reaching a smaller size than populations from Brazil. They reported a very low
frequency of occurrence of the commensal crab Dissodactylus crinitichelis
between oral surface spines.
Tripylaster philippii is a spatangoid echinoid known from the region. This
species is found in greater depths with a maximum at 135 m. It is associated with
substrates having high percentages of silt–clay fraction and very fine sands with a
clear predominance of the mud fraction. Despite the existence of suitable substrates in large areas at SJG, this species is absent from those areas, probably
because the fine sediments of SJG are not stable enough to allow its establishment,
or perhaps summer temperatures are too high (Zaixso and Lizarralde 2000).
Tripylaster philippii is also found at great depths of NG (Carriquiriborde et al.
1983). At SJOG it is part of an important benthic association with the colonial
coelenterate Renilla sp. (Roux 2000).
The heart urchin Abatus cavernosus (Fig. 11.4a) is another spatangoid that
occurs off RP to TF. This species also occurs in the Subantarctic and Antarctic
Islands (Bernasconi 1953, 1966b; Schatt and Féral 1991; López Gappa and Sueiro
2007). It inhabits unconsolidated, muddy and sandy sediments and has a wide
bathymetric range, from the intertidal to 700 m (Bernasconi 1966b). In RP, this
species has a mean density of 63.9 ind m
-2 and can be found buried in sand,
muddy sand, and pebbly sand substrata, from midlittoral to the shallow subtidal,
and within soft-bottom tidepools (Dolcemascolo and Zaixso 2000; Gil et al. 2009).
According to Gil et al. (2009), A. cavernosus is a deposit-feeder that broods its
young inside four dorsal ambulacral pouches. Sex ratio was not homogeneous
across size classes and females were found to be larger than males. The temporal
pattern in the size-adjusted gonad dry weight demonstrated that reproduction in the
littoral population of A. cavernosus is seasonal and synchronous. Spawning occurs
from May to July, and females breed within a 9-month period from May to
February, followed by a 1–2 month lag between gonad development and
decreasing seawater temperature. Spawning occurred when the seawater temperature decreased and may be influenced by spring and summer periods of sediment
enrichment. Juveniles develop directly without formation of larval structures, and
are released during the austral summer. The heart urchin exhibited some reproductive characteristics that differ from those of other Subantarctic and Antarctic
spatangoids, including relatively large egg size and low fecundity. Tolosano et al.
(2006) found that gonopore diameter may be used to sex immature heart urchins,
using a breakpoint regression model. The epibiont bivalve Waldo parasiticus is
usually found among the spines of A. cavernosus (Zelaya and Ituarte 2002).
According to Zaixso et al. (2003) the number of bivalves per host is not influenced
by sea urchin sex, but it is positively related to size. Epibionts were mainly found
on the ventral ambulacral areas but can also occupy other areas on the ventral and
dorsal side. Maximum abundance of epibionts per host was found during summer
and lowest values in winter. The relation between these two species should be
tentatively classified as inquiline or commensal.
11 Echinoderms from Argentina
379
frequency of occurrence of the commensal crab Dissodactylus crinitichelis
between oral surface spines.
Tripylaster philippii is a spatangoid echinoid known from the region. This
species is found in greater depths with a maximum at 135 m. It is associated with
substrates having high percentages of silt–clay fraction and very fine sands with a
clear predominance of the mud fraction. Despite the existence of suitable substrates in large areas at SJG, this species is absent from those areas, probably
because the fine sediments of SJG are not stable enough to allow its establishment,
or perhaps summer temperatures are too high (Zaixso and Lizarralde 2000).
Tripylaster philippii is also found at great depths of NG (Carriquiriborde et al.
1983). At SJOG it is part of an important benthic association with the colonial
coelenterate Renilla sp. (Roux 2000).
The heart urchin Abatus cavernosus (Fig. 11.4a) is another spatangoid that
occurs off RP to TF. This species also occurs in the Subantarctic and Antarctic
Islands (Bernasconi 1953, 1966b; Schatt and Féral 1991; López Gappa and Sueiro
2007). It inhabits unconsolidated, muddy and sandy sediments and has a wide
bathymetric range, from the intertidal to 700 m (Bernasconi 1966b). In RP, this
species has a mean density of 63.9 ind m
-2 and can be found buried in sand,
muddy sand, and pebbly sand substrata, from midlittoral to the shallow subtidal,
and within soft-bottom tidepools (Dolcemascolo and Zaixso 2000; Gil et al. 2009).
According to Gil et al. (2009), A. cavernosus is a deposit-feeder that broods its
young inside four dorsal ambulacral pouches. Sex ratio was not homogeneous
across size classes and females were found to be larger than males. The temporal
pattern in the size-adjusted gonad dry weight demonstrated that reproduction in the
littoral population of A. cavernosus is seasonal and synchronous. Spawning occurs
from May to July, and females breed within a 9-month period from May to
February, followed by a 1–2 month lag between gonad development and
decreasing seawater temperature. Spawning occurred when the seawater temperature decreased and may be influenced by spring and summer periods of sediment
enrichment. Juveniles develop directly without formation of larval structures, and
are released during the austral summer. The heart urchin exhibited some reproductive characteristics that differ from those of other Subantarctic and Antarctic
spatangoids, including relatively large egg size and low fecundity. Tolosano et al.
(2006) found that gonopore diameter may be used to sex immature heart urchins,
using a breakpoint regression model. The epibiont bivalve Waldo parasiticus is
usually found among the spines of A. cavernosus (Zelaya and Ituarte 2002).
According to Zaixso et al. (2003) the number of bivalves per host is not influenced
by sea urchin sex, but it is positively related to size. Epibionts were mainly found
on the ventral ambulacral areas but can also occupy other areas on the ventral and
dorsal side. Maximum abundance of epibionts per host was found during summer
and lowest values in winter. The relation between these two species should be
tentatively classified as inquiline or commensal.
11 Echinoderms from Argentina
379