Phylum: Cnidaria Obelia longissima Class: Hydrozoa, Hydroidolina Order: Leptothecata A floating dock hydroid Family: Campanularidae Taxonomy: Obelia longissima was first de- about 0.5 mm in diameter; as they mature, scribed by Pallas in 1766. Synonymous in- they grow to 5 mm in diameter (Cornelius clude Campanularia flabellata, Gonothyraea 1975; Kozloff 1983). longissima, Laomeda flabellata, L. longissi- Color: Medusae are primarily clear. ma, O. flabellata, Sertularia longissima Their tentacle bases, mouths, gonads, and (WoRMS 2015). O. lucifera may also be a stomachs are sometimes yellow to brown, synonym (especially of the medusa form), while their gonads and mouths can be bright but further research is necessary to be sure. green (Puget Sound) (Arai and Brinckmann- There has been much debate over the spe- Voss 1980). cies identities within the genus Obelia Body: (Cornelius 1975; Arai and Brinckmann-Voss Bell: The bell is very thin and flat, with 1980). The taxonomy above was taken from a small stomach, no peduncle, and a rudi- the World Register of Marine Species mentary velum (Fig. 3). It is eversible (Arai (WoRMS 2015). In addition to confusion in and Brinckmann-Voss 1980). the lower taxonomy, the higher taxonomy Radial canals: There are four has undergone revision. The order Hydroida straight radial canals, each containing a glob- was determined to be synonymous with sub- ular gonad (Fig. 3). class Hydroidolina in 2004 (Schuchert Ring canal: The ring canal is 2015). narrow, with eight statocysts (balance struc- tures) (Arai and Brinckmann-Voss 1980) and Description no ocelli (Fig. 3). General Morphology: Obelia longissima Mouth: The mouth has 4 small, has two forms. The sexual form is a gelati- simple lips (Arai and Brinckmann-Voss 1980); nous hydromedusa. It has radial canals that in mature specimens these contain nemato- run from the top of the peduncle to the bell cysts (Boero et al. 2007). margin, where they are connected by a ring Tentacles: Tentacles are nu- canal. Suspended from the inside of the bell merous, solid (as opposed to hollow), and by a peduncle is the manubrium, or mouth. short. There are usually 16-26 in young me- A velum rings the inside of the bell margin dusae (Ricketts et al. 1985; Mills and Strath- (Fig. 3). Its asexual morphology is a large mann 1987), and more develop as they ma- polyp. Each polyp has a stem (hydrocaulus), ture. and most have a sheathed (thecate) hy- Velum: Reduced (Arai and dranth with a mouth (manubrium), stomach, Brinckmann-Voss 1980) and tentacles. Rather than having hy- Gonads: There are 4 round gonads on dranths, some polyps have gonothecae (Fig. the middle of each radial canal (Arai and 2). Brinckmann-Voss 1980). Medusa: Polyp: Size: Newly-released medusae are Size: Each colony can be up to 60 cm A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] Piazzola, C.D. and T.C. Hiebert. 2015. Obelia longissima. In: Oregon Estuarine Invertebrates: Rudys' Illustrated Guide to Common Species, 3rd ed. T.C. Hiebert, B.A. Butler and A.L. Shanks (eds.). University of Oregon Libraries and Oregon Institute of Marine Biol- ogy, Charleston, OR. long (Mills et al. 2007) (Fig. 1). Older side lack operculum. The genus Obelia includes branches are all about the same length species that have a nearly flat and eversible (towards the base), but younger branches bell, a reduced velum, eight statocysts, and gradually get shorter near the growing tip solid (rather than hollow) tentacles (Arai and (Mills et al. 2007). Brinckmann-Voss 1980). Within the genus, Color: The polyp is transparent white the species look so similar (especially in the when young, while old, mature colonies look hydromedusa stage) that it is common for re- dirty. Stems are brown to black (Mills el al. searchers to identify only to the genus level 2007). (Arai and Brinckmann-Voss 1980). Body: Lacks nematophores (non- There are two very closely related spe- feeding defensive polyps) (Mills et al. 2007) cies of Obelia: (Fig. 4) Pedicel: Stems are thread-like Obelia geniculata has a central zig zag and often monosiphonic (consist of a single stem, thickened at the joints, while O. longis- tube or row of cells) except sometimes near sima has a much straighter and narrower the base. Internodes of the stem are straight stem. Obelia geniculata has a rather conical or curve very slightly (Mills et al. 2007). hydrothecae (as opposed to bell-shaped) that Each stem has many branches, which are is only slightly longer than wide, and has plain ringed at the joints, alternate, and have short margins rather than cusped (Fig. 4a). Colony stalks (Fig. 2). Stems hold many hydranths size is also a difference; the maximum size of rather than just one (Mills et al. 2007). an O. geniculata colony is 2 cm (Cornelius Hydranth: The hydranth is 1975), much smaller than the 60 cm maxi- covered by a theca (hydrothecae), which is mum of O. longissima. This size also gives O. campanulate (bell-shaped, hence family geniculata a delicate white and fuzzy appear- name) and deep enough to contain the hy- ance (Kozloff 1983). The species are similar dranth when contracted. The margin is cren- in that they both have ringed branches and ulate rather than cusped, and lacks an oper- axillary gonothecae that are urn-shaped, with culum (Mills et al. 2007). Each hydranth is a raised center and a short ringed stalk. Their on the end of a long, ringed pedicels (Fig. hydromedusae are also similar enough that 2d, f). they are often not identified past the genus Gonangium: The reproductive level (Arai and Brinckmann-Voss 1980). buds (gonotheca, gonangia) asexually pro- Obelia dichotoma is very similar to O. duce medusae (Fig. 2c) (Ricketts et al. longissima. Both have slender and annulated 1985). These buds are axillary (i.e., grow out stems, though O. dichotoma has curved inter- of the angle between the stem and the hy- nodes and is irregularly branched while O. drotheca). They are oval-shaped and longissima has a straight stem and is alter- smooth, with a raised central aperture nately branched. These are essentially the (Parker et al. 1951) and a terminal collar only differences. Both have hydrothecae that (Mills et al. 2007). are broad, bell-shaped, with slightly sinuated margins; their gonothecae are axillar, slender, Possible Misidentifications and smooth. They widen from the base, and The family Campanulariidae includes end in a “raised, somewhat conical aper- leptomedusae possessing four radial canals, ture” (Russell 1953) (Fig. 4b). No one has yet and lacking excretory pores and marginal or been able to definitively identify differences in lateral cirri. The polyps in the family have a the hydromedusae forms. club or trumpet-shaped manubrium, and A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] Other hydroids, which have stalks, was found down to 12 (Cornelius 1975). and thecae within which their hydranths can Temperature: Specimens have been found in be retracted (Fig. 2b) include those of the cold and temperate waters; settling may occur families Campanulinidae and Phialellidae in cooler temperatures during the year (Rees and Hand 1975), which are very small (Standing 1976). and have tubular thecae with a pointed oper- Tidal Level: They are most abundant in mid- culum. Other Campanularidae (bell-shaped intertidal and just below, and have been found hydrothecae) include Phialidium sp. and from low tide to 128 m (Mills et al. 2007). Campanularia sp. both of which have colo- Associates: The hydroid colonies serve as a nies of less than 2 cm in height, and are good habitat for many epibionts. Some of rarely branched. these include caprellid and garnmarid amphi- The genus most closely related to pods; asellote isopods; copepods; diatoms; Obelia is Gonothyraea, which does not re- the sea slug Eubranchus; nudibranchs Den- lease free medusae, but retains them within dronotus frondosus and Phidiana crassicornis the gonotheca (Kozloff 1983). (Bodega Bay); and pycnogonid Halosoma veridintestinale. The medusa form plays host Ecological Information to pycnogonid larvae of Anaphia (England). Range: The type locality is Belgium (Ralph Despite both species’ prevalence as fouling 1957). The genus Obelia is found worldwide. organisms, barnacle larvae cannot settle Obelia longissima is found from Alaska to where O. longissima growth is heavy San Pedro, California (Ricketts et al. 1985). (Standing 1976). Local Distribution: All three closely related Abundance: Obelia longissima is particularly species (O. longissima, O. geniculata, O. common in harbors in northern California dictotoma) are found from northern Califor- (Rees and Hand 1975), British Columbia, and nia to Puget Sound, Washington; other spe- Puget Sound (Arai and Brinckmann-Voss cies may be present as well, some of them 1980; Ricketts et al. 1985). In the Strait of introduced (Rees and Hand 1975). Georgia, it is collected from March to Sept., Habitat: This is a frequent fouling organism, and common from April to June (Arai and common in harbors (Mills et al. 2007), on Brinckmann-Voss 1980). Medusae are re- docks, kelp, and floats in bays, and in eel- leased primarily during summer, but also in grass beds (Elkhorn Slough, CA) (Ricketts smaller quantities throughout year (Ricketts et et al. 1985). Healthy colonies are found on al. 1985). Medusa blooms and subsequent exposed pilings, particularly where water is massive shoreline settlement are not common clean and fast-moving. Medusae are found but do occasionally occur (Genzano et al.