David Russel
ABSTRACT
The hydroids Aglaophenia struthionides (Murray, 1860), Eucopella
everta (Clark, 1896), and Abietinaria spp. in the low intertidal zone
at Mussel Point, Pacific Grove, Monterey Bay, California, often bear
the adult pycnogonids Tanystylum duospinum (Hilton, 1939) and Lecytho¬
rhyncus hilgendorfi (Bohm, 1879). Larval stages of these pycnogonids
were found living and feeding on Eucopella everta. The larvae of
Tanystylum duospinum are ectoparasitic. Those of Lecythorhyncus
hilgendorfi are endoparasitic in the gastrovascular cavities of
hydranths. Larvae of both species feed on the gut contents of Eucopella
everta, often on the same hydroid colony.
INTRODUCTION
Hydroid colonies found in the low intertidal zone on rocky
California shores often bear pycnogonids. At Mussel Point, Pacific
Grove, adults of the sea spider Tanystylum duospinum are frequently
found on the hydroid Aglaophenia struthionides, and less often on
other hydroids, bryozoans and tunicates. Adults of the pycnogonid
Lecythorhyncus hilgendorfi, less abundant at Mussel points during this
study, were found on the hydroids Eucopella everta, Aglaophenia
struthionides, and Abietinaria spp.. Larval stages of both pycnogonid
species were found only on Eucopella everta, often on the same colonies.
The occurrence of two similar forms undergoing larval development
on the same substrate provided an opportunity to make comparisons
and to examine the possibility of resource partitioning.
David Russel
MATERIALS AND METHODS
All material was collected from low intertidal areas at Mussel
Point, Pacific Grove, Monterey Bay, California, between 18 April and
30 May, 1980. Hydroid colonies were generally collected on red algae
(Prionitis, Gastroclonium, and Gigartina spp.), but occasionally
were taken from rocks. Both homogeneous hydroid clusters (colonies
consisting of a single species) and heterogeneous clusters (consisting
of two or more species growing closely together) were kept separate
both during collecting and later in the laboratory. Samples brought
back to the laboratory were observed while fresh. Some colonies were
kept for up to one week without feeding, in aquaria and finger bowls
with constantly running seawater at approximately 14°C, during which
time additional observations were made.
Identification of larval Tanystylum duospinum was made by arrang-
ing the stages found into a continuous developmental series, and then
working backwards from the adult. The earliest larval form observed
possessed three pairs of larval appendages and lacked any adult
walking legs.
RESULTS
TANYSTYLUM DUOSPINUM
Adult Tanstylum duospinum were found most abundantly on fronds
of Aglaophenia struthionides, but were collected from a variety of
substrates including the hydroids Abietinaria and Sertularella, the
tunicate Archidistoma, and an arborescent bryozoan. Females with
developed eggs in their legs and males carrying egg masses were
David Russel
occasionally found. No males carrying protonymphon larvae were seen.
Protonymphon larvae with clear exoskeletons were found, firmly
attached by the grip of the two chelifores, on the stolons, stalks,
and hydrothecae of Eucopella everta. When larvae were gently pulled
from the substrate, breaking the grip of the chelifores, they retained
a connection with the hydroid via a thread formed by secretion of the
cement gland. The thread, which may be at least twice the length of
the larva, acts as a safety line, preventing the animal from being
swept from the substrate if the chelifores lose their grip. In still
water a larva was observed to slowly ascend the line, until it
regained the substrate. A larva dislodged in a current was swung
in an arc, and rapidly regained a foothold on the substrate.
Undisturbed protonymphon larvae found attached to E. everta as
noted above, were often observed to have the proboscis inserted
through the perisarc and body wall into the hydroid coelenteron. In
one such individual, observed over a ten minute period, feeding was
seen to be suctorial. Expansion of the pycnogonid's gut from
anterior to posterior caused fluid to be drawn from the hydroid's
coelenteron into the pycnogonid. After a variable length of time a
contraction of the pycnogonid gut, starting at the posterior and
moving anteriorly, forced much of the ingested material back into
the hydroid coelenteron. The process was repeated at irregular
intervals. More advanced larvae (stages V and VI of Dogiel, 1913)
possessing their first and second pairs of walking appendages and
an unpigmented exoskeleton (Figs 1 and 2) were found attached to the
host hydroid in the same sorts of places and appeared to feed in the
David Russel
same manner. T. duospinum with three and four pairs of walking legs
and a clear exoskeleton (Figs. 3 & 4) was not observed feeding, nor
was an attachment thread seen in these stages.
The most advanced stage of T. duospinum found on E. everta had a
maximum leg span of only 4 mm, still possessed functional chelifores
and a clear exoskeleton, and had an incompletely developed fourth pair
of walking legs. This corresponds closely with the earliest stage of
T. duospinum found on Aglaophenia struthionides. Larger T. duospinum
with four fully developed pairs of walking legs and a leg span of up
to 10 mm were found on A. struthionides, Abientinaria and Sertularella
but were not found on E. everta. The smaller eight-legged individuals
were light straw in color, while the larger specimens were a darker
golden brown and were occasionally fouled by epiphytes.
LECYTHORHYNCUS HILGENDORFI
The earliest developmental stage of L. hilgendorfi seen occurred
within aberrant, enlarged hydranths of Eucopella everta. Both hydranth
and parasite were encased in an exoskeletal capsule attached within
the original hydrotheca but extending outward considerably beyond the
hydrothecal margin (Fig. 5). Each hydranth contained a small pycnogonid
(Stage IV, Dogiel, 1913) with a proboscis, chelicerae, a small pair of
appendages, and three pairs of short walking legs (Fig. 6). The gut
was clearly branched. Within the hydranth the pycnogonid proboscis
was inserted into the coelenteron of the hydroid stalk, the chelicerae,
directed forward, held the pycnogonid in place. Slightly larger
pycnogonids with longer legs (Stage V, Dogiel, 1913) were also found
in an identical orientation (Fig. 7 & 8).
David Russel
At the next observed stage the capsule, and often most or all of
the hydrotheca and hydranth of the infected hydroid were absent (Fig.
9). The three pairs of walking legs of the now exposed pycnogonid
were present as stubs which no longer contained the branches of the
gut seen in the two preceding stages. Occasionally two pycnogonids
at this stage were found at the end of a single hydroid stalk (Fig. 10).
In the next developmental stage the pycnogonid's walking legs were
well developed (Fig. 11). These legs still lacked extensions of the
gut; they waved slowly in walking motions similar to those of a normal
adult pycnogonid. Pycnogonids at this stage were also found in double
infections, occasionally sharing a stalk with a pycnogonid at the pre¬
viously described stage.
The last pre-adult stage seen was found detached from the stalk,
and walking slowly over the stolons, stalks, and hydrothecae of the
E. everta colony (Fig. 12). The three pairs of walking legs still
lacked extensions of the gut, and the chelifores were now achelate.
Active feeding of the encysted pycnogonid on the hydroid was
not observed. However, a direct connection between the pycnogonid
and hydroid gut was demonstrated in live whole-mount preparations.
Pressure on the pycnogonid forced a flow of fluid from the gut directly
into the coelenteron of the hydroid stalk. Pressure on the stalk
caused fluid to flow back into the pycnogonid gut. In neither case
was there any leakage of fluid at the junction of host and parasite.
David Russel
DISCUSSION
Some pycnogonid species are known to be associated with character¬
istic organisms (Hedgpeth, 1980; King, 1973). Such associations have
been reported for adults of 36 and larvae of 21 pycnogonid species
(King, 1973). Only 13 of the approximately 500 species of pycnogonids
have previously been observed feeding while on a host organism (King,
19/3). The host itself may be the food source, being parasitized
(Allman, 1862; Dogiel, 1913; King, 1973) or preyed on (King, 1973),
or the host may simply provide a surface from which food such as small
attached plants, animals, microorganisms, or detritus is obtained,
The present study shows that the early developmental stages of
Tanystylum duospinum and Lecythorhyncus hilgendorfi feed directly from
the gut contents of the hydroid Eucopella everta. Both species of
pycnogonid often occur on the same hydroid colony. The early develop¬
mental stages of T. duospinum are ectoparasitic, while those of L.
hilgendorfi are endoparasitic. While adults of neither species were
seen to feed, adults of T. duospinum were found in abundance on only
one substrate, Aglaophenia struthionides, a hydroid whose color is
matched by the color of the adult pycnogonid.
By adopting different strategies (ecto-, and endoparasitism)
while employing basically similar suctorial feeding methods, T. duospinum
and L. hilgendorfi effectively divide a given hydroid colony into two
non-overlapping resources for food and space, and thus avoid competi¬
tion.
David Russel
ACKNOWLEDGMENT.
I thank everyone who helped make the Spring Quarter at the
Hopkins Marine Station such an enjoyable learning experience. Special
thanks to Chuck Baxter for his jocular skepticism, to Donald P. Abbott
for his time, energy and interest in this project, and to Nancy Oku for
her assistance and patience.
David Russel
FIGURES
PLATE I:
Fig. 1. Tanystylum duospinum; larva (stage V, Dogiel 1913),
ventral view.
Fig. 2. T. duospinum; larva (stage VI, Dogial 1913), dorsal
view.
PLATE II:
Fig. 3. T. duospinum; larva (stage VII, Dogiel 1913) dorsal
view.
Fig. 4. T. duospinum; adult, dorsal view.
PLATE III:
Fig. 5. Lecythorhyncus hilgendorfi; larva (stage IV, Dogiel
1913), encysted in hydranth of Eucopella everta.
Fig. 6.
L. hilgendorfi; larva (stage IV, Dogiel 1913), removed
from cyst, ventral view. Gut extensions are present
in three pairs of folded walking legs and in bases of
chelifores.
Fig. 7.
L. hilgendorfi; larva (stage V, Dogiel 1913), encysted
in E. everta.
L. hilgendorfi; larva (stage V, Dogiel 1913), removed
Fig. 8.
from cyst, ventral view. Legs separated, gut extending
into legs and chelifores.
David Russel
PLATE IV:
Fig. 9.
Fig. 10.
Fig. 11.
Fig. 12.
FICURES CONT'D
L. hilgendorfi; larva (stage uncertain) attached
to deteriorating hydranth of E. everta. Legs con¬
sisting of three pairs of stubs without extensions
of the gut.
L. hilgendorfi; larvae (same as fig. 9.), double
infestation of a single E. everta hydranth.
L. hilgendorfi; larva (stage VII, Dogiel 1913), attached
to remnant of E. everta hydranth by chelifores. Dis¬
tended gut extending into the chelifores but not into
the three pairs of walking legs.
L. hilgendorfi; larva (stage uncertain), detached from
hydroid stalk. Chelifores achelate, gut extending
into chelifores but not into walking legs.
David Russel
10
PLATE I


Fig. 1

7.









Fig. 2.
David Russel






PLATE II
Fig. 3.

Fig. 4.

3
f

11
David Russel
Fig. 5.

Fig. 7.
PLATE III
Fig. 6.
Fig. 8.
David Russel
Fig. 9.
Fig. 11.
PLATE IV
Fig. 10.
Fig. 12
9
David Russel
14
LITERATURE CITED
Allman, G.J., 1862. "On a Remarkable form of Parasitism Among the
Pycnogonidae," Ann. Mag. Nat. Hist., 9(3), 36-43.
Dogiel, V., 1913. "Embryologische Studien an Pantopoden." Z. Wiss.
Zool., 107(4), 575-741; pl. 17-22.
Hedgpeth, J.W. & Haderlie, E.C., 1980. "Pycnogonida: the Sea Spiders'
Chapter 27, pp. 638-639, in Intertidal Invertebrates of California,
Stanford University Press. Stanford, CA.
King, P.E., 1973. Pycnogonids, pp. 7-144. Hutchinson & Co., London.