollis were studied using allozyme, morphological, and cytological data together, it was found that in reality they were two divergent lineages; the Caribbean lineage, which was named S. hartmani, and the Pacific lineage, which named S. sabogae (Boury-Esnault et al., 1999).
In conclusion, given the limited number of morphological characters available to study marine sponges, it is not possible to decide whether Pacific M. aff. magnirhaphidifera is an invasive species in the Pacific, or if it is part of a sibling species. Such facts will only be demonstrated if we combine the classical methods of morphological measurement with the extensive use of genetic approaches.