Aplidiopsis Lahille, 1890

Lambert, Gretchen, 2003, New records of ascidians from the NE Pacific: a new species of Trididemnum, range extension and redescription of Aplidiopsis pannosum (Ritter, 1899) including its larva, and several non-indigenous species, Zoosystema 25 (4), pp. 665-679 : 671-674

Genus Aplidiopsis Lahille, 1890

Aplidiopsis pannosum ( Ritter, 1899) ( Figs 4 ; 5 )

Polyclinum pannosum Ritter, 1899: 519 , figs 17, 18 (type locality: Alaska).

Polyclinum globosum Ritter, 1899: 518 , figs 14-16.

Aplidiopsis pannosum – Hartmeyer 1924: 187. — Van Name 1945: 66-68, fig. 27. — Tokioka 1960: 194, 195. — Nishikawa 1990: 80, 81. — Sanamyan 1998: 107.

Aplidiopsis helenae Redikorzev, 1927: 382 .

MATERIAL EXAMINED. — USA. Alaska, dredged off Point Barrow , 42 m, 9.IX.1948, G. E. MacGinitie coll., ident. D. P. Abbott, 2 small colonies ( USNM 10893 ) ; 43m, 15.IX.1948, G. E. MacGinitie coll., ident. D. P. Abbott, 4 small colonies ( USNM 10964 ) ; 37 m, 16.IX.1948, G. E. MacGinitie coll., ident. D. P. Abbott, 2 small colonies ( USNM 10894 ) .

San Juan Archipelago , Rock Pt, off W side of Lopez I., dredged at 74 m, 9. VI.1999, C. Lambert & D. Duggins coll., 3 colonies, larvae present ( USNM 1006924 ) ; same data, 1.VII.1999, D. Duggins coll., 8 colonies, the largest 4.6 cm in diameter with 2 cormidia, attached to a sabellarid tube, larvae present ( MNHN A1 APL A 21 ) ; same data, 3.VIII.1999, D. Duggins coll., 11 colonies, the largest 2 cm in diameter, no larvae ( CASIZ 162517 ) together with numerous large colonies of Distaplia occidentalis full of mature larvae ( CASIZ 162516 , 3 colonies) ; 80 m, 29. VI.2001, D. Duggins coll., 11 colonies, the largest 16 mm in diameter, resting stage, no larvae ( CASIZ 162518 ) .

Pt Caution, San Juan Channel, dredged at 96-114 m, mud, sand, shell bottom, 8.VIII.2001, D. Duggins coll., 1 small colony 16 mm in diameter, 20 mm tall, no larvae ( USNM 1006925).

DISTRIBUTION. — Japan Sea ( Nishikawa 1990); Kurile I. ( Sanamyan 1998); Bering Sea ( Ritter 1899; Sanamyan 1998); Point Barrow (USNM specimens); Montague I., Gulf of Alaska ( Van Name 1945); Sea of Okhotsk ( Redikorzev 1927; Sanamyan 1998); Kamchatka ( Redikorzev 1927; Tokioka 1960; Sanamyan 1998); off Lopez I., San Juan Archipelago, WA (present study). Depth: intertidal to 535 m; usually less than 100 m.

DESCRIPTION

Colonies are globular or irregularly capitate, attached basally by a very short stalk or peduncle that is not always apparent. The tunic is smooth, clean, translucent and without any embedded sand. Colonies may be whitish or pale yellow, mostly due to the color (or lack of color) of the zooids; the tunic may contain a few scattered reddish brown pigment granules. The largest colony collected is 4 cm in diameter across the upper surface and 3.4 cm in height including a stalk length of 1.4 cm.

The zooids are arranged in approximately circular systems around the common cloacal openings; the largest colonies contain many systems. Zooids are 19-20 mm in length if not contracted, though many are only 10-12 mm in preserved colonies. The oral siphon is short with six lobes and bears numerous fine circular muscles. The non-lobed atrial opening is surmounted by a wide, cowl-like broad languet with many circular muscles and with three small teeth at its tip (see Ritter 1899: fig. 16). There are about 16-18 irregular longitudinal muscles on each side of the thorax. The majority of mature zooids ( Fig. 4 ) have 12 rows of stigmata, with up to about 28 stigmata/half row in the middle rows of the largest zooids; those at the ends of the rows are very small. The number of stigmata/half row is

A

C

D

quite variable. Papillae are absent from the transverse vessels. The dorsal languets are long, broad and triangular and do not extend as far across the branchial sac as described by Ritter (1899) except in contracted zooids; they originate to the left of mid-dorsal. The esophagus is short, funnelshaped, and twisted to enter the smooth globular stomach on its dorsal side ( Fig. 4 ) ( Ritter 1899: fig. 18). A prominent typhlosole extends from the entrance of the esophagus to the posterior end of the stomach. An abruptly narrowed poststomach about as long as the stomach itself ends equally abruptly in the widened intestine that forms the posterior loop and then extends anteriorly into the atrium to the level of the sixth row of stigmata. The end of the rectum is flared out and bilobed (see Ritter 1899: fig. 18). In living zooids the stomach and proximal region of the intestine are a dark yellowish orange, but the post-stomach is colorless. The postabdominal ovary and testis are not developed in any of these specimens. The heart is at the posterior end of the postabdomen and has two beating chambers in living zooids.

In those colonies with larvae, eight to 10 could be found in the atrial chamber of a single zooid ( Fig. 4 ), the most mature being closest to the atrial opening. Rather than exhibiting a continuous series of developmental stages, there are groups of two or three at approximately the same stage of development: the three closest to the atrial opening are mature or nearly so, three at mid-atrial chamber are at early tailbud with small otolith and ocellus beginning to form, and three at the base of the chamber are at pre-tailbud stage. This may indicate that on average three eggs or embryos are released into the atrial chamber each day or at a single spawning. Some mature swimming larvae were released in the laboratory in July 1999. They are large, with an elongate trunk up to 1.3 mm long ( Fig. 5 ) and three large adhesive organs each on a long stalk. There are large lateral and ventral ampullae with multiple fingerlike branches. In addition, numerous tiny ectodermal vesicles are scattered over most of the lateral, anterior and ventral areas in a pattern corresponding to the “multiple arc of vesicles” in some Aplidium species ( Kott 1992: 508). The tadpoles have a wide tail fin. The siphons appear to be open in the most advanced larvae even before hatching. A large ocellus and otolith are present.

Matching Ritter’s (1899) description of his specimens, the post-abdomens are filled with opaque mesenchymatous cells and the gonads have apparently regressed as the larvae matured. Parasitic copepods were present in several of the colonies from Rock Pt, and one in the stomach of a zooid from the Pt Caution colony.

REMARKS

Although this is the first record of this species in the NE Pacific south of Alaska, the colonies are easily mistaken for pale-colored Distaplia occidentalis Bancroft, 1899 until examined under a microscope. The collection of 3.VIII.1999 contains numerous D. occidentalis and only a few A. pannosum . The region of collection has been a popular dredge site for the Friday Harbor Laboratories for many years for Balanus nubilis , so it is likely that at least a few colonies of A. pannosum were collected over the years along with D. occidentalis . Huntsman (1912a: 115) wrote that “Dall has remarked that the fauna of the inner channels of the British Columbian archipelago is of a distinctly more northern character than that of the open coast. This is well shown in the ascidians. The list from Departure Bay includes arctic forms that are not represented at Ucluelet and among the Ucluelet species are a number of southern forms that do not occur at Departure Bay”.

Huntsman did not collect any Aplidiopsis pannosum from the Departure Bay area but it is apparently abundant and widespread in the North Pacific (see Distribution). The northernmost record is the samples from Point Barrow collected in 1948. These differ from the present samples in having sand grains embedded on and in the tunic, though Ritter (1899) states that there may or may not be sand present. This is a variable character in the NE Pacific species Aplidium californicum ( Ritter & Forsyth, 1917) and could also be variable in Aplidiopsis pannosum .

The larvae described by Sanamyan (1998) are somewhat different from those in the present collections and are probably immature, based on the appearance of his groups of epidermal vesicles and lateral ampullae, as well as the lack of siphons in his figure 1C and the very small siphons in his figure 1D. In the 9.VI.1999 colonies of the present study the siphons are very prominent in the most mature larvae. In an examination of various larval stages I found that the ampullar blocks of the immature larva subdivide at their tips during maturation, forming fingerlike projections that remain joined at the base ( Fig. 5B, C ). There are two discrete clumps of vesicles in immature larvae similar to those illustrated by Sanamyan (1998); these spread out anteriorly and laterally as the larva matures ( Fig. 5B, C ). Epidermal vesicles form at the ampullar tips and are pinched off late in larval development, breaking the connection between vesicle and ampulla and leaving the vesicles free in the tunic (R. Cloney pers. comm.) ( Fig. 5D ). Larval morphology, now a crucial part of species identifications, was not considered important by the early taxonomists and thus there are very rarely any comments about the larvae other than possibly their presence or absence. The above observations on morphological changes that occur as the larvae mature show the necessity of having fully mature larvae available, a requirement that is unfortunately often lacking.

Japan Sea specimens that have been assigned to this species have a higher number of stigmatal rows and lower number of longitudinal muscles than the present specimens ( Nishikawa 1990). In the present collections, some small immature colonies have zooids with fewer than 12 rows of stigmata but none have more than 12 rows. Curiously, the largest colony from 1.VII.1999, with the largest cormidium 4 cm in diameter, contains zooids that appear to be immature or may have regressed. Gonads are lacking, the postabdomen is not well developed, and some zooids have an empty gut and intestine indicating that they were not feeding.

Figures

FIG. 4. — Aplidiopsis pannosum (Ritter, 1899), zooid 14 mm in length with incubated larvae in the atrial chamber.

FIG. 5. — Aplidiopsis pannosum (Ritter, 1899), unhatched larvae from early to hatching stage of development. Scale bar: 0.5 mm.