Tricellaria inopinata, d'Hondt, 1985

Dyrynda, P. E. J., Fairall, V. R., Ambrogi, A. Occhipinti & d’Hondt, J. - L., 2000, The distribution, origins and taxonomy of Tricellaria inopinata d’Hondt and Occhipinti Ambrogi, 1985, an invasive bryozoan new to the Atlantic, Journal of Natural History 34 (10), pp. 1993-2006 : 1994-1998

The arrival of Tricellaria inopinata in the north-east Atlantic

A substantial population of putative T. inopinata was discovered in Poole Harbour on the central southern coast of England in August 1998 (fi gure 1). Morphological comparisons with Adriatic material indicated this taxon to be identical to T. inopinata as previously described from the Venice Lagoon (d’Hondt and Occhipinti Ambrogi, 1985). Detailed surveys of the Poole Harbour area (August 1998 to November 1999) indicate that this taxon is well represented within fouling assemblages colonizing anthropogeni c substrata within the port and several marinas. It also occurs on natural shores, both within and outside the Harbour, indicating an ability to colonize brackish and fully saline environments (20-33½). With respect to micro-habitat preferences, T. inopinata has been found within the infralittoral fringe (lowest shore/shallow subtidal zone), colonizing primary (abiotic) substrata, most notably fl oats associated with berthing pontoons, but also infralittoral boulders. Secondary (biotic) substrata colonized within the port area, marinas and natural shores include macro-algae such as Chondrus crispus Stackhouse and Sargassum muticum (Yendo) Fensholt and sessile invertebrates such as Styela clava Herdman. The latter two species are also invasive taxa of Pacific origin ( Eno, 1996).

A subsequent autumn/winter survey of vulnerable habitats (notably marina pontoon fl oats) at a succession of localities along the south coast of England indicated this taxon to be localized to central southern England (fi gure 1). It was found to be abundant within ports and marinas in Chichester Harbour, Portsmouth Harbour, the Solent and Poole Harbour and to occur on the open coast at Swanage. To date it has not been found further east or west.

The precise chronology of invasion to date is not known, but the population increase within Poole Harbour is considered to have been recent and rapid. This species was not detected by routine sampling of pontoons in the summer of 1996 in the Port of Poole (P. E. J. Dyrynda, unpublished), a site at which it is now abundant. Surveys of T. inopinata were aided by the persistence of large conspicuous colonies through autumn and winter. In the absence of vigorous growth, colonies became fouled and deteriorated progressively through this period and those inhabiting well illuminated surfaces were subsequently masked by spring fl ushes of fi lamentous macro-algae. In Poole Harbour vigorous growth and larval production recommenced in early summer (June), 1999. The invasion ecology of this taxon within the region will be described in more detail elsewhere.

Tricellaria inopinata in the Adriatic

The distribution and habitat preferences of T. inopinata in the Venice Lagoon have been studied by d’Hondt and Occhipinti Ambrogi (1985), Occhipinti Ambrogi (1991), Occhipinti Ambrogi and d’ Hondt (1994) and Occhipinti Ambrogi (in press). Initially (in 1982) T. inopinata was found to be restricted to the central lagoon in the vicinity of the old city, and it subsequently, gradually radiated out to most parts of the lagoonal basin. The population reached maximum abundance and geographical spread c. 1988-89 ( Occhipinti Ambrogi, 1991), but more recently a reduction in abundance and a fragmentation of distribution have been evident (Occhipinti Ambrogi, in press). The distribution within different salinity zones within the lagoon indicates a salinity optimum in the range 20-35½. Quantitative surveys have been facilitated by wooden navigable channel markers known as briccole which are widely distributed across the lagoon and are readily colonized by T. inopinata . Colonies occur on lower shore facets, most characteristically beneath a cover of Enteromorpha intestinalis (L.). Tricellaria inopinata also colonizes secondary substrata in the lagoon including the byssal threads of the mussel Mytilus galloprovincialis Lamarck , the sponges Hymeniacidon perleve (Montagu) and Mycale contareni (Martens) , and the ascidian Styela plicata (Lesueur) . During the last survey in July 1999 it was also found growing on the brown alga Undaria pinnati W da (Harvey) Suringar, an invasive species recently introduced into the lagoon. The presence of T. inopinata on the lagoonal side of the lock gates at the entrance to the lagoon and its absence on the seaward side may indicate a preference here for brackish conditions.

Until recently the Venice Lagoon was the only documented location for T. inopinata in the Adriatic. In 1998 a substantial population of T. inopinata was discovered within Marano Lagoon, the entrance of which is c. 70 km to the east of that of the Venice Lagoon. There it shows ecological requirements comparable to those of the Venice population, thriving best in channels featuring relatively strong tidal currents and brackish salinities. Species richness of hard substratum communities is less than within the Venice Lagoon and T. inopinata is here mainly associated with Mytilus galloprovincialis .

The ecological characteristics of the Adriatic and north-east Atlantic populations of T. inopinata concur to a large degree and indicate this to be a robust opportunisti c taxon capable of enduring a broad spectrum of temperate zone water temperatures. In this context it may be signi fi cant that the climate of the northern Adriatic is cooler and salinities generally lower than those of much of the main Mediterranean basin where it is yet to be recorded. Its ability to colonize a wide range of anthropogenic and natural substrata has also, no doubt, facilitated its dispersal.

Identity and global distribution of Tricellaria inopinata

Bryozoan taxonomy is almost entirely reliant on the determination of morphospecies using qualitative and to a lesser extent quantitative exoskeletal characteristics. Many morphospecies are readily discernable according to the presence of highly consistent features ( Hayward and Ryland, 1998). Tricellaria inopinata , on the other hand, is characterized in certain respects by a high level of morphological variability within an individual colony. We believe that this variability has been the source of very considerable confusion within the Pacific literature pertaining to T. occidentalis (Trask) , T. occidentalis var. catalinensis (Robertson) and, to a lesser extent, T. porteri (MacGillivray) .

All of the Tricellaria specimens examined share the following characteristics: an erect colony morphology composed of unilaminar branches with anchoring rhizoids; individual branches bifurcate at intervals and consist of biserial autozooid lineages; autozooids bear frontal spines and a single over-arching scutum; at least some autozooids bear lateral avicularia but frontal avicularia and vibracula are absent. Descriptions of the terminology used to distinguish cheilostomatous bryozoan morphospecies are provided by Hayward and Ryland (1998).

The type descriptions of T. occidentalis (as Menipea occidentalis ) by Trask (1857) and T. porteri (as Menipea porteri ) by MacGillivray (1889) are much briefer than that of T. inopinata (d’Hondt and Occhipinti Ambrogi, 1985) and are probably based on the examination of small amounts of material. We believe, however, that these enable T. inopinata to be distinguished from the two aforementioned Pacific morphospecies. Key morphological characters include internode length (the number of autozooids between branch bifurcations), morphology of the scutum (a modi fi ed spine over-arching the frontal membrane of the autozooid), presence or absence of a bi fi d (forked) spine on some autozooids within each colony, and the number and size of the spines prominent in branch axils ( table 1 ).

Ideally, the current study would have provided the opportunity to redescribe T. occidentalis and T. porteri . In view of the paucity of the type descriptions of these species and the limited quantities of material (mostly very old) available to us for examination (including the non-existence of type material of T. occidentalis ), redescriptions would be premature. They would require the procurement and examination of signi fi cant quantities of new material ideally coupled with morphometric and possible molecular analyses.