Tricellaria inopinata d’Hondt and Occhipinti Ambrogi, 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 : 2001-2003

Tricellaria inopinata d’Hondt and Occhipinti Ambrogi, 1985

Tricellaria inopinata is distinguishable from T. occidentalis and T. porteri by a combination of characters and particularly the remarkable degree of morphological variability shown by the scutum, the considerable variability in the number of autozooids within internodes, the presence of some bi®d spines in every colony and the number of spines prominent in branch axils ( table 1, ®gure 2). In view of the absence of this taxon from Paci®c and Atlantic bryozoan synopses and guides, and in order to emphasize the key diOEerences between this and the other discernible morphospecies, we provide the following updated description based on the examination of material from the Venice Lagoon and Poole Harbour.

Well developed colonies, conspicuous to the naked eye, are cream to buOE in colour and arborescent in form. Individual branches are unilaminar (single layer of autozooids) and branch dichotomously. Individual autozooids within the biserial zooid lineages that form each internode (section between branch dichotomies) are staggered relative to each other, with an additional autozooid wedged between the distal-most pair. The internode length is characteristically highly variable, consisting of between three and 19 autozooids, odd numbers only, with three, ®ve and seven being most frequent and with longer internodes at the distal ends of branches. Autozooids vary in size and shape according to their position within the internode and whether or not they bear an ovicell, but all are elongate and taper proximally. Mean values for nonovicellate and ovicellate autozooids dimensions are given in table 2.

All autozooids, except the ancestrula (the ®rst zooid of the colony), have a scutum, which is characteristically highly variable in form and size within an

individual colony. Morphologies vary from a slender projection, which may be forked, to an extensive, often cervicorn-like structure, typically with a wavy or spiky margin (®gure 2). All autozooids have marginal spines around the rim of the opesia. Generally there are three disto-lateral spines on the outer ¯ank of the autozooid, the most proximal of which is slender and often bi®d, the second is slender and generally straight, and the third is generally more robust, longer, and orientated distally. On the inner side there are generally only two disto-lateral spines, both slender, straight and upright. Additionally, axillary autozooids bear a long near median spine which is prominent in each branch axil. Triangular lateral avicularia occur distally on the marginal autozooids, varying in size from 86 to 210 m m long, and from 57 to 172 m m wide. Lateral avicularia are most numerous within the relatively short proximal internodes where non-ovicellate autozooids predominate. In the majority of internodes, the most distal pair of autozooids usually bear lateral avicularia. There are no frontal avicularia or vibracula. The ovicells are sub-globular and especially numerous in long distal internodes. The ectooecium (outer surface) has a scattering of pores, usually c. 15, situated on a smaller number of radiating sutures. The living embryo is rose to crimson pink in colour. Colonies are anchored to the supporting substratum by tubular rhizoids, mainly arising from the more proximal internodes. The ancestrula, the founder zooid of the colony, remains present only in young colonies. It is c. 200 m m high and 152 m m wide, lacks a scutum, and the opesia is surrounded by about eight thin straight spines.

Our own evaluations of specimens and descriptions lead us to believe that T. inopinata from the Adriatic and northeast Atlantic is morphologically indistinguishable from material referred to as T. occidentalis from Japan and New Zealand ( Mawatari, 1951; Gordon, 1986; Gordon and Mawatari, 1992), and may be identical to T. occidentalis var. catalinensis known from the Paci®c coast of North America. We believe that this taxon diOEers fundamentally from the original T. occidentalis described by Trask (1857) which has been recorded on western North American coasts through to recent times (see above). Detailed analyses of samples of T. inopinata from a number of Atlantic sites and from the Adriatic have failed to produce evidence of developmental, temporal or spatial intergradation between typical T. inopinata and typical T. occidentalis . All colonies examined resembled the former only. We therefore conclude that the name T. inopinata should apply to Paci®c material conforming to the morphology of Adriatic and Atlantic T. inopinata but which was originally recorded as T. occidentalis . This may involve records from North America, Japan, Australia and New Zealand. It should also, probably, apply to records for T. occidentalis var. catalinensis . This involves material collected at many locations and over many decades prior to the discovery of T. inopinata in the Adriatic.

The localization of putative T. inopinata to temperate regions of the Paci®c is consistent with the known distribution of T. inopinata in northeast Atlantic and Adriatic waters. Although ecological information for Paci®c populations is limited, evidence suggests that habitat preferences are similar. In the Paci®c this taxon is known from the infralittoral fringe/shallow subtidal zone where it colonizes primary substrata including anthropogeni c materials, and secondary substrata, notably algae. In Japan it is known to be a vigorous colonizer of set ®shing nets and the hulls of boats ( Mawatari, 1951), and in New Zealand it is known as a fouling organism within ports.

Tricellaria inopinata exhibits the characteristics of a vigorous invasive species and is now gaining cosmopolitan status (®gure 1). The prevalence of Atlantic T. inopinata in marinas and ports highlights the likelihood of shipping on all scales as the principal vector facilitating long-distance and regional anthropogeni c dispersal. In view of the fact that T. inopinata is not known from Atlantic America, its most likely route of introduction to central southern England from the Paci®c would be via Venice, involving either small recreational and/or large commercial vessels. The involvement of as yet unknown intermediate locations cannot be ruled out. It is believed that the most likely mode of introduction from the Paci®c to Venice was in association with imported consignments of live oysters deposited within the lagoon. Now that this taxon has arrived in the Atlantic, we would anticipate progressive spread along temperate Atlantic European coasts and ultimately translocation to other temperate regions including Atlantic North America. The authors would welcome information on any new geographic records for this taxon.